At high altitudes the boundary layer of the blades of low-pressure turbine of small gas turbines for aircraft propulsion could be dominated by laminar flow and susceptible to flow separation and secondary vortices, with a reduction of turbine performance. The present study concerns the active flow control using plasma actuators to reattach the simulated separation flow over the suction surface of a low-pressure turbine blade at low Reynolds number. Different actuator geometries have been studied: a macro single dielectric barrier discharge (SDBD), a micro SDBD and a micro linear plasma synthetic jet (L-PSJ) with and without thrust vectoring. In particular, the micro plasma actuator was realized and experimentally characterized by measuring the induced wall-jet with Particle Image Velocimetry (PIV) and by evaluating the electrical power consumption. The numerical modelling was used to assess and compare the performances of the different configurations in the separation control. In presence of these active flow control devices the separated flow was successfully reduced.

This paper presents a data acquisition system oriented to detect bubble collapse time and pressure losses in water cavitation in an internal orifice. An experimental campaign on a cavitating flow of water through an orifice has been performed to analyze the flow behavior at different pressures and temperatures. The experiments were based on visual observations and pressure fluctuations frequency analysis. Comparing the visual observations and the spectral analysis of the pressure signals, it is evident that the behavior of the different cavitating flows can be correlated to the frequency spectrum of the upstream, downstream and differential pressure fluctuations. The further reduction of the cavitation number and the consequent increase in the width of the cavitating area are related to a corresponding significant increase of the amplitude of typical frequency components. The spectrogram analysis of the pressure signals leads to the evaluation of the bubble collapse time, also compared with the numerical results calculated by the Rayleigh–Plesset equation.

The present study aims at the implementation of a Matlab/Simulink environment to assess the performance (thrust, specific fuel consumption, aircraft/engine mass, cost, etc.) and environmental impact (greenhouse and pollutant emissions) of conventional and more electric aircrafts. In particular, the benefits of adopting more electric solutions for either aircrafts at given missions specifications can be evaluated. The software, named PLA.N.E.S, includes a design workflow for the input of aircraft specification, kind of architecture (e.g. series or parallel) and for the definition of each component including energy converter (piston engine, turboprop, turbojet, fuel cell, etc.), energy storage system (batteries, super-capacitors), auxiliaries and secondary power systems. It is also possible to setup different energy management strategies for the optimal control of the energy flows among engine, secondary equipment and storage systems during the mission. The tool is designed to be integrated with a multi-objective optimization environment. In the present investigation the tools has been applied to a regional airliner (ATR 72-600) as a case study and two options for the propulsion system were considered: conventional and More Electric Aircraft. In order to validate the proposed turboprop model, the results obtained with PLA.N.E.S. were compared to nominal literature data and numerical values obtained with the Gas Turbine Simulation Program (GSP).

The identification of the water cavitation regime is an important issue in a wide range of machines, as hydraulic machines and internal combustion engine. In the present work several experiments on a water cavitating flow were conducted in order to investigate the influence of pressures and temperature on flow regime transition. In some cases, as the injection of hot fluid or the cryogenic cavitation, the thermal effects could be important. The cavitating flow pattern was analyzed by the images acquired by the high-speed camera and by the pressure signals. Four water cavitation regimes were individuated by the visualizations: no-cavitation, developing, super and jet cavitation. As by image analysis, also by the frequency analysis of the pressure signals, different flow behaviours were identified at the different operating conditions. A useful approach to predict and on-line monitoring the cavitating flow and to investigate the influence of the different parameters on the phenomenon is the application of Artificial Neural Network (ANN). In the present study a three-layer Elman neural network was designed, using as inputs the power spectral density distributions of dynamic differential pressure fluctuations, recorded downstream and upstream the restricted area of the orifice. Results show that the designed neural networks predict the cavitation patterns successfully comparing with the cavitation pattern by visual observation. The Artificial Neural Network underlines also the impact that each input has in the training process, so it is possible to identify the frequency ranges that more influence the different cavitation regimes and the impact of the temperature. A theoretical analysis has been also performed to justify the results of the experimental observations. In this approach the nonlinear dynamics of the bubbles growth have been used on an homogenous vapor - liquid mixture model, so to couple the effects of the internal dynamic bubble with the other flow parameters.

Depending on their input, wind power forecasting models are classified as physical or statistical approaches or a combination of both. Physical models use physical considerations, as meteorological information (Numerical Weather Prediction) and technical characteristics of the wind turbines (hub height, power curve, thrust coefficient). Statistical models use explanatory variables and online measurements, usually employing recursive techniques, like recursive least squares or artificial neural networks (ANNs) which perform a non-linear mapping and provide a robust approach for wind prediction. In this paper a new hybrid method (mixing physical and statistical approaches) is proposed, based on the wavelet decomposition technique and on artificial neural networks, in order to predict power production of a wind farm in different time horizons: 1, 3, 6, 12 and 24 hours. In particular, two approaches are compared, both based on the time series of on-line measured wind power and on the Numerical Weather Predictions; in the first approach, the forecast is carried out only through the training of a neural network which, in the second approach is, instead, used in combination with the wavelet decomposition technique, improving the performance especially over the short time horizons. The error of the different forecast systems is investigated for various forecasting horizons and statistical distributions of the error are calculated and presented.

In this work it is applied a CFD analysis to study the suppression of the boundary layer separation in to a highly - loaded subsonic compressor stator cascade, by active flow control techniques, as zero net mass flux Synthetic Jet (SJA) actuation and Plasma actuation. Active flow control techniques have the potential to delay separation and to increase the pressure ratio. Several works have investigated the use of synthetic jet and plasma actuators on the airfoil, but only few studies have compared the effect of these two devices. Concerning the synthetic jet actuator, a suction/blowing type boundary condition is used, imposing a prescribed sinusoidal velocity depending on velocity amplitude, jet frequency and jet angle of ejection with respect to the wall. Concerning the plasma actuation, the effect is modeled into numerical flow solvers by adding the paraelectric force that represents the plasma force into the momentum equation. The plasma, generated by Dielectric Barrier Discharge, acts as a momentum source to the boundary layer allowing it to remain attached throughout a larger portion of the airfoil. The timeaveraged body force component, acting on the fluid, depends on the frequency and on the applied voltage, the charge density, the electrical field and the dimensional properties of the actuator, like width of the electrodes and gap between the electrodes. Using this numerical model, the effect of plasma actuators to suppress the flow separation over the blade has been investigated, increasing the turbo-machinery performance too. Finally, the comparison between these two devices shows that, reducing the secondary flow structures, both actuation techniques beneficially affect the performance of the stator compressor cascade, even if in the steady jet the costs are relevant.

This study is focused on the active flow control in the internal flow of turbo-jet engines by Dielectric Barrier Discharges Actuators. In particular the application of this techniques has been studied with the aim of improving the aerodynamic behavior of compressor blades by reducing, or even eliminating, flow separation. A numerical electrostatic model has been implemented in the CFD code, to investigate the aerodynamic behaviour of a transonic compressor rotor with and without plasma actuators.

The present work introduces a method for flow and noise control using plasma actuation. The Single Dielectric Barrier Discharge Plasma Actuator (SDBDPA) device is object of study. A discussion of potential applications in flow and noise control in aerospace field is initially done. Then experimental results on separation control applications are presented. The investigated, SDBDPA was manufactured by means of photolithographic technique. Particular attention was paid in materials selection because of possible degradation in plasma environment. The device separation control authority was investigated locating it on a curved plate with a shape designed to reproduce the suction surface of a low pressure turbine (LPT) rotor blade. The changes in the device performances with aging were quantified by monitoring in time the actuator power consumption. Scanning electron microscope (SEM) images on the new and used device were also used to complement the investigation.

Many industrial and transportation applications use combustion in dedicated chambers. Combustion implies, depending upon the nature and the amount of precursors, production of carbon dioxide, pollutants and dusts in terms of particulate matters. With the aim of reducing emission, lean combustion is of great interest. However flame stability within the combustor chamber is a key issue under lean conditions. In fact under lean conditions burners exhibit flame instability, flashback or lean blow out, until the flame extinction. Hence the online monitoring of these phenomena related to combustion instability is essential. One the most used techniques is to check temperature and flame stability by means of sensing probes resisting to high temperatures. Increasing the number of probes, it is possible to perform a 2D and 3D monitoring. However since these probes are costly and require heavy maintenance procedures, it could be wise to exploit imaging processing through cameras directed to portholes across which we can see inner parts, and atmosphere of the furnace/chamber. This paper illustrates findings related to monitoring the flame behaviour different operating conditions chamber by an advanced image processing. A specific algorithm has been developed to characterize the flame, hence, to perform measurements. Myriad filters have been utilized to enhance flame features.

Several active and passive flow control systems are studied to improve the performances of fluid machineries and to increase aerodynamic efficiency of propulsion systems. Among all the well-known active flow control devices, the dielectric barrier discharge plasma actuator (PA) is in full expansion and of great interest in the scientific community. A PA modifies the following behaviour of a fluid by providing an electronically controllable disturbance that brings to drag reduction, flow separation control, enhanced mixing, and noise suppression. PA is potentially easy to construct, has no moving parts and has low power requirements. This leads to its possible applications for separation control in low pressure turbine blade and compressor cascade, tip clearance flow control and compressor stability range extension. The present work reports the design and fabrication of cheap Kapton-based flow turbulence capacitive sensors able to be embedded into aircraft wing profiles and airfoil structures for critical turbulence conditions detection and early-detected separated flows control. The embedded system will provide a Kapton-foil based pressure detection and linear/synthetic jet plasma actuators working in feedback, for prevention and active reduction of separated flow for regional aircraft applications.

Identification of cavitating regime is an important issue in a wide range of fluid dynamic systems. The cavitation behavior is affected by several parameters, as the operating pressure and the fluid temperature. In the present study the cavitating behavior of water inside an orifice was analyzed by images analysis and by pressure signals. Four cavitation regimes were characterized: no-cavitation, developing cavitation, super cavitation and jet cavitation. A three-layer Elman neural network was designed to predict the cavitation regime, from the frequency content of the pressure fluctuations, recorded upstream and downstream the internal orifice. Cavitation regimes were successfully predicted. The designed neural networks were useful also to underline the influence of each operating parameter on the phenomena under investigation; in particular it was possible to identify the frequency ranges that characterize the different cavitation regimes and the influence of the fluid temperature.

This paper consists of a technical and economic feasibility study of a CHCP system for the cogeneration of heat, cooling and power to service a residential cluster consists of 30 terraced houses. The study provides a computational tool that realistically simulates the functioning and the performances of the first engine (a micro-turbine powered by natural gas) in different operating conditions, its economic behaviour - in terms of payback period - and its energy and environmental prospects of the tri-generation plant. The simulations have been carried out using the computer code TRNSYS16, which allowed the valuation of energy demand through the dynamic simulation of heat and energy transfer in buildings by their climatic zone and stratigraphy, and software CYCLE-TEMPO, which allowed the check of first engine’s performances in the nominal conditions provided by manufacturer and the forecast of the system’s performances in off-design operation. In particular, the influence of the environmental boundary conditions (such as temperature and relative humidity’s outside air) has been analyzed, allowing the valuation of the electrical and thermal efficiency’s variation and, consequently, the performances of the entire tri-generation system in terms of primary energy saving. Finally, a methodology for comprehensive evaluation of the tri-generation system is introduced, which includes both technical, energy and economic aspects.

