Thermoacoustic combustion instabilities affect modern gas turbines equipped with lean premixed dry low emission combustion systems. In the case of annular combustion chambers, experimental test cases carried out on small scale test rigs equipped with single burner arrangements fail to give adequate indications for the design of the full scale combustion chamber, since they are unable to reproduce the interaction of the flame fluctuation with the azimuthal pressure waves. Therefore there is a large interest in developing techniques able to make use of data gathered from tests carried out on a single burner for predicting the thermoacoustic behavior of the combustion chamber at full scale with its actual geometry. A hybrid technique based on the use of the finite elements method and the transfer matrix method is used to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations at the onset of instability, under the hypothesis of linear behavior of the acoustic waves. This approach is able to model complex geometries such as annular combustion chambers equipped with several burners. Heat release fluctuations are modeled through a classical n-τ Flame Transfer Function (FTF). In order to model the acoustic behavior of the burners, the computational domain corresponding to each burner is substituted by a mathematical function, that is the Burner Transfer Matrix (BTM), that relates, one to each other, pressure and velocity oscillations at either sides of the burner. Both the FTF and the BTM can be obtained from experimental tests or from CFD simulations. The use of the transfer matrix permits us to take into account parameters, such as the flow velocity and the viscous losses, which are not directly included in the model. This paper describes the introduction of the burner transfer matrix in the combustion chamber model. Different geometries of combustion chamber and burner are tested. The influence of the parameters characterizing the transfer matrix is investigated. Finally the application of the BTM to an actual annular combustion chamber is shown.

Lean premixed combustion chambers fuelled by natural gas and used in modern gas turbines for power generation are often affected by combustion instabilities generated by mutual interactions between pressure fluctuations and heat oscillations produced by the flame. Due to propagation and reflection of the acoustic waves in the combustion chamber, very strong pressure oscillations are generated and the chamber may be damaged. This phenomenon is generally referred as thermoacoustic instability, or humming, owing to the cited coupling mechanism of pressure waves and heat release fluctuations. Over the years, several different approaches have been developed in order to model this phenomenon and to define a method able to predict the onset of thermoacoustic instabilities. In order to validate analytical and numerical thermoacoustic models, experimental data are required. In this context, an experimental test rig is designed and operated in order to characterize the propensity of the burner to determine thermoacoustic instabilities. In this paper, a method able to predict the onset of thermoacoustic instabilities is examined and applied to a test rig in order to validate the proposed methodology. The experimental test is designed to evaluate the propensity to thermoacoustic instabilities of full scale Ansaldo Energia burners used in gas turbine systems for production of energy. The experimental work is conducted in collaboration with Ansaldo Energia and CCA (Centro Combustione e Ambiente) at the Ansaldo Caldaie facility in Gioia del Colle (Italy). Under the hypotheses of low Mach number approximation and linear behaviour of the acoustic waves, the heat release fluctua- tions are introduced in the acoustic equations as source term. In the frequency domain, a complex eigenvalue problem is solved. It allow us to identify the frequencies of thermoacoustic instabilities and the growth rate of the pressure oscillations. The Burner Transfer Matrix (BTM) approach is used to characterize the influence of the burner characteristics. Furthermore, the influence of different operative conditions is examined considering temperature gradients along the combustion chamber.

The influence of the introduction of a Helmholtz resonator as a passive damper in a gas turbine combustion chamber on the bifurcation mechanism that characterizes the transition to instability is investigated. Bifurcation diagrams are tracked in order to identify the conditions for which the machine works in a stable zone and which are the operative parameters that bring the machine to unstable conditions. This work shows that a properly designed passive damper system increases the stable zone, moving the unstable zone and the bistable zone (in the case of a subcritical bifurcation) to higher values of the operative parameters, while have a limited influence on the amplitude of limit cycle. In order to examine the effect of the damper, a gas turbine combustion chamber is first modeled as a simple cylindrical duct, where the flame is concentrated in a narrow area at around one quarter of the duct. Heat release fluctuations are coupled to the velocity fluctuations at the entrance of the combustion chamber by means of a nonlinear correlation. This correlation is a polynomial function in which each term is an odd-powered term. The corresponding bifurcation diagrams are tracked and the passive damper is designed in order to increase the stability zone, so reducing the risk to have an unstable condition. Then both plenum and combustion chamber are modeled with annular shape and the influence of Helmholtz resonators on the bifurcation is examined.