The present work is focused on the investigation of an alternate current driven single dielectric barrier discharge plasma actuator (AC-SDBDPA) for the control of separated flow at Reynolds numbers up to 2·104. Laminar boundary layer separation typically occurs on the suction surface of the low pressure turbines (LPT) blades when operating at high altitude cruise conditions, as the Reynolds number can drop below 2.5·104. In this context, the implementation of an active boundary layer control system able to operate in suppressing separation — only at the critical Reynolds numbers — is of great interest. The SDBDPA was manufactured by means of the photolithographic technique, which ensured a thin metal deposition with high manufacturing reliability control. Actuator operation under sinusoidal voltage at 8 kV amplitude and 2 kHz frequency was considered. Investigations were performed in a closed loop wind tunnel. A curved plate with a shape designed to reproduce the suction surface of a LPT was mounted directly over the bottom wall of the test section. The SDBDPA was inserted in a groove made at the middle of the curved plate, located at the front side of the adverse pressure gradient region. The flow pattern and velocities in absence of actuation were experimentally measured by a two-dimensional (2-D) particle image velocimetry (PIV) system and a laser Doppler velocimetry (LDV) system. PIV measurements were performed in presence of actuation. Simultaneously to the velocity measurements, the voltage applied to the AC-SDBDPA and the discharge current flowing through the circuit were acquired in order to determine the power dissipated by the device. The experimental data were supported by computational fluid dynamics (CFD) simulations based on the finite volume method. In order to deeply investigate the effect of flow separation control by the AC-SDBDPA on the LPT blade performances, the viscous and unsteady Reynolds-averaged Navier-Stokes equations were solved to predict the characteristics of the flow with and without actuation. The actuation effect was modelled as a time-constant body force calculated prior to the fluid flow simulations by using the dual potential algebraic model. The experimental data were used to calibrate and successfully validate the numerical model. An unsteady RANS (URANS) approach, using the k-ω Lam and Bremhorst Low-Reynolds turbulence model was employed, accounting with the main transient flow structures. Results showed that the mixing action of the streamwise fluid with higher momentum and the boundary layer fluid with the lower momentum -due to the AC-SDBDPA-led, depending on the tested Reynolds number, to the alleviation or suppression of the boundary layer flow separation which occurred on the suction surface of the LPT blade. The validated numerical model will allow expanding the study of the actuation effect including different locations and multiple devices, saving considerably experimental efforts.

The cavitation phenomenon interests a wide range of machines, from internal combustion engines to turbines and pumps of all sizes. It affects negatively the hydraulic machines’ performance and may cause materials’ erosion. The cavitation, in most cases, is a phenomenon that develops at a constant temperature, and only a relatively small amount of heat is required for the formation of a significant volume of vapor, and the flow is assumed isothermal. However, in some cases, such as thermosensible fluids and cryogenic liquid, the heat transfer needed for the vaporization is such that phase change occurs at a temperature lower than the ambient liquid temperature. The focus of this research is the experimental and analytical studies of the cavitation phenomena in internal flows in the presence of thermal effects. Experiments have been done on water and nitrogen cavitating flows in orifices at different operating conditions. Transient growth process of the cloud cavitation induced by flow through the throat is observed using high-speed video images and analyzed by pressure signals. The experiments show different cavitating behaviors at different temperatures and different fluids; this is related to the bubble dynamics inside the flow. So to investigate possible explanations for the influence of fluid temperature and of heat transfer during the phase change, initially, a steady, quasi-one-dimensional model has been implemented to study an internal cavitating flow. The nonlinear dynamics of the bubbles has been modeled by Rayleigh–Plesset equation. In the case of nitrogen, thermal effects in the Rayleigh equation are taken into account by considering the vapor pressure at the actual bubble temperature, which is different from the liquid temperature far from the bubble. A convective approach has been used to estimate the bubble temperature. The quasisteady one-dimensional model can be extensively used to conduct parametric studies useful for fast estimation of the overall performance of any geometric design. For complex geometry, three-dimensional CFD codes are necessary. In the present work good agreements have been found between numerical predictions by the CFD FLUENT code, in which a simplified form of the Rayleigh equation taking into account thermal effects has been implemented by external user routines and some experimental observations. DOI: 10.1115/1.4000367

The present work focuses on the numerical modeling of combustion in liquid-propellant rocket engines. Pressure and temperature are well above thermodynamic critical points of both the propellants and then the reactants show liquid-like characteristics of density and gas-like characteristics for diffusivity. The aim of the work is an efficient numerical description of the phenomena and RANS simulations were performed for this purpose. Hence, in the present work different kinetics, combustion models and thermodynamic approaches were used for combustion modeling first in a trans-critical environment, then in the sub-critical state. For phases treatment the pure Eulerian single phase approach was compared with the Lagrangian/Eulerian description. For modeling combustion, the Probability Density Function (PDF) equilibrium and flamelet approaches and the Eddy Dissipation approach, with two different chemical kinetic mechanisms (the Jones-Lindstedt and the Skeletal model), were used. Real Gas (Soave-Redlich-Kwong and Peng-Robinson) equations were applied. To estimate the suitability of different strategies in phenomenon description, a comparison with experimental data from the literature was performed, using the results for different operative conditions of the Mascotte test bench: trans-critical and subcritical condition for oxygen injection. The main result of this study is the individuation of the DPM approach of the most versatile methods to reproduce cryogenic combustion adapted for different operating conditions and producing good results.

Several forecast systems based on Artificial Neural Networks have been developed to predict power production of a wind farm located in a complex terrain, where geographical effects make wind speed predictions difficult) in different time horizons: 1,3,6,12 and 24 h. In the first system, the neural network has been used only as a statistic model based on time series of wind power; later it has been integrated with numerical weather predictions, by which an interesting improvement of the performance has been reached, especially with the longer time horizons. In particular, a sensitivity analysis has been carried out in order to find those numerical weather parameters with the best impact on the forecast. Then, after the implementation of forecast systems based on a single ANN, the two best prediction systems individuated through the sensitivity analysis, have been employed in a hybrid approach, made up of three different ANNs. Besides, a prediction system based on the wavelet decomposition technique has been also carried out in order to evaluate its contribute on the forecast performance in two time horizons (1 and 24 h). The error of the different forecast systems is investigated and the statistical distributions of the error are calculated and presented.

Experimental investigations were performed on a non-premixed liquid fuel-lean burner. The present work aims to the development of a methodology for the recognition of flame instability regimes in industrial and aeronautical burners. Instability, in fact, is an unpleasant aspect of combustive system that negatively impacts on combustion efficiency. The online monitoring of the occurrence of instability conditions, permits to adjust combustion parameters (as fuel or air mass flow, temperature, pressure, etc.) and to stabilize again the flame. High speed visualization systems are promising methods for on-line combustion monitoring. In this study two high speed visualization systems in the visible range and in the infrared spectral region were applied to characterize combustion efficiency and flame stability. Different processing techniques were used to extract representative data from flame images. Wavelet Decomposition and Spectral analysis of pixel intensities of flame images were used for feature extraction. Finally a statistical analysis was performed to identify the most unstable regions of the flame by the pixel intensity variance.

Questo lavoro ha come obiettivo il controllo attivo di flussi aerodinamici mediante l’impiego di attuatori a getto sintetico (SJA – Synthetic Jet Actuators), per possibili applicazioni in campo aeronautico. In particolare, é stato caratterizzato il campo di moto in prossimità di un profilo alare NACA 0015 in condizioni di stallo aerodinamico, con l’obiettivo di analizzare le prestazioni aerodinamiche in presenza di un attuatore a getto sintetico allocato al 12% della lunghezza della corda a partire dal bordo di attacco. Lo studio, dapprima nel caso non controllato, è stato condotto per angoli di attacco compresi fra 12° e 22° a numero di Reynolds pari a Re =8.96 x 105. È stata inoltre effettuata un’analisi di sensibilità dei parametri dell’attuatore (angolo di inclinazione del getto rispetto alla parete del profilo, frequenza ed ampiezza del segnale di attuazione) in corrispondenza di un angolo di attacco pari a 16° (condizione di post-stallo), con l’intento di aumentare il coefficiente di portanza e di ritardare il distacco della vena fluida. Nella scelta dei valori ottimali si è tenuto conto non solo della configurazione con la migliore prestazione aerodinamica, ma anche del fatto che, al fine di poter applicare tale tecnica su un sistema aeronautico, ci sono ulteriori considerazioni da tener farsi. Relativamente all’ampiezza del getto, un picco troppo elevato nel profilo di velocità del getto, deve poter essere compatibile con il peso e le dimensioni dell’attuatore, e la sua realizzazione. Analogamente frequenze di attuazione troppo elevate possono comportare un aumento della potenza richiesta dal dispositivo, senza un aumento eccessivo delle prestazioni aerodinamiche del getto sintetico. Si ottiene, in questo modo, una configurazione ottimale del dispositivo con un aumento significativo della portanza massima (+38% rispetto al caso non controllato) e un ritardo del verificarsi del fenomeno di stallo.

A cavitating two-phase flow of water in a pipe with area shrinkage was experimentally investigated, acquiring at high sampling rate pressure signals and images of the cavitating flow field. The time series of the pressure fluctuations was analyzed in terms of power spectral density and related to the cavitation regimes. Furthermore, the fluctuations of the pressure measurements were also decomposed using the wavelet transform to analyze the frequency distribution of the signals energy with respect to the flow behavior. The energy content at each frequency band of the acquire signals is well related to cavitation flow-field behavior. Moreover, the artificial neural network and the least squares support vector machine (LS-SVM) were implemented to identify the cavitation regime, using, as inputs, the power spectral density distributions of the pressure fluctuations, and some features of the decomposed signals, as the wavelet energy for each decomposition level and wavelet entropy. Results indicate the most accurate model to be used in the cavitation regime identification, underlining the enhanced capability of LS-SVM trained with the input data set based on the wavelet decomposition features

The following work details the experimental and numerical characterization of jet flow control in a coaxial Bunsen burner obtained by an alternate current driven dielectric barrier discharge plasma actuator (AC-DBDPA). Fuel and oxidizer flows could be inverted between the inner circular tube and outer annular tube. Use of the ACDBDPA was intended to improve lean combustion performance. The high voltage (HV) electrode of the ACDBDPA was powered with sinusoidal voltage excitation at 20 kHz frequency and peak-to-peak voltage of about 12 kV. Two burner fueling configurations were tested, characterized by different flow rates in the inner and annular tubes. Values of actuator power dissipation, which were deduced through electrical characterization, ranged between 25 and 28 W. A preliminary analysis of the air-methane flame behavior in presence and absence of plasma actuation was performed. Images of the flame and chemiluminescence emissions showed that the flame luminosity changed and OH radical production increased in presence of the discharge. The thermal and fluid dynamic effects of the plasma actuation on the flow were investigated by means of a non-reacting flow characterization (absence of combustion), where air was used as both fuel and oxidizer. The flow pattern and turbulence in both presence and absence of actuation were studied by laser Doppler velocimetry (LDV) measurements. Flow temperature was acquired using thermocouples. The non-reacting flow experimental results showed that, for a fixed inlet volumetric flow rate, the plasma actuation increased jet velocity and air temperature. Interesting results were obtained from a spectral analysis using the time resolved velocity signals: plasma actuation affected jet mixing and impacted energy distribution between the different flow scales. Complementary numerical simulations identified increasing temperature as primarily responsible for increasing flow velocity. Moreover, numerical predictions permitted better identification of flow regimes under different test conditions.

The aim of the present work is to compare different approaches for modeling combustion in cryogenic LOx/CH4 rockets. The oxidizer is liquid oxygen, delivered to combustion chambers as a spray of droplets; the fuel is gaseous methane. Pressure and temperature are well above thermodynamic critical points of both the propellants and so the reactants show liquid-like characteristics for density and gas-like characteristics for diffusivity. A so complex behavior leads to some difficulties in choosing the most appropriate modeling approach for phenomenon description. In literature it is possible to find a large number of experimental works concerning cryogenic combustion of liquid oxygen and gaseous hydrogen under sub-critical and trans-critical conditions but also for methane as fuel. In the present work LOx-methane combustion for G1 and G2 cases of RCM3-VO4 test plane of ONERA have been investigated to compare different approaches in numerical modeling of cryogenic flame. The most important aspects taken into account are combustion models, kinetics descriptions and thermodynamic properties. The Eddy Dissipation combustion model and the Probability density function PDF model were compared; the Jones- Lindstedt and the Skeletal mechanisms were used for kinetics description for the Eddy Dissipation model; for thermodynamic approach, in the current work gas was modeled using both ideal gas and real gas equation of state (Peng-Robinson and SRK) and liquid oxygen was treated with a Lagrangian approach in a hybrid Eulerian- Lagrangian approach for mixture. Numerical predictions were compared with experimental data from literature.