This paper compares different operating strategies for small scale (100 kWe) combined heat and power (CHP) plants fired by natural gas and solid biomass to serve a residential energy demand. The focus is on a dual fuel micro gas turbine (MGT) cycle. Various biomass/natural gas energy input ratios are modelled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) higher primary energy savings and revenues from feedin tariff available for biomass electricity fed into the grid. The strategies of baseload (BL), heat driven (HD) and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average and mild climate conditions. On the basis of the results from thermodynamic assessment and simulation at partial load operation, CAPEX and OPEX estimates, and Italian energy policy scenario (incentives available for biomass electricity, on-site and high efficiency CHP), the maximum global energy efficiency, primary energy savings and investment profitability is found, as a function of biomass/ natural gas ratio, plant operating strategy and energy demand typology. The thermal and electric conversion efficiency ranged respectively between 46 and 38% and 30 and 19% for the natural gas and biomass fired case studies. The IRR of the investment was highly influenced by the load/CHP thermal power ratio and by the operation mode. The availability of high heat demand levels was also a key factor, to avoid wasted cogenerated heat and maximize CHP sales revenues. BL operation presented the highest profitability because of the higher revenues from electricity sales. Climate area was another important factor, mainly in case of low load/CHP ratios. Moreover, at low load/CHP power ratio and for the BL operation mode, the dual fuel option presented the highest profitability. This is due to the lower cost of biomass fuel in comparison to natural gas and the high subsidies available for biomass electricity by feedin tariffs. The results show that dual fuel MT can be an interesting option to increase efficiencies, flexibility and plant reliability at low cost in comparison to only biomass systems, facilitating an integration of renewable and fossil fuel systems.

A three-dimensional finite element code is used for the eigenvalue analysis of the thermoacoustic combustion instabilities modeled through the Helmholtz equation. A full annular combustion chamber, equipped with several burners, is examined. Spatial distributions for the heat release intensity and for the time delay are used for the linear flame model. Burners, connecting the plenum and the chamber, are modeled by means of the transfer matrix method. The influence of the parameters characterizing the burners and the flame on the stability levels of each mode of the system is investigated. The obtained results show the influence of the 3D distribution of the flame on the modes. Additionally, the results show what types of modes are most likely to yield humming in an annular combustion chamber. The proposed methodology is intended to be a practical tool for the interpretation of the thermoacoustic phenomenon (in terms of modes, frequencies, and stability maps) both in the design stage and in the check stage of gas turbine combustion chambers.

CHP) plants fired by natural gas and solid biomass. The focus is on dual fuel gas turbine cycle, where compressed air is heated in a high temperature heat exchanger (HTHE) using the hot gases produced in a biomass furnace, before entering the gas combustion chamber. The hot air expands in the turbine and then feeds the internal pre-heater recuperator, Various biomass/natural gas energy input ratios are modeled, ranging from 100% natural gas to 100% biomass. The research assesses the trade-offs between: (i) lower energy conversion efficiency and higher investment cost of high biomass input rate and (ii) higher primary energy savings and revenues from bio-electricity feed-in tariff in case of high biomass input rate. The influence of fuel mix and biomass furnace temperature on energy conversion efficiencies, primary energy savings and profitability of investments is assessed. The scenarios of industrial vs. tertiary heat demand and baseload vs. heat driven plant operation are also compared. On the basis of the incentives available in Italy for biomass electricity and for high efficiency cogeneration (HEC), the maximum investment profitability is achieved for 70% input biomass percentage. The main barriers of these embedded cogeneration systems in Italy are also discussed.

Modern gas turbines equipped with lean premixed dry low emission combustion systems suffer the problem of thermoacoustic combustion instability. The acoustic characteristics of the combustion chamber and of the burners, as well as the response of the flame to the fluctuations of pressure and equivalence ratio, exert a fundamental influence on the conditions in which the instability may occur. A three dimensional finite element code has been developed in order to solve the Helmholtz equation with a source term that models the heat release fluctuations. The code is able to identify the frequencies at which thermoacoustic instabilities are expected and the growth rate of the pressure oscillations at the onset of instability. The code is able to treat complex geometries such as annular combustion chambers equipped with several burners. The adopted acoustic model is based upon the definition of the Flame Response Function (FRF) to acoustic pressure and velocity fluctuations in the burners. In this paper, data from CFD simulations are used to obtain a distribution of FRF of the k-t type as a function of the position within the chamber. The intensity coefficient, k, is assumed to be proportional to the reaction rate of methane in a two-step mechanism. The time delay t is estimated on the basis of the trajectories of the fuel particles from the injection point in the burner to the flame front. The paper shows the results obtained from the application of FRF with spatial distributions of both k and t. The present paper also shows the comparison between the application of the proposed model for the FRF and the traditional application of the FRF over a concentrated flame in a narrow area at the entrance to the combustion chamber. The distribution of the intensity coefficient and the time delay proves to have an influence, both on the eigenfrequency values and on the growth rates, in several of the examined modes. The proposed method is therefore able to establish a theoretical relation of the characteristics of the flame (depending on the burner geometry and operating conditions) to the onset of the thermoacoustic instability.