An important issue in the study of active flow control by dielectric barrier discharge plasma actuators is the implementation of a model that accurately predicts the induced flow. In this study different numerical plasma models were compared, by estimating the Lorentz force generated by the plasma without spatially resolving the plasma chemistry directly. The body force distribution computed with each model was introduced in the CFD code to compute the resulting velocity field induced by the plasma actuators on a quiescent flow on a flat plate. The limit of many of these models is the necessity to calibrate some parameters, as the charge distribution on the dielectric surface. In the present study a unique method has been defined to establish this parameter, by applying an unsteady plasma model based only on the solution of the electrostatic potential field. Steady linear body fields were also estimated based on results of the previous unsteady plasma models and applied to simulate plasma effects on a highly loaded compressor cascade. By the active flow control the secondary flow is successfully reduced, resulting in a decreased total pressure loss and a rise in the static pressure.

A high penetration of wind energy into the electricity market requires a parallel development of efficient wind power forecasting models. Different hybrid forecasting methods were applied to wind power prediction, using historical data and numerical weather predictions (NWP). A comparative study was carried out for the prediction of the power production of a wind farm located in complex terrain. The performances of Least-Squares Support Vector Machine (LS-SVM) with Wavelet Decomposition (WD) were evaluated at different time horizons and compared to hybrid Artificial Neural Network (ANN)-based methods. It is acknowledged that hybrid methods based on LS-SVM with WD mostly outperform other methods. A decomposition of the commonly known root mean square error was beneficial for a better understanding of the origin of the differences between prediction and measurement and to compare the accuracy of the different models. A sensitivity analysis was also carried out in order to underline the impact that each input had in the network training process for ANN. In the case of ANN with the WD technique, the sensitivity analysis was repeated on each component obtained by the decomposition.

A numerical investigation is performed to analyse the suppression of the boundary layer separation first on a NACA 0015 airfoil and subsequently in a highly loaded subsonic compressor stator cascade using two different active flow control techniques: synthetic jet actuators (SJA) and continuous jet actuators (CJA). In particular, the effort is concentrated towards understanding the physics of the phenomenon, which makes one configuration perform better than the other one as an active flow control (AFC) system. The analysis of the interaction of the jet with the boundary layer was performed, with a description of the vortical structures, which are beneficial for the mixing of the boundary layer and the entrainment of energy from the external flow towards the most inner layers. Regarding airfoils, the comparisons of the SJA with the CJA were performed considering two similarity conditions: identical momentum coefficients and identical amounts of energy fed into the cross flow. Finally, the flow behaviour and the topology structure in a highly loaded compressor cascade with and without AFC are examined. Active flow control using synthetic jet actuators proved to be attractive because it could exploit the unsteady phenomena, inhibit the separation, improve and control the aerodynamics of the flow in both external aerodynamics and turbomachinery components. Furthermore, comparing the CJA and the SJA for active flow control on the compressor at similar momentum coefficients shows that the relative reduction of the total pressure losses for the SJA is approximately twice as large as that for the CJA. It should also be remarked that even if the two AFC configurations present similar effects on the reduction of secondary flow structures, the SJA is more advantageous than the CJA from the regaining energy viewpoint.

Recent advances in gas turbine combustor design aimed at achieving low NOx emissions have focused on locally leaner combustion by rapid mixing of fuel and air. Multipoint injection leads to a fast and efficient mixing with the control of the spatial fuel distribution. In the present work, an experimental study on combustion phenomena in a liquid fuel burner, which operates in non-premixed (single point injection) and partially-premixed regimes (multipoint injections), was carried out in order to investigate the effect of the injection mode. In both the cases the lean combustion behavior was investigated in proximity of the blow-out limit. An intensified high rate CCD was used for flame imaging in the ultraviolet spectral range. It was equipped with different optical filters to selectively record single species chemiluminescence emissions (e.g. OH∗, CH∗). Analogous filters were also used in association with photomultiplier (PMT) tubes. Finally the NOx emissions were monitored. Furthermore, preliminary computational fluid dynamic (CFD) simulations were also performed under the typical combustor operation conditions to provide insight into the mixing of the air and the fuel under the different injection modes and the related flame pattern.

The forecasting techniques are affected by the renewable sources randomness. Improvements of the prediction models with more accurate results and lower error are necessary for future development of the microgrids projects and of the economic dispatch sector. The LS-SVM (Least Square Support Vector Machine), a relatively unexplored neural network known as GMDH (Group Method of Data Handling) and a novel hybrid algorithm GLSSVM (Group Least Square Support Vector Machine), based on the combination of the first two models, were implemented to forecast the PV (Photovoltaic) output power at several time horizons up to 24 h. In order to improve the forecasting accuracy, each model was combined with three strategies for multi-step ahead forecast (Direct, Recursive and DirRec). A detail analysis of the normalized mean error is carried out to compare the different forecasting methods, using the historical PV output power data of a 960 kWP grid connected PV system in the south of Italy. The outcomes demonstrate the GLSSVM method with the DirRec strategy can give a normalized error of 2.92% under different weather conditions with evident improvements respect to the traditional ANN (Artificial Neural Network)

Wind forecasting models are divided in two main categories, physical and statistical. The former are based on Numerical Weather Prediction (NWP). The statistical models, on the other hand, use on-line measurements. In this paper we use hybrid models, which combine elements of both types. In particular three forecast systems based on Artificial Neural Networks have been developed in order to predict power production of a wind farm in different time horizons: 1, 3, 6, 12 and 24 hours. In the first forecast system, the neural network has been used only as a statistic model based on time series of on-line measured wind power, while in the second and third forecast systems different combinations of measured data and numerical weather predictions have been used, improving the performance in the predictions, especially over long time horizons. The error of the different forecast systems is investigated for various forecasting horizons and statistical distributions of the error are calculated and presented.

This paper presents an investigation of LOX/CH4 Liquid Rocket Engines injector flames to investigate the impact of real gas effects and the combustion models on the predictions. In liquid-propellant rocket engines the combustion occurs at operating conditions well above of the thermodynamic critical points of the fluid where reactants properties show liquid-like densities, gas-like diffusivities, and pressure-dependent solubility. So using real gas properties as accurately as possible is a key issue in the preliminary design of LRE injectors and combustion chambers. In the numerical study of LOX/CH4 jet flames a critical aspect is the choice of the combustion model, that should be accurate and a well compromise between phenomena description and computational costs. Complex, simplified and reduced kinetics scheme could be implemented in the CFD modeling of cryogenic spray and to make a well choice of the modeling approach it is necessary to estimate the mixing and kinetic time scales in the case of study. In the rockets it’s generally possible to assume that the chemistry is infinitely fast and that burnt gas conditions can be approximated like similar to the chemical equilibrium condition. Under these conditions a simple finite rate combustion model is less realistic; otherwise an Eddy Dissipation Approach or Flamelet model could more realistically model phenomena. In this work LOX/CH4 jet flames at high pressure have been simulated by implementing different kinetic mechanisms usually presented in literature for the methane/air flame, starting from the simple one step mechanism up to the detailed Skeletal model derived from the Grimech 3.0. Then results obtained using some simplified mechanisms, as the Modified Jones-Lindstedt kinetics model, more accurate for combustion in presence of pure oxygen have been compared with the previous ones. EDC and Non- Premixed Combustion Model, including flamelet approaches are implemented in the CFD Fluent v.13.0 code, like in the present work, it is not possible to take into account real gas effetcs with the non-premixed model. Real gas effects have been also considered in the case of EDC combustion model. In a liquid rocket engine at high pressures real gas effect needs to be modelled by using a Real Gas Equation of State (RG EOS). In the current work the real gas effects have been modelled by the Soave-Redlich-Kwong (SRK) real gas model.

In order to satisfy the requirements of Directive 2010/31/EU for Zero Energy Buildings (ZEB), innovative solutions were investigated for building HVAC systems. Horizontal air-ground heat exchangers (HAGHE) offer a significant contribution in reducing energy consumption for ventilation, using the thermal energy stored underground, in order to pre-heat or pre-cool the ventilation air, in winter and summer, respectively. This is particularly interesting in applications for industrial, commercial and education buildings where keeping the indoor air quality under control is extremely important. Experimental measurements show that, throughout the year, the outside air temperature fluctuations are mitigated at sufficient ground depth (about 3 m) because of the high thermal inertia of the soil, the ground temperature is relatively constant and instead higher than that of the outside air in winter and lower in summer. The study aims to numerically investigate the behavior of HAGHE by varying the air flow rate and soil conductivity in unsteady conditions by using annual weather data of South-East Italy. The analysis shows that, in warm climates, the HAGHE brings a real advantage for only a few hours daily in winter, while it shows significant benefits in the summer for the cooling of ventilation air up to several temperature degrees, already by a short pipe.

The aim of the present work was to investigate the influence of pressures and temperature on cavitation in an orifice flow. In particular image analysis and frequency analysis of the experimental pressure signals were used to identify different flow behaviors at different operating conditions. . In order to show the effect of the temperature at different cavitation numbers, the waterfall diagrams of the frequency components of the downstream and upstream pressure at 293-348 K were investigated. Bubbles growth and collapse generate pressure fluctuations so frequency spectra can be related to cavitation behavior. The amplitude of the upstream FFT is higher than the downstream one. In particular the frequency range 0-10 kHz is investigated. The amplitude of FFT were used to training an ANN. ANN permitted to identify the different cavitation regimes and to lightly the influence of each input parameters about the learning of the network and then about the cavitation phenomenon. Following, a theoretical analysis was performed to justify the results of the experimental observations. In this approach the nonlinear dynamics of the bubbles growth were described by the Rayleigh-Plesset equation in a one-dimensional code, so to couple the effects of the internal dynamic bubble with the other flow parameters (pressure, velocity, void fraction, temperature, etc..).

The weather data have a relevant impact on the photovoltaic (PV) power forecast, furthermore the PV power prediction methods need the historical data as input. The data presented in this article concern measured values of ambient temperature, module temperature, solar radiation in a Mediterranean climate. Hourly samples of the PV output power of 960kWP system located in Southern Italy were supplied for more 500 days. The data sets, given in , were used in DOI: 10.1016/j.enconman.2015.04.078, M.G. De Giorgi, P.M. Congedo, M. Malvoni, D. Laforgia (2015) [1] to compare Artificial Neural Networks and Least Square Support Vector Machines. It was found that LS-SVM with Wavelet Decomposition (WD) outperforms ANN method. In DOI: 10.1016/j.energy.2016.04.020, M.G. De Giorgi, P.M. Congedo, M. Malvoni (2016) [2] the same data were used for comparing different strategies for multi-step ahead forecast based on the hybrid Group Method of Data Handling networks and Least Square Support Vector Machine. The predicted PV power values by three models were reported in .

The data concern the photovoltaic (PV) power, forecasted by a hybrid model that considers weather variations and applies a technique to reduce the input data size, as presented in the paper entitled "Photovoltaic forecast based on hybrid pca-lssvm using dimensionality reducted data" (M. Malvoni, M.G. De Giorgi, P.M. Congedo, 2015) [1]. The quadratic Renyi entropy criteria together with the principal component analysis (PCA) are applied to the Least Squares Support Vector Machines (LS-SVM) to predict the PV power in the day-ahead time frame. The data here shared represent the proposed approach results. Hourly PV power predictions for 1,3,6,12, 24 ahead hours and for different data reduction sizes are provided in Supplementary material.

The quality of combustion process has an impact on combustion itself and mainly on emissions. This1 latter is one of the major concerns in an environmental viewpoint; for instance, the amount of oxygen is an indicator of bad and good combustion. It is also a constraint for regulating pollutant production, in particular dust that is also a vector transporting harmful micropollutants. The paper illustrates combustion quality detection by means of imaging. The work aims at retrieving possible precursors of combustion deterioration, and instability and allowing decision makers to provide accordingly. Images have been taken from an experimental setup.

The combustion phenomena in liquid-propellant rocket engines are highly complex. The combustion occurs at operating conditions well above of the thermodynamic critical points of the fluid where reactants properties show liquid-like densities, gas-like diffusivities, and pressure-dependent solubility. Actually, there is a great interest in the development of reusable liquid rocket engines that operates with methane and liquid oxygen as propellants. In the numerical study of LOX/CH4 jet flames there are some critical aspects to be taken into account. The choice of the combustion model is a critical point: it should be accurate in the phenomena description but it should also characterized by a low computational cost. In the present study different combustion models were used as the Eddy-dissipation finite-rate approach based on Arrhenius chemical kinetics, the equilibrium mixture fraction model (PDF) and the Steady State Flamelet approaches. In the case of reacting models based on chemical kinetics, both simplified and more complex kinetics models can be used to numerically describe the flames but the critical point in the choice is the individuation of the best compromise between computational cost and accuracy. In this work different chemical kinetics schemes were used, as the Skeletal mechanism and the Jones- Lindstedt mechanism, that permit to limit the number of reactions and species but taking into account also the intermediate species in the flame. Finally a purpose of this work is also to develop a pure Eulerian (i.e., single-phase) methodology by using both ideal gas and real gas equation of state and to compare with the discrete phase approach that uses an Eulerian description of the gas phase and Lagrangian equations for the dilute spray. For all the models used, a comparison with experimental data from literature was performed.

In recent years, single dielectric barrier discharge (SDBD) plasma actuators have gained great interest among all the active flow control devices typically employed in aerospace and turbomachinery applications. Compared with the macro SDBDs, the micro single dielectric barrier discharge (MSDBD) actuators showed a higher efficiency in conversion of input electrical power to delivered mechanical power . This article provides data regarding the performances of a MSDBD plasma actuator . The power dissipation values and the experimental and numerical induced velocity fields [6] are provided. The present data support and enrich the research article entitled "Optimization of micro single dielectric barrier discharge plasma actuator models based on experimental velocity and body force fields" by Pescini et al.

The reduction of nitric oxides (NOx) in aircraft engines, gas turbines, or internal combustion engines is a main issue in the design of novel combustion systems. The reduction of the NOx emissions might be reached by lean combustion. However, the major issue is the stabilization of the flame under lean conditions. In this context, the present work investigates the possibility of increasing the combustion efficiency of a lean flame through the employment of a plasma actuator, operated by both nanosecond repetitively pulsed high voltage (NRPP) and sinusoidal DBD high voltage (HV). Different actuation conditions have been tested to stabilize and improve the efficiency of a lean non premixed methane/air flame in a Bunsen-type coaxial burner with central fuel jet. An image processing approach was used to characterize the flame behavior near blowout conditions

A dielectric barrier discharge plasma actuator (PA) was designed and manufactured with microscale dimensions using photolithographic process on fibre glass substrate. AC operation under sinusoidal voltage was investigated experimentally by means of electrical characterisation, smoke flow visualisations and particle image velocimetry. The performances of the micro PA were evaluated and compared with the ones of a macro PA found in this literature. The velocity induced by the micro PA was comparable with the macro PA one, but with lower applied voltage, electrical power dissipation and actuator size. This is particularly interesting for potential applications in turbomachinery.

This study aims to develop effcient, reproducible and durable plasma actuators. Thin metal deposition and high manufacturing reliability control were gained by adopting the photographic technique. As dielectric material degradation and electrodes corrosion are significant issues, emphasis was put in selecting materials that could withstand the plasma environment. Gold was selected as electrode material for all the geometric configurations. A Schott alkali-free borosilicate glass substrate was chosen as dielectric because of its resis- tance to material degradation in the plasma environment. Measurements of the actuator power consumption and capacitance were used to quantify the change in actuator performance over time. Moreover, scanning electron microscope images and energy dispersive X-ray spectroscopy analysis were utilised on the used devices to help explain the observed changes in actuator performance. After usage, unexpected degradation effects were ob- served on the gold electrodes, in both the front side and back side of each actuator. The electrode edges retracted and residual gold remained on the glass substrate. In particu- lar, the exposed electrode width reduced by up to approximately 50%, depending on the actuator working conditions. Moreover, the morphology of the electrode surfaces changed and melting/crystalline reorganization of the gold layer was observed at the borders of the shrunk electrodes. As a consequence, a decrease of the actuator cold capacitance with age was evident. Furthermore, a rise in the actuator power consumption and effective capac- itance was noted with increasing actuation time. At the same time, the actuator aging process produced an increase in its effective capacitance at a given dissipated powe

This work reports on finite element method (FEM) design and fabrication of low cost capacitive pressure sensors for aircraft applications; this work is part of a research activity aimed to develop an embedded sensor - Actuator system composed by multi-measure points pressure sensors for flow turbulence detection and coupled plasma actuators for control of separated flows on aircraft wings structures. The system uses sensors feedback information to provide fast reattachment of boundary layer separation flow on the suction surface of regional aircraft vehicles. Flow separation has great impact on the performance and safety of an aircraft and it can be predicted by quantifying the pressure gradients along the wing wall. Considering the absolute pressure values on a NACA 0012 profile as a function of the angle of attack, high sensitivity measurements of differential pressure can be obtained by positioning the sensor-nodes at points on the airfoil surface where the P/Pstall ratio between the absolute pressure at different angles of attack and the pressure measured in stall condition is maximized

The advancement of photovoltaic (PV) energy into electricity market requires efficient photovoltaic power prediction systems. Furthermore the analysis of PV power forecasting errors is essential for optimal unit commitment and economic dispatch of power systems with significant PV power penetrations. This study is focused on the forecasting of the power output of a photovoltaic system located in Apulia - South East of Italy at different forecasting horizons, using historical output power data and performed by hybrid statistical models based on Least Square Support Vector Machines (LS-SVM) with Wavelet Decomposition (WD). Five forecasting horizons, from 1 h up to 24 h, were considered. A detailed error analysis, by mean error and statistical distributions was carried out to compare the performance with the traditional Artificial Neural Network (ANN) and LS-SVM without the WD. The decomposition of the RMSE into three contributions (bias, standard deviation bias and dispersion) and the estimation of the skewness and kurtosis statistical metrics provide a better understanding of the differences between prediction and measurement values. The hybrid method based on LS-SVM and WD out-performs other methods in the majority of cases. It is also evaluated the impact of the accuracy of the forecasting method on the imbalance penalties. The most accurate forecasts permit to reduce such penalties and thus maximize revenue. © 2015 Elsevier Ltd. All rights reserved.

The integration of wind farms in power networks has become an important problem. This is because the electricity produced cannot be preserved because of the high cost of storage and electricity production must follow market demand. Short-long-range wind forecasting over different lengths/periods of time is becoming an important process for the management of wind farms. Time series modelling of wind speeds is based upon the valid assumption that all the causative factors are implicitly accounted for in the sequence of occurrence of the process itself. Hence time series modelling is equivalent to physical modelling. Auto Regressive Moving Average (ARMA) models, which perform a linear mapping between inputs and outputs, and Artificial Neural Networks (ANNs) and Adaptive Neuro-Fuzzy Inference Systems (ANFIS), which perform a non-linear mapping, provide a robust approach to wind power prediction. In this work, these models are developed in order to forecast power production of a wind farm with three wind turbines, using real load data and comparing different time prediction periods. This comparative analysis takes in the first time, various forecasting methods, time horizons and a deep performance analysis focused upon the normalised mean error and the statistical distribution hereof in order to evaluate error distribution within a narrower curve and therefore forecasting methods whereby it is more improbable to make errors in prediction.

Experiments on a water cavitating orifice were conducted to investigate the influence of pressure and temperature on flow regime transition due to cavitation. The thermal effects could be important in cases with cryogenic cavitation or hot fluid injection. The investigations were based on CCD observations and a pressure fluctuations frequency analysis. The high-speed photographic recordings were used to analyze the cavitation evolution and individuate the frequency content of the two-phase flow by processing the pixel-intensity time-series data. The cavitating structures showed different behaviors and characteristics with variations in operating conditions, as the pressure inside the orifice and the flow temperature . The flow regime map for the cavitating flow was obtained using experimental observations to analyze the occurrence of the different two-phase flow regime transitions at various operating conditions. As the pressure at the orifice inlet increased, at the same downstream pressure, cavitation inception occurred. The decrease of the cavitation number brought a significant increase in cavitation zone extension. As the pressure drop inside the orifice increased, the cavitation was characterized by an evident increase in cavitation zone length to the outlet of the orifice. With a further cavitation number decrease, the transition to jet cavitation was evident. The temperature influenced both the cavitation intensity and the cavitation number at which different two-phase flow regime transitions occurred, which tended to increase with temperature. The vapor fraction was estimated using an image processing algorithm. The frequency content given by the pressure fluctuations was analyzed and compared with the frequency spectra obtained from the visual observations. The behavior of the different cavitating flows could be correlated to the frequency spectrum of the pressure fluctuations measured upstream and downstream of the orifice. The cavitation number reduction and consequent increase in cavitating area width were related to a corresponding significant increase in the amplitude of typical frequency components. The transition to jet cavitation was characterized by a significant increase in the first peak in the frequency spectrum; weaker spectral peaks were also present at high cavitation numbers.

Nowadays several active flow control systems, particularly dielectric barrier discharge plasma actuators, appear to be effective for the control of flow stream separation and to improve performance of turbomachinery. However these applications require high actuation strength, higher than the one generated by conventional macro plasma actuators. Research is actually improving the design of plasma actuator in order to enhance the flow control capability and reduce the power consumption. In this contest, this work concerns the implementation of a micro plasma actuator for the active control in a compressor cascade. For this aim, firstly the micro actuator was developed and an experimental characterization of the flow induced by the device was done. The induced flow field was studied by means of Particle Image Velocimetry and Laser Doppler Velocimetry. The dissipated power was also evaluated. Experimental results were used to validate a multi-physics numerical model for the prediction of the body forces induced by the plasma actuator. Finally, the obtained body force field was used for modeling the separation control by means of the micro plasma actuator in a highly-loaded subsonic compressor stator.

In the present work the performance of a multipoint lean direct injection strategy for low emission aero-propulsion systems has been experimentally and numerically investigated, and compared with the single point injection strategy. A swirler liquid fueled combustor was designed and used in experiments to investigate the flame behavior in lean and ultra-lean conditions for both the single-point and the multi-points injection strategies. Multipoint injection has been realized injecting an amount of fuel upstream the swirler inlet and using also the central injector as a “pilot” injection. As regarding the experimental facilities, the combustor is equipped with four optical accesses for high speed flame imaging and with pressure and temperature sensors. Experimental data on flame characteristics and pollutant emissions are obtained. The characterization of the flame was realized using intensified high rate CCD camera for the acquisition in the ultraviolet spectral range. In front of the camera various combinations of optical filters were installed to selectively record the respective chemiluminescent species (OH* and CH*). Computational fluid dynamic (CFD) simulations were also performed for a deeper understanding of the flame characteristics under the two injection strategies. The typical combustor operations were reproduced to more deeply understand the differences between the injection modes and the related flame patterns. The numerical results show different temperature and species fields predicted for the non-premixed and the partially premixed cases and furnish relevant information about the fluid dynamics in the combustion chamber in both the injection conditions.

High altitude cruise represents a crucial issue for small size low pressure turbines (LPT), commonly used in the propulsion of unmanned air vehicles (UAVs). The Reynolds number can drop below 25000, which in turn can lead to laminar boundary layer separation on the suction surface of the blades. This makes the turbine working in off-design conditions with very poor performances. Modifying the blade shape to counteract the boundary layer separation is not a feasible solution since the performance of the turbine will be adversely affected at the engine design conditions (take-off and landing). Therefore, the implementation of a boundary layer control system on the suction side of the turbine able to operate only at low Reynolds number is the most practical solution. The present study investigates experimentally and numerically the potential of an alternate current (AC) driven Single Dielectric Barrier Discharge Plasma Actuator (AC-SDBDPA) to reattach the separated flow at a Reynolds number around 2·104. The SDBDPA was designed and manufactured by means of lithographic technique, which ensured a thin metal deposition with high manufacturing reliability control. The experimental approach comprised the actuator testing over a curved plate with a shape designed to reproduce the suction surface of a LPT. A closed loop wind tunnel was employed. The curved plate was mounted directly over the bottom wall of the test section. The AC-SDBDPA was placed in a grove made at the middle of the curved plate and located at the front side of the adverse pressure gradient region. Sinusoidal voltage excitation was tested. The flow measurements –with and without actuation– were carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Planar measurements were performed over the curved plate at the midspan plane. Simultaneously to the velocity measurements the applied voltage and the discharge current were acquired in order to determine the device dissipated power. The experimental data was complemented with CFD simulations based on the finite volume method. The actuator effect was modelled as a time-constant body force calculated prior to the fluid flow simulation by using a dual potential algebraic model. Reynolds Averaged Navier Stokes (RANS) method was used to consider the turbulence effect. The validity of the numerical model allows to expand the study of the actuation effect including different locations and multiple devices, saving considerably experimental efforts.