The paper shows novel technologies for heat and power generation for residential and industrial end users. The paper is focused on micro gas turbines of about 100 kWe and shows their profitability in the Italian scenario, making use of thermoeconomic methodologies. The paper explores different options that make use of biomass and natural gas in dual fuel arrangements. The possibility to set up combined cycle schemes by coupling the micro gas turbine to a bottoming organic Rankine cycle is also examined. The work is the basis of the research project “CLEVER” submitted to the European Union within the program Horizon 2020.

A method for predicting the onset of acoustically driven combustion instabilities in gas turbine combustor is examined. The basic idea is that the governing equations of the acoustic waves can be coupled with a flame heat release model and solved in the frequency domain. The paper shows that a complex eigenvalue problem is obtained that can be solved numerically by implementing the governing equations in a finite element code. This procedure allows one to identify the frequencies at which thermo-acoustic instabilities are expected and the growth rate of the pressure oscillations, at the onset of instability, when the hypothesis of linear behavior of the acoustic waves can be applied. The method can be applied virtually to any three-dimensional geometry, provided the necessary computational resources that are, anyway, much less than those required by computational fluid dynamics methods proposed for analyzing the combustion chamber under instability condition. Furthermore, in comparison with the "lumped" approach that characterizes popular acoustics networks, the proposed method allows one for much more flexibility in defining the geometry of the combustion chamber. The paper shows that different types of heat release laws, for instance, heat release concentrated in a flame sheet, as well as distributed in a larger domain, can be adopted. Moreover, experimentally or numerically determined flame transfer functions, giving the response of heat release to acoustic velocity fluctuations, can be incorporated in the model. To establish proof of concept, the method is validated at the beginning against simple test cases taken from literature. Over the frequency range considered, frequencies and growth rates both of stable and unstable eigenmodes are accurately evaluated. Then the method is applied to a much more complex annular combustor geometry in order to evaluate frequencies and growth rates of the unstable modes and to show how the variation in the parameters of the heat release law can influence the transition to instability.

A renewed interest on Vertical Axis Wind Turbines (VAWTs) arose from their ability to be effectively integrated within urban contests in the spirit of distributed generation. In order to improve their performance, a deeper comprehension of their fluid dynamic behavior is necessary. In the last years, at Politecnico di Bari a great effort has been addressed toward the numerical and experimental investigation of both lift- and drag-driven VAWTs. In particular, constant temperature hot wire anemometry (CTA) is used for the evaluation of the unsteady flow field downstream the VAWT prototypes tested in the subsonic closed-loop wind tunnel of the Politecnico di Bari, whilst, torque measurements are obtained directly from the servo amplifier monitor. Furthermore, by means of CFD analysis, a deep insight into the complex fluid-dynamics of the VAWTs has been obtained. All the acquired experience ended in the development of an innovative lift-driven VAWT prototype currently under investigation

The introduction of piezo-electric sensor on mass produced engines for in-cylinder pressure measurement represents a near future step in order to improve combustion control. Piezo-electric sensors provide pressure measurements affected by offset error and drift due to thermal sensitivity: a pressure reference is needed for each cycle in order to obtain the actual pressure value (pegging). Three methods for the evaluation of the offset, based on the hypothesis of polytropic compression, have been analyzed in this work: (1) a three-point referencing method, (2) a linear least-square method and (3) a non-linear least-square method. From such a comparison the three-point referencing method appeared the best suited for on-board calculation since it has the lowest computational cost even if it suffers from noise sensitivity. Hence, the accuracy and the efficiency of this method have been improved by means of an original methodology. The proposed method has been applied both to simulated and to experimental signals gathered from a 4-stroke spark-ignited engine. The results are compared to those obtained by using the other two methods showing much better accuracy in all the examined tests. Finally, a computational cost analysis proves the feasibility of the proposed method for efficient on-board pegging calculation with modern engine control units.

Aim of this paper is to provide a deep insight into the dynamic behavior of the flow through a Savonius rotor by means of computational fluid dynamics (CFD). The analysis is carried out solving the incompressible Unsteady Reynolds Averaged Navier-Stokes equations, providing fundamental information concerning the complex unsteady flow field in and around the rotor. The motivation for employing a numerical approach relies on the consideration that detailed analysis of wind turbines, aiming to improve their design, cannot be easily performed by means of experimental full-scale field-testing due to the lack of control on the test conditions. At the same time, few are the wind tunnels where large turbine prototype testing is possible, so that experimental tests are usually carried out inside wind tunnels having dimensions comparable with those of the prototype. Moreover, if the available wind tunnel has a confined test section, the turbine performance could be quite different from those expected