The article presents the data related to the flame acquisitions in a liquid-fuel gas turbine derived burner operating in non-premixed mode under three different equivalence fuel/air ratio, which corresponds to a richer, an intermediate, and an ultra-lean condition, near lean blowout (LBO). The data were collected with two high speed visualization systems which acquired in the visible (VIS) and in the infrared (NIR) spectral region. Furthermore chemiluminescence measurements, which have been performed with a photomultiplier (PMT), equipped with an OH* filter, and gas exhaust measurements were also given. For each acquisition the data were related to operating parameters as pressure, temperature and equivalent fuel/air ratio. The data are related to the research article “Image processing for the characterization of flame stability in a non-premixed liquid fuel burner near lean blowout” in Aerospace Science and Technology

In the present work, an experimental investigation was performed to characterize lean combustion flames in a liquid fuel burner. Two different regimes were investigated: non-premixed and partially-premixed combustion modes. The fuel mass flow rate was fixed and the air mass flow was reduced until the blow-out limit was reached. A high rate CCD and a PMT tube equipped wit1h an OH* filter were used for the acquisitions. Statistical and spectral post-processing methods were applied obtaining variance maps, trends of the averaged value of variance with respect to the equivalent fuel/air ratio and trends of the wavelets energy contents with respect to the frequency ranges. Results underline that the onset of flame instability occurs at higher fuel/air ratio in the non-premixed combustion regime compared to the partially premixed mode. Furthermore the rise of CO emissions starts at the leaner conditions in the case of partially premixed combustion. The present work also shows that imaging techniques are suitable to individuate this instability incipience using spectral and statistical parameters extracted by the temporal series of flame images, hence they might be implemented in online monitoring systems.

With the aim to characterize the flame behavior when ultra-lean combustion conditions are reached, an experimental investigation was performed on a liquidfuel gas turbine derived burner, at different fuel/air ratios and comparing different fuel injection modes. Ultra lean conditions have a negative impact on combustion efficiency for the instabilities insurgence. High speed acquisitions by a CCD camera were performed to investigate the behavior of the spray and the flame close to lean blowout. Statistical and spectral analyses were also applied to the flame acquisitions to extract suitable parameters for blowout recognition.

Reduction of nitric oxides (NOx) in aircraft engines and in gas turbines by lean combustion is of great interest in the design of novel combustion systems. However, the stabilization of the flame under lean conditions is a main issue. In this context, the present work investigates the effects of sinusoidal dielectric barrier discharge (DBD) on a lean inverse diffusive methane/air flame in a Bunsen-type burner under different actuation conditions. The flame appearance was investigated with fixed methane loading (mass flux), but with varying inner airflow rate. High-speed flame imaging was done by using an intensified (charge-coupled device) CCD camera equipped with different optical filters in order to selectively record signals from the chemiluminescent species OH*, CH*, or CO2* to evaluate the flame behavior in presence of plasma actuation. The electrical power consumption was less than 33 W. It was evident that the plasma flame enhancement was significantly influenced by the plasma discharges, particularly at high inner airflow rates. The flame structure changes drastically when the dissipated plasma power increases. The flame area decreases due to the enhancement of mixing and chemical reactions that lead to a more anchored flame on the quartz exit with a reduction of the flame length.

The effects of active separation control on a highly loaded subsonic compressor stator cascade were numerically studied by comparing the behavior of different devices: continuous jet actuator (CJA), zero-net mass-flux synthetic jet actuator (SJA), and plasma actuator (PA). For the jet actuator modeling, a suction/blowing type boundary condition was used, imposing a time-constant velocity for the CJA case and a prescribed sinusoidal time-varying velocity for the SJA case. For the PA case, the body force, which represents the effect of the plasma actuator on the flow, was added to the momentum equations in the computational fluid dynamics (CFD) code. The PA is slightly more efficient for the reduction of flow separation in the region just downstream of the blade actuators. However, at the same mechanical power delivered by the actuator to the fluid, the SJAs are more advantageous than the CJAs and slightly outperform plasma actuator application from the pressure loss reduction and pressure rise viewpoints. The fluidic jets have low power requirements, whereas the power consumption would be prohibitive for the PA configuration that shows low fluid mechanic efficiency

Stochastic nature of weather conditions influences the photovoltaic power forecasts. The present work investigates the accuracy performance of data-driven methods for PV power ahead prediction when different data preprocessing techniques are applied to input datasets. The Wavelet Decomposition and the Principal Component Analysis were proposed to decompose meteorological data used as inputs for the forecasts. A time series forecasting method as the GLSSVM (Group Least Square Support Vector Machine) that combines the Least Square Support Vector Machines (LS-SVM) and Group Method of Data Handling (GMDH) was applied to the measured weather data and implemented for day-ahead PV generation forecast.

Monitoring and characterization of combustion flames by digital image processing is an active research topic. This study experimentally investigates the feasibility of high speed visualization techniques for combustion instability monitoring in a swirl liquid-fueled lean combustor for different air/fuel ratios. Instability, in fact, is an unpleasant aspect in the combustive system that negatively impacts on combustion efficiency. This work investigates methods for extracting significant parameters using the geometrical and luminous data of the flame images; some flame features are related to the combustion regimes. The stability of the flame is identified using spectral and wavelet-based analysis of the pixel intensities of the flame images. In particular the most flame unstable regions were identified by analyzing the two dimensional maps of different physical quantities. The impact of the fuel/air ratio on the stability of the flame is investigated also by a Monochromator/Photomultiplier system (PMT). The results support the potential of the methods described for flame monitoring.

Improving energy efficiency in buildings and promoting renewables are key objectives of European energy policies. Several technological measures are being developed to enhance the energy performance of buildings. Among these, geothermal systems present a huge potential to reduce energy consumption for mechanical ventilation and cooling, but their behavior depending on varying parameters, boundary and climatic conditions is not fully established. In this paper a horizontal air-ground heat exchanger (HAGHE) system is studied by the development of a computational fluid dynamics (CFD) model. Summer and winter conditions representative of the Mediterranean climate are analyzed to evaluate operation and thermal performance differences. A particular focus is given to humidity variations as this parameter has a major impact on indoor air quality and comfort. Results show the benefits that HAGHE systems can provide in reducing energy consumption in all seasons, in summer when free-cooling can be implemented avoiding post air treatment using heat pumps.

Abstract In the present work, an experimental investigation was performed by varying the fuel/air ratio of a liquid-fuel gas turbine derived burner in the non-premixed mode, until an ultra-lean combustion condition was reached. In this condition, flame instabilities occur with negative impacts on combustion efficiency. Two high speed visualization systems in the visible range and in the infrared spectral region were used. Moreover, they were supported by an OH⁎ chemiluminescence measurement and by gas exhaust measurements. Different techniques were used starting from the luminosity signal of each pixel: the Wavelet Decomposition to calculate the wavelet energy, the frequency analysis of pixel intensities of the flame images to estimate the dominant frequency, finally the statistical analysis to calculate the pixel intensity variance. Both the statistical and frequency analyses were applied to the OH⁎ chemiluminescence data. One of the most important results of the present work regarded the capability of imaging techniques to individuate the instability insurgence and to be used as a predictive tool. Furthermore 2D maps of some parameters, extracted by the wavelet-based analysis of flame images, permitted to investigate local unsteadiness in the flame area.

In the present work cavitation in liquid hydrogen and nitrogen was investigated by using the open source toolbox OpenFOAM. Simulations were performed by means of a mass transfer model, based on the homogeneous mixture approach in combination with the Volume of Fluid (VOF) method for the reconstruction the liquid-vapor interface. Two additional transport equations were considered, i.e. the liquid volume fraction advection and the temperature equation. The implementation of an extended Schnerr- Sauer model allowed for the introduction of the thermal effects in terms of latent heat release/absorption and convective heat transfer inside the liquid-vapor interface. A set of Antoine-like equations relate the saturation conditions to the local conditions.

Abstract Low environmental impact is a main issue in the design of novel combustion systems, as aircraft engines. In this context, the present work investigates the possibility to increase the combustion efficiency of a lean flame through the use of sinusoidally driven dielectric barrier discharge (DBD) plasma actuator. The effect of the plasma discharge on a lean non premixed methane/air flame in a Bunsen-type burner has been studied for two different configurations: the normal diffusive flame (NDF) and the inverse diffusive flame (IDF). The flame behavior was investigated by chemiluminescence imaging through an intensified CCD camera. Optical filters were installed in front of the camera, aiming to selectively record signal from the chemiluminescent species OH*, CH*, or CO2*. This allowed evaluating the changes occurring in presence of plasma actuation in term of flame emissions. It was shown that the plasma effects are significantly influenced by the burner and DBD configuration. A plasma power of approximately 25 W permitted to increase the air mass flow rate at which lean blowout appears; it rose up to 30% for low methane flow rate and up to 10% at high fuel flow rate.

The performance assessment of photovoltaic (PV) systems is a complex process. Several meteorological data sources are available to evaluate the PV system generation. Different computing models can be applied to determinate the solar irradiance on the plane of the array (POA). The cooling effect of the PV module due to the wind speed should not be neglected. The present study may support several users to perform more accurate PV energy predictions, providing important suggestions to develop future PV system projects with more high reliability. Perez and Hay-Davies models for the computing of the irradiance on tilted surfaces are combined with three meteorological datasets, characterized by different monitoring period and meteo station location, to estimate the POA irradiance, the module temperature and PV energy output for a PV system located in the Mediterranean climate area. Prediction results are performed by the PVsyst tool and compared with the actual data. Simulations are carried out taking into account the wind effects on the PV module performance. Results demonstrate that the geographic features of the location, in which the weather station is located, have higher impact on the estimations of the PV system performance than the distance between the PV system and the meteo station. Perez and Hay-Davies models provide predictions of the PV energy and the module temperature with a difference up to 3% and 1% respectively. Yearly average wind speed in the range 2–4 m/s fosters a cooling effect up to 3% higher than one due to the wind magnitude less of 2 m/s, increasing the PV energy up to 1%

The accuracy of numerical simulations for the prediction of cavitation in cryogenic fluids is of critical importance for the efficient design and performance of turbopumps in rocket propulsion systems. One of the main remaining challenges is efficiency in modeling of the physics, handling the multi-scale properties involved and developing robust numerical methodologies. Such flows involve thermodynamic phase transitions and cavitation bubbles that are on a smaller scale than the global flow structure. Cryogenic fluids are thermo-sensitive, and therefore, thermaleffects and strong variations in fluid properties can alter the cavitation properties. The aim of this work is to address the challenge posed by thermaleffects. The Rayleigh–Plesset equation is modified by the addition of a term for convectiveheattransfer at the interface between the liquid and the bubble coupled with a bubbly flow model to assess the prediction of thermaleffects. We perform a parametric study by considering several values of and models for the convectiveheattransfer coefficient, hb, and we compare the resulting temperature and pressure profiles with the experimental data. Finally, the results of a 2D simulation with a commercial CFD code are presented and compared with the previous results. We note the importance of the choice of hb for the correct prediction of the temperature drop in the cavitating region, and we assess the most efficient models, underlining that the choice of hbestimation model in a cryogeniccavitatingflow is more important in the bubble growth phase than in the bubble collapse phase.

Wireless sensors networks enable the chance to investigate with enhanced freedom physical phenomena, aiming to increase the informative content obtained by sensors measurements. In this work we will focus on a system allowing to experimentally measure pressure profiles obtained from sensor nodes deployed on a NACA0012 aircraft wing model. By exploiting measurements gathered from sensors, allowing to measure pressure fluctuations of ±600Pa with a resolution of 4Pa, together with results obtained by Computational Fluid Dynamics (CFD) models, the system enables extracting flow profile, thus obtaining information on flow separation and stall phenomenon. Wireless measures are delivered with an enhanced version of IEEE802.15.4e, allowing to decrease power consumption by a factor of 7. Packet routing, based on Routing Protocol for Low-Power and Lossy Networks (RPL), has been improved by means of a newly introduced Lifetime and Latency Aggregatable Metric (L2AM) leading to a 18% increased network lifetime.

The present study intends to investigate the boundary layer characteristics to assess the potentiality of the single dielectric barrier discharge plasma actuators (SDBDPAs) to reattach the separated flow at low Reynolds numbers. The effect of the actuator geometrical parameters and of the Reynolds number on the device control authority was experimentally investigated. For this aim, a curved wall plate, which profile shape was designed to reproduce the suction surface of a low-pressure turbine (LPT) blade, was installed in the test section of a closed loop wind tunnel and a groove was made over it, at the front of the adverse pressure gradient region, for allocating a SDBDPA. Three actuators, characterized by different streamwise width, were manufactured by photolithography technique and they were tested. The velocity flow field, in both presence and absence of external flow, was investigated by particle image velocimetry (PIV) measurements. When the actuator was turned on, a sinusoidal voltage excitation with amplitude of 8 kV and frequency of 2 kHz was applied and the dissipated power (View the MathML source) was retrieved by electrical characterization. The effect of the active flow control was firstly estimated by analyzing the plasma induced velocity fields in absence of external flow. Subsequently the wind tunnel inlet free stream velocity (View the MathML source) was set to 1.54 m/s. The velocity, turbulence intensity (Tu) and vorticity (ωz ) fields together with the boundary layer shape factor (H12) and momentum coefficient (cμ) were evaluated in both presence and absence of actuation. All the aforementioned analyses together with the estimation of the device electrical-to-fluidic energy conversion efficiency (ηfm ) allowed identifying the best actuator geometry. Then, that configuration was chosen to investigate the effects of the wind tunnel velocity on the device control authority. The tested View the MathML source values ranged from 1.54 m/s up to 3.16 m/s. In absence of actuation, a large reverse flow and high turbulence intensity was observed in the separation region. Considering the actuated cases, it was found that at View the MathML source ≈ 7 W, the SDBDPA operation always implied a reduction of the separated region, of the flow angle and of the turbulence intensity. Moreover, the plasma induced jet had a larger impact on the flow at lower velocities and a low flow control effect was noticed at the highest View the MathML source values. The H12 factor evaluation confirmed the flow regimes at the different tested velocities (i.e. cμ values). The whole data set allowed to evaluate the actuator success for separation control and to identify a threshold value of the cμ coefficient delimiting the still detached flow from the reattached one.

The present study intends to investigate the potentiality of the single dielectric barrier discharge plasma actuators (SDBDPAs) to reattach the separated flow, occurring at low Reynolds numbers. For this aim, a curved wall plate, which profile shape was designed to reproduce the suction surface of a low-pressure turbine (LPT) blade, was installed in the test section of a closed loop wind tunnel and a SDBDPA was placed in a groove made over it, at the front of the adverse pressure gradient region. The flow behavior in absence of actuation has been experimentally investigated by particle image velocimetry (PIV) and laser Doppler velocimetry measurements (LDV). Moreover, sinusoidal voltage excitation with amplitude of 8 kV and frequency of 2 kHz was applied to the SDBDPA and PIV measurements were also performed in presence of actuation. The applied voltage and the discharge current have also been recorded simultaneously, and they have been used for the determination of the device dissipated power. Different wind tunnel free stream velocities were investigated, both in absence and presence of actuation. The effect of the active flow control was then studied in the entire measurement domain by analyzing the velocity fields, the turbulence intensity (Tu) values, the momentum coefficient ( cμ ) and the boundary layer shape factor (H12 ). In absence of actuation, a large reverse flow and a turbulence intensity up to ≈60% was observed in the separation region. Good agreement was found between the flow results obtained by the two velocity measurement techniques. Considering the actuated cases, it was found that, in all the tested operating conditions, actuation implied a reduction of the separated region and the turbulence intensity, even if a low flow control effect was noticed at the highest tested velocity

This paper reports a multitechnique investigation of a micro dielectric barrier discharge plasma actuator (DBDPA) as a promising system to control separated flows. The device was manufactured through a photolithographic technique and its performances and capabilities were compared with the ones of conventional macro DBDPAs. Alternate current operation under sinusoidal voltage excitation was studied in the absence of external flow by means of many experimental techniques like discharge imaging, flow visualizations, particle image velocimetry, infrared thermography, and electrical characterization. The influence of the operating parameters was investigated. The main results underlined that an increase in the voltage amplitude or frequency brought to a rise in the maximum induced velocity, electrical power dissipation, and actuator surface temperature. Moreover, it was assessed that the small heating of the micro DBDPA did not affect the actuated flow. A jet velocity up to 1.36 m/s was obtained at a 9.01 W/m electrical power dissipation per unit electrode length. The device realized by microelectronic fabrication technology allowed reaching a flow velocity magnitude comparable with the one of conventional macro DBDPAs, with a reduction in applied voltage, power dissipation, and actuator size. Furthermore, the induced wall jet was more confined in the area in proximity of the device, because of the limited plasma discharge extension. © 2015 IEEE.

SIBA Get it!(opens in a new window)|View at Publisher| Text export | Download | Save to list | More... IEEE Sensors Journal Volume 16, Issue 10, 15 May 2016, Article number 7426717, Pages 3896-3903 Lean Blowout Sensing and Plasma Actuation of Non-Premixed Flames (Article) De Giorgi, M.G.a , Sciolti, A.a , Campilongo, S.a , Pescini, E.a , Ficarella, A.a , Lovascio, S.b , Dilecce, G.c a Department of Engineering for Innovation, University of Salento, Lecce, Italy b Department of Chemistry, University of Bari, Bari, Italy c Istituto di Nanotecnologie, Consiglio Nazionale delle Ricerche, University of Bari, Bari, Italy View additional affiliations View references (31) Abstract The aim of this paper is the use of optical sensors to recognize lean blowout in a non-premixed methane/air burner, Bunsen-type, and the use of plasma actuators for flame control and stabilization. The burner is optically accessible to permit the imaging acquisitions of the flame region. The plasma actuation regards alternatively the air flow and the fuel flow. The electric field is generated using a fixed configuration of plasma actuator and the dielectric barrier discharge (DBD) but using two different power supplies: a nanosecond repetitively pulsed high voltage (HV) and a sinusoidal DBD HV. The comparison between the two types of actuation is the core of this paper, together with the analysis of the results obtained when actuation acted on the air or on the fuel. For the analysis, the lean blowout (LBO) limits recorded in the presence and absence of plasma actuation to investigate the plasma actuation success. The flame behavior is acquired using a compact digital camera, an intensified charge-coupled device (CCD) in order to capture the differences between the baseline conditions and the actuated cases. It is shown that the plasma significantly allows stabilizing the flame under lean conditions where it would not exist without plasma

This paper investigates the performance of a 960 kWP photovoltaic (PV) system, located in southern Italy. Monitoring data over a 43 months period are used to evaluate the monthly average of energy yields, losses and efficiency. The performance ratio and the capacity factor were 84.4% and 15.6%, respectively. The yearly average module efficiency was 15.3% and the system efficiency was 14.9%, whereas the capture and system losses were 0.6 h/day and 0.1 h/day respectively. The degradation rate of the PV system, after continuous outdoor exposure, has been estimated at about 1.48%/year by applying the Classical Seasonal Decomposition (CSD) method. A comparison in terms of degradation rate is made with other PV plants installed in different climates to address the need of standard methods to determine this crucial parameter. The actual performance of the studied PV system is compared with the expected outcomes by using two widely used PV simulation tools, SAM and PVsyst. A detailed difference analysis is carried out to evaluate the accuracy of the SAM and PVsyst models. Results show that the yearly average energy injected into the grid was under-estimated by 3.0% by SAM and by 3.3% by PVsyst, but overall PVsyst outperforms the SAM tool. Results provide reliable data on the plant and demonstrate its good performance compared to other plants located in the same climate. The results can be used to improve the prediction of the performance of future PV of systems in the Mediterranean climate, and also provide a useful dataset for comparative studies

Abstract The present study aims to investigate, by numerical simulations, the potentiality of alternate current (AC) driven plasma actuators to reattach the separated flow along a low pressure turbine blade operating at low-Reynolds number. The flow over a curved wall plate, installed in a wind tunnel, to simulate the suction surface of a low-pressure turbine blade, was examined. Different plasma actuator geometries have been studied: a macro single dielectric barrier discharge (SDBD), a micro SDBD and a micro linear plasma synthetic jet (L-PSJ) without and with thrust vectoring. Numerical simulations were performed in absence and in presence of actuation. In presence of actuation, the plasma induced force was modelled and introduced as a source term in the momentum Navier-Stokes equation. The numerical flow simulations were validated with the experimental data. To compare the different plasma actuator geometries effects, the velocity profiles have been considered. The micro SDBD and the micro L-PSJ with thrust vectoring led to a reduction of recirculation and a substantial decrease of the boundary layer thickness. The reattachment of the flow was also evident by analyzing the wall shear stress profiles and the vortical flow structure using the Q-criterion. The characteristics of the boundary layer (shape factor, displacement and momentum thickness) in presence of the different actuation techniques were also studied.

Dielectric Barrier Discharge (DBD) plasma devices have been designed and manufactured with micro scale dimensions through photolithographic process on fiber glass substrate. AC operation under sinusoidal voltage up to 14 kVpp and carrier frequency up to 2.5 kHz has been investigated experimentally by means of smoke flow visualizations and Particle Image Velocimetry. Velocity profiles, maximum induced velocity and induced body force have been calculated. A comparison between the microactuator and a conventional macroactuator has been performed. It has been demonstrated that the microactuator produces velocities on the order of the macro scale actuator with a significant reduction in inception voltage, size and mass. This leads to a simpler and a less intrusive dispositive.

Preventing the flow separation could enhance the performance of propulsion systems and future civil aircraft. To this end, a fast detection of boundary layer separation is mandatory for a sustainable and successful application of active flow control devices, such as plasma actuators. The present work reports on the design, fabrication and functional tests of low-cost capacitive pressure sensors coupled with dielectric barrier discharge (DBD) plasma actuators to detect and then control flow separation. Finite element method (FEM) simulations were used to obtain information on the deflection and the stress distribution in different-shaped floating membranes. The sensor sensitivity as a function of the pressure load was also calculated by experimental tests. The results of the calibration of different capacitive pressure sensors are reported in this work, together with functional tests in a wind tunnel equipped with a curved wall plate on which a DBD plasma actuator was mounted to control the flow separation. The flow behavior was experimentally investigated by particle image velocimetry (PIV) measurements. Statistical and spectral analysis, applied to the output signals of the pressure sensor placed downstream of the profile leading edge, demonstrated that the sensor is able to discriminate different ionic wind velocity and turbulence conditions. The sensor sensitivity in the 0-100 Pa range was experimentally measured and it ranged between 0.0030 and 0.0046 pF Pa-1 for the best devices

In questa memoria è preso in esame il controllo attivo del flusso su un profilo aerodinamico attraverso attuatori al plasma. Tali dispositivi generano plasma nello strato limite del profilo, inducendo una scarica a barriera a pressione atmosferica, permettendo il trasferimento di energia dal campo elettrico alle particelle neutre dello strato limite, dove il flusso è in procinto di separazione dal profilo (interazione Elettroidrodinamica – EHD), e influenzando il profilo di velocità nello strato limite. Gli elettrodi eccitati ionizzano, seppure debolmente, l’aria circostante ed il campo elettrico prodotto dagli stessi genera un vettore forza che agisce sul flusso esterno. Questa forza può essere espressa in termini di potenziale applicato ed essere considerata nelle equazioni di Navier-Stokes. Numerosi sono i lavori che hanno analizzato l’effetto di attuatori al plasma attraverso l’implementazione di modelli numerici semplificati che studiano l’effetto del plasma come forza volumetrica costante nel tempo e nello spazio, oppure modelli che analizzano la forza elettro-idro-dinamica che genera il plasma, forza elettrica linearizzata e dipendente dal campo elettrico. Nel presente lavoro, invece, è stato implementato un modello numerico in cui la forza, generata dal plasma, è ottenuta come prodotto della densità di carica e del campo elettrico, risolvendo l’equazione di Maxwell per ottenere il campo elettrico generato dal potenziale applicato agli elettrodi e un’ulteriore equazione che tiene in considerazione la densità di carica, che rappresenta la densità del plasma. Inizialmente il suddetto modello è stato simulato su una piastra con un attuatore al plasma, composto da due elettrodi e da materiale dielettrico. Successivamente, la modellazione numerica è stata implementata su un profilo NACA 0015, con attuatore al plasma posizionato in prossimità del bordo di attacco, che ha comportato una quasi scomparsa della zona di separazione del fluido dalla superficie ed una riduzione delle zone di turbolenza del fluido in movimento sul profilo alare.

A cavitating two-phase flow of water in a pipe with area shrinkage was experimentally investigated, acquiring at high sampling rate pressure signals and images of the cavitating flow-field. The fluctuations of the pressure measurements in the internal orifice and of the intensity level of the images' pixels were decomposed using the wavelet transform to analyze the frequency distribution of the signals energy with respect to the flow behavior and to highlight the influence of temperature on the phenomena under investigation. It was shown that the energy content at each frequency band of the acquired signals is well related to cavitation flow-field behavior.

The present work aims at the numerical prediction of the performance of a Contra-Rotating Propellers (CRP) system for a Remotely Piloted Aerial Vehicles (RPAV). The CRP system was compared with an equivalent counter-rotating propellers configuration which was set by considering two eccentric propellers which were rotating at the same speed. Each contra-rotating test case was built by varying the pitch angle of blades of the rear propeller, while the front propeller preserved the original reconstructed geometry. Several pitch configurations and angular velocities of the rear propeller was simulated. Comparisons showed an improvement of the propulsive efficiency of the contra-rotating configuration in case of larger pitch angles combined with slower angular velocities of the rear propeller.

Super-critical mixing and combustion phenomena involves a large spectrum of interconnected physical processes which determine complexity in the problem formulation. Near critical or trans-critical condition the reacting mixture exhibits large variation in thermodynamic and transport properties, which affect drastically the mixing and combustion processes. The aim of this investigation is the implementation by user defined routines of thermodynamically consistent real gas mixing equation of state into the commercially available CFD-code ANSYS Fluent and the validation using experimental data. CFD simulations have been also performed by using different approaches: the Soave Redlich-Kwong real gas model and Peng Robinson have been implemented to model the physical properties of the species. This work is divided in two parts: in the first part a numerical analysis has been conducted to study the nitrogen high pressure flow in liquid rocket near the critical point , in the second the numerical study of the LOX/H2 and LOX/CH4 injection, mixing and combustion in liquid rocket engines with shear coaxial injectors, at supercritical conditions by using one-step reaction mechanism and detailed chemical kinetic reaction mechanism.

Different input waveforms applied to a Single Dielectric Barrier Discharge Plasma Actuator (SDBDPA) were compared for flow separation control on low-pressure turbines (LPTs). The investigated Reynolds number (Re) was 2·104. The work aim was the device optimization in terms of materials and excitation conditions for enhancing its durability and performances. The SDBDPA was manufactured by microfabrication techniques. Device materials that could withstand the plasma environment were selected. Sine, square and triangle waveforms were compared in terms of actuator dissipated power and induced velocity. At comparable peak-to-peak applied voltage, the sinus outperformed the other waveforms, while the square dissipated the most.

Recently, the Micro Single Dielectric Barrier Discharge Plasma Actuator has become attractive for application in aeronautics and micropopulsion thrusters. The present work carried out a preliminary characterization of such device, acting on initially quiescent air by experimental and numerical approaches. Sinusoidal voltage excitation with amplitude up to 7 kV and frequency up to 2.5 kHz was applied. The induced flow was investigated by particle image velocimetry and the measured velocity fields were used to estimate experimentally the time-averaged induced body force distributions by a differential method. Plasma induced forces were modeled by following three different approaches, later implemented as a source term in the Navier-Stokes equations for the fluid flow simulations. Potentialities, advantages and disadvantages of the considered force modeling methods were investigated. Quantitative comparison of the experimental and numerical induced force, as well as of the velocity fields, allowed establishing which model best predicted the actuator effects. The algebraic Dual Potential Model provided a good agreement between experimental and simulated results, in terms of flow velocities and thickness of the induced wall-jet. The downstream decay of the wall-jet velocity, experimentally observed, was also successfully predicted. A maximum induced velocity of ≈2 m/s was obtained and a jet thickness of ≈3 mm.

Auto Regressive Moving Average (ARMA) models, which perform a linear mapping between inputs and outputs, and Artificial Neural Networks (ANNs) and Adaptive Neuro-Fuzzy Inference Systems (ANFIS), which perform a non-linear mapping, provide a robust approach to wind power prediction. In this paper a new hybrid method is proposed, based on the application of the six Daubechies wavelet employed to do the 3rd level discrete wavelet decomposition of the original hourly wind power time series, in combination with ANNs, ARMA and ANFIS models, in order to predict the power production of a wind farm located in Southern Italy in different time horizons: 1, 3, 6, 12 and 24 hours. In particular, the results obtained with and without the wavelet decomposition are compared for each of the aforementioned techniques (ANNs, ARMA and ANFIS), by investigating the error of the different prediction systems for various forecasting horizons; the statistical distributions of the error are calculated and presented.

The study is focused on the monitoring of the performances of a photovoltaic system, influenced of climatic characteristics of a particular geographical area, in which the PV system is installed. This paper presents the results obtained by monitoring, from March to October 2012, a 960 kWp photovoltaic system installed on the parking lots in the campus of the University of Salento, Italy and divided in two subfields with different tilt angle (3–15°) and different nominal powers (353.3 kWp and 606.6 kWp). The values of energy and power generated, final yield, reference yield, photovoltaic system efficiency, performance ratio and cell temperature losses are analyzed and linked to the climatic characteristics of the site, such as insolation (for the two distinct subfield), ambient temperature and wind speed. This study is part of the European Project “Buildings Energy Advanced Management System, BEAMS”, and its aim is to develop an advanced, integrated management system for buildings and special infrastructures of public use. This work offers a tool to estimate the performances of plants to be installed in sites with climatic characteristics similar to South-eastern Italy, and it is also useful as comparison to investigate the productivity of plants placed in different areas.

This work deals with the computational modeling of the single dielectric barrier discharge (SDBD) plasma actuator and its applications as a flow actuator. In the literature, plasma actuators have been used especially in order to control boundary layer separation. The plasma acts as a momentum source to the boundary layer allowing it to remain attached throughout a large portion of the airfoil. The RANS simulations are performed using a CFD code in which the plasma force have been modeled as paraelectric force acting on the charged particles in the working flow Using this numerical model, different cases have been simulated on NACA 0015 airfoil, depending on the direction of the force, to study the effect of the force on the flow and on the boundary layer. The best flow control solutions have been displayed when body force component in the direction straight along the flow is positive and the component normal to the flow is considered. Finally, this numerical simulation methodology has been used for the investigations on the potential of plasma actuators, to suppress the flow separation over a compressor blade. Specifically, the analysis has been focused to evaluate the increasing of the compressor performance depending on the actuator strength and position on the blade.

An important issue for the growth and management of grid-connected photovoltaic (PV) systems is the possibility to forecast the power output over different horizons. In this work, statistical methods based on multiregression analysis and the Elmann artificial neural network (ANN) have been developed in order to predict power production of a 960 kWP gridconnected PV plant installed in Italy. Different combinations of the time series of produced PV power and measured meteorological variables were used as inputs of the ANN. Several statistical error measures are evaluated to estimate the accuracy of the forecasting methods. A decomposition of the standard deviation error has been carried out to identify the amplitude and phase error. The skewness and kurtosis parameters allow a detailed analysis of the distribution error.

The power forecasting plays a significant role in the electrical systems. Furthermore the high-dimensional data reduction without losing essential information represents an important advantage in the forecasting models. Low computational costs and short execution time together with high predicted performance are the main goals to be reached in the development of a prediction method. In this paper a hybrid method based on an active selection of the support vectors, using the quadratic Renyi entropy criteria in combination with the principal component analysis (PCA), is shown to dimensionally reduce the training data in the forecasting models. The reduced data have been used to implement the Least Squares Support Vector Machines (LS-SVM) in order to predict the photovoltaic (PV) power in the day-ahead time horizon. The model has been validated using historical data of a PV system in the Mediterranean climate. Additionally the weather variations have been taken into account to evaluate the outcome of the sunny and cloudy condition in the PV forecasting models. The proposed technique gives fulfill results. A training data size same as 30% original dimension allows to improve the forecasting accuracy and reduces the computational time of 70% respect to an implementation without dimensionality reduction data

Aim of the present paper is the characterization of a non-premixed methane/air microburner, Bunsen-type, equipped with a plasma actuator for the flame stabilization and the blowoff control. The burner is optically accessible to permit imaging acquisitions of the flame region. The plasma actuation regards the air flow, while the fuel flow is protected by the action of the electric field. The limits of flashback and blowoff were recorded in presence and in absence of the plasma actuation. The flame behavior was acquired using a digital camera in order to capture the differences between the baseline conditions and the actuated cases. The results appear very encouraging for future application in powering micro-devices for aerial and industrial application. It was shown that the plasma significantly allows stabilizing the flame under lean conditions where it would not exist without plasma.

The present work was performed to investigate the employment of micro dielectric barrier discharge plasma actuators for mitigating separation, thereby decreasing wake losses and increasing efficiency, on a highly loaded compressor cascade. To this aim, the experimental characterization of the control device was initially done. A dedicated activity was devoted to microelectronic technology adoption for micro plasma actuator fabrication, together with batch production of electrodes with photolithographic techniques. The actuation effect on quiescent flow was evaluated by measuring the induced wall-jet with particle image velocimetry. The actuator power consumption was estimated by recording the applied voltages and resulting currents. Experimental results were then used to calibrate a multi-physics numerical model, for the prediction of the body forces induced by plasma actuator. Different algebraic models were compared. Numerical modelling was applied to predict the capability of micro plasma actuation to suppress flow separation into a highly-loaded subsonic compressor stator cascade. At first, simulations of the compressor cascade without active flow control were carried out and the results were compared with the literature experimental data. A good agreement was found between the experimental and the numerical results. Active flow control by the micro plasma actuator was then tested under different sinusoidal voltage amplitudes. It was found that the compressor pressure losses were reduced by increasing the applied voltage; actuation brought to a reduction in the pressure loss coefficient up to 14% and to an increase in static pressure up to 3%. When the actuator was on, the isosurface of the Q-criteria showed the reduction of secondary flow structures and the shape factor at the trailing edge of the midspan section was always lower than 2.2, confirming a reattachment of the flow. Furthermore, a conventional macro actuator found in the literature was also modelled and its actuation effect was compared to the one of the micro plasma actuator. In conclusion, the analysis of the actuation cost underlined that the adoption of micro actuation allowed reaching a higher gain when operating at lower voltage and same frequency

Abstract In recent years, the application of plasma actuators in different engineering fields was considered particularly interesting. It was successfully applied for the cold flow control in aero engines and turbo-devices. One important application concerns the use of non-equilibrium plasma for plasma-assisted ignition and combustion control. The reduction of nitric oxides (NOx) in aircraft engines, gas turbines, or internal combustion engines has become a major issue in the development of combustion systems. A way to reduce the NOx emissions is to burn under homogenous lean conditions. However, in these regimes the flame becomes unstable and it leads to incomplete combustion or even extinction. Thus, the major issue becomes to stabilize the flame under lean conditions. In this context the present work aims to demonstrate the possibility to increase the combustion efficiency of a lean flame through the use of nanosecond repetitively pulsed plasma (NRPP). A NRPP produced by electric pulses with amplitude up to 40 kV, pulse rise time lower than 4 ns and repetition rate up to 3.5 kHz has been used to stabilize and improve the efficiency of a lean non premixed methane/air flame in a non-premixed Bunsen-type burner. The burner is optically accessible permitting the imaging acquisitions of the flame region. The flame behavior was acquired using a high rate CCD camera in order to capture the differences between the baseline conditions and the actuated cases. Moreover a post-processing technique showing the jagging of the flame in different conditions was applied to evaluate the changes occurring in presence of plasma actuation in term of flame area weighted respect to the luminosity intensity. It was shown that the plasma significantly allows stabilizing the flame under lean conditions where it would not exist without plasma.

Purpose – Reynolds number in small-size low-pressure turbines (LPT) can drop below 2.5 · 104 at high altitude cruise, which in turn can lead to laminar boundary layer separation on the suction surface of the blades. The purpose of this paper is to investigate the potential of an alternate current (AC)-driven Single Dielectric Barrier Discharge Plasma Actuator (AC-SDBDPA) for boundary layer control on the suction side of a LPT blade, operating at a Reynolds number of 2 · 104. Design/methodology/approach – Experimental and numerical analyses were conducted. The experimental approach comprised the actuator testing over a curved plate with a shape designed to reproduce the suction surface of a LPT blade. A closed loop wind tunnel was employed. Sinusoidal voltage excitation was tested. Planar velocity measurements were performed by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). The device electrical power dissipation was also calculated. Computational fluid dynamics (CFD) simulations using OpenFOAM© were conducted, modelling the actuator effect as a body force calculated by the dual potential algebraic model. Unsteady RANS (Reynolds Averaged Navier-Stokes equations), also known as URANS approach, with the k- Lam-Bremhorst Low-Reynolds turbulence model was used. Findings – The AC-SDBDPA operation brought to a reduction of the separation region; in particular, the boundary layer thickness and the negative velocity values decreased substantially. Moreover, the flow angle in both the main flow and in the boundary layer was reduced by the plasma control effect. The actuation brought to a reduction of the 17 per cent in the total pressure loss coefficient. The pressure coefficient and skin friction coefficient distributions indicated that under actuation the reattacnment point was displaced upstream, meaning that the flow separation was effectively controlled by the plasma actuation. Adopting slightly higher actuation parameters could bring to a full reattachment of the flow. Practical implications – The work underlines the potentialities of an AC-SDBDPA to control separation in LPTs of aeroengines. Originality/value – The present work sets a methodological framework, in which the validated procedure to obtain the body force model combined with CFD simulations can be used to study a configuration with multiple actuators allocated in array without requiring further experiments.

Recent advances in gas turbine combustor design are aimed at achieving low exhaust emissions, hence modern aircraft jet engines are designed with lean-burn combustion systems. In the present work, we report an experimental study on lean combustion in a liquid fuel burner, operated under a non-premixed (single point injection) regime that mimics the combustion in a modern aircraft engine. The flame behavior was investigated in proximity of the blow-out limit by an intensified high rate Charge-Coupled Device (CCD) camera equipped with different optical filters to selectively record single species chemiluminescence emissions (e.g., OH*, CH*). Analogous filters were also used in combination with photomultiplier (PMT) tubes. Furthermore this work investigates well-mixed lean low NOx combustion where mixing is good and generation of solid carbon particulate emissions should be very low. An analysis of pollutants such as fine particles and gaseous emissions was also performed. Particle number concentrations and size distributions were measured at the exhaust of the combustion chamber by two different particle size measuring instruments: a scanning mobility particle sizer (SMPS) and an Electrical Low Pressure Impactor (ELPI). NOx concentration measurements were performed by using a cross-flow modulation chemiluminescence detection system; CO concentration emissions were acquired with a Cross-flow modulation Non-dispersive infrared (NDIR) absorption method. All the measurements were completed by diagnostics of the fundamental combustor parameters. The results herein presented show that at very-lean conditions the emissions of both particulate matter and CO was found to increase most likely due to the occurrence of flame instabilities while the NOx were observed to reduce.

A numerical evaluation of the effects of volcanic ash ingestion in a turbofan engine was carried out, with particular regard to the prediction of the erosion damage to fan blades. The ash concentration level examined in the study was below the flight limit because the aim of this study is to investigate the damage due to long-term exposure to low concentration levels. The work aims to the implementation of a numerical methodology that takes into account the geometry change of the fan blades during the exposure to volcanic ash. A dimensional and morphological characterization of a real volcanic ash sample from the Mount Etna volcano has been performed to model the particle flow dynamics using a computational fluid dynamics (CFD) code. The fan performance in terms of the total pressure increase was calculated for both the baseline and damaged geometries to quantify the performance deterioration trend with respect to the particle exposure time. For the calculation of the eroded fan performance, two different numerical approaches were considered. In the first approach, the erosion rate (ER) was evaluated based on the initial blade geometry and was held constant. In the second approach, the ER was updated as the erosion of the blade continued. The second approach shows a higher deterioration of the pressure rise across the fan, suggesting that the variation of the ER due to the blade shape modification cannot be neglected in the calculations.

Small engines will be finding increasing applications in unmanned aerial vehicles (UAVs), drones and helicopters. However, their turbomachines exhibit lower efficiencies than those of large scale engines. In this context, the aerodynamic losses in the low-pressure turbines (LPTs) are largely accountable to flow separation at low Reynolds numbers operation, i.e. in cruise conditions. Active flow control is a promising technology to suppress separation, thus reducing losses, fuel consumption rates and therefore emissions. The present paper is focused on the experimental investigation of the potentialities of a Single Dielectric Barrier Discharge Plasma Actuator (SDBDPA) to reattach the separated flow at a Reynolds number of 2 ·104. The influence of the high voltage (HV) waveform supplying the SDBDPA on both flow separation control and device power dissipation was studied. The investigated SDBDPA was manufactured by microfabrication techniques. Photolithography ensured thin metal deposition with high manufacturing reliability control. Due to the possible device degradation during operation, emphasis was put in selecting thin film materials that could withstand the plasma environment. Schott alkali-free borosilicate glass substrate was chosen as dielectric, while a multilayer tungsten (W)/titanium nitride (TiN) as electrode material. The experimental approach comprised the actuator testing over a curved wall plate, designed with a shape to reproduce the suction surface of a LPT rotor blade and installed in closed loop wind tunnel test section. The SDBDPA was located at the front side of the adverse pressure gradient area, in order to control flow separation. Different HV excitation waveforms (sinus, triangle and square) and amplitudes were tested and compared, aiming to identify the input signal that gave the best flow control authority and device energy conversion efficiency. The applied voltage and the discharge current were acquired in order to determine the actuator dissipated power. Two-dimensional (2-D) flow velocity measurements were carried out by laser Doppler velocimetry (LDV) and particle image velocimetry (PIV). Velocity results showed that the extension of the separation area was reduced by actuation. Moreover, when the actuator was on, the boundary layer thickness and the negative velocity magnitude decreased. Their reduction increased with the applied voltage (i.e. higher power dissipations). At comparable peak-to-peak applied voltages, the sinus waveforms slightly outperformed the other waveforms; however, while the sinus and triangle ones had comparable power dissipation, the square wave always dissipated the most.

Statistical methods based on Multiregression Analysis and Artificial Neural Networks (ANNs) have been developed in order to predict power production of a 960 kWp grid-connected photovoltaic (PV) plant in the campus of the University of Salento, Italy. The neural network has been used only as a statistic model based on time series of PV power and meteorological variables, as module temperature, ambient temperature and irradiance on module’s plain. In particular, a sensitivity analysis has been carried out in order to find those weather parameters with the best impact on the forecasting.

Nel presente lavoro è stata analizzato l'utilizzo del plasma generato da una scarica dielettrica a barriera, usato come attuatore elettroidrodinamico EHD, per controllare il flusso in movimento su un profilo alare, aspetto di notevole interesse in ambito aeronautico, per il vantaggio di non richiedere parti meccaniche e di avere una rapida risposta al controllo. È stato caratterizzato il campo di moto in prossimità di un profilo alare, in presenza di tali attuatori, con gli obiettivi di analizzare il grado di sensibilità del flusso su un profilo NACA 0015 e di identificare il miglior dominio disponibile. Analizzata l'influenza di un campo di forze esterne applicate a differenti domini sul profilo per il controllo della separazione del flusso, con velocità di 35 m/s e angolo di attacco α=23°, si è proceduto all’analisi delle forze, con diversi moduli e direzioni, e dei migliori risultati ottenuti per ogni dominio. Quindi, è stato implementato un modello numerico per la forza paraelettrica agente sulle particelle cariche presenti nel fluido di lavoro ed esaminato l’impatto della forza sul fluido in funzione di differenti valori e direzioni della stessa. Considerando l’attuatore EHD tangente al profilo secondo un angolo di 27°, è stata implementata la modellazione delle componenti del campo elettrico e della forza rispetto al sistema di riferimento x, y, localizzato in corrispondenza del bordo di attacco del profilo NACA. Successivamente, sono stati individuati 4 differenti casi di analisi in funzione della direzione della forza, per studiare l’effetto che ha la stessa sul flusso in movimento sul profilo e quindi sul bordo di attacco. L’analisi condotta ha rivelato un miglior controllo del fluido sul profilo nei casi in cui la componente della forza considerata è quella lungo la direzione x e si ampli il campo di applicazione, in particolare lungo l’asse verticale y.

The aim of the present work is the investigation of the combustion phenomenon in liquid-propellant rocket engines. The combustion of liquid oxygen and gaseous methane of a shear coaxial injector under supercritical pressure was analyzed. To realize an efficient numerical description of the phenomena, it is important to treat the LOx jet in a manner which takes into account its real behavior. In the present work different kinetics, combustion models and thermodynamics approaches were used in association with the description of the jet as a discrete phase. For all the approaches used, a comparison with experimental data from literature was performed.

Performance of photovoltaic (PV) systems degrades due to the technology and the operating conditions. The degradation of is one of the key indicators for reliability assessment of a PV system. This paper presents a degradation study of the grid connected PV system located in the campus of the University of Salento. A comparative analysis of actual and theoretical output power is carried out over a monitoring period of five years. PVsyst software is chosen to simulate the output power using actual meteorological data. The hourly expected power generation index is introduced to investigate on degradation and reliability.

The present study focuses on the formation of cavitation in cold and hot water and in cryogenic fluid, characterized by strong variations in fluid properties caused by a change in temperature. Cavitation phenomenon is investigated in water and nitrogen flows in a convergent-divergent nozzle through pressure measurements and the optical visualization method. High-speed photographic recordings have been made, the cavitation phenomena evolution and the related frequency content are investigated by means of pixel intensity time series data. The results obtained concur with those obtained with the spectral analysis of the pressure signals. In the case of cryogenic fluid frequency peaks are shifted towards lower frequencies, with respect to cold water and the magnitude of the signal rises, in particular at low frequencies, for nitrogen and hot water. This can be due to thermal effects that contribute also to the low frequencies in the case of cryogenic fluid. To verify the validity of this assumption, a simple model based on the resolution of Rayleigh equation is used.

The aim of the present investigation is the characterization of the behavior of a lean partially-premixed liquid fuel gas turbine near lean blowout limit. At this combustion regime the onset of instability will occur with negative impacts on combustion efficiency. The identification of the instability occurrence permits an efficient flame control adjusting the combustion parameters (as fuel or air mass flow, temperature, pressure, etc.) to stabilize the flame or designing opportunely flame control system. High-speed images of the flame under stable and near blowout condition were captured in conjunction with simultaneous optical data in order to better understand the phenomenology of the flame blowout process and the onset of instability. In particular the experimental characterization was performed through a High Speed Digital Camera, an Infrared camera and a Photomultiplier Tube (PMT) in association with the use of optical filter (OH*). The data collected with these instrumentations produce useful features for the development of an efficient tool for the flame control in industrial and aeronautical burners. The images acquired by the different cameras were processed considering the luminosity signal of each pixel and evaluating the frequency behavior, the variations of amplitude of the signals and some other descriptive parameters able to define the regime of the flame. Spectral analysis and Wavelet transform of pixel intensities of flame images were used and entropy and energy contents were evaluated. The spatial maps of the different spectral and statistical parameters were shown at different fuel/air equivalence ratio. The OH* emissions data measured by the PMT were processed and compared with the data obtained from the images processing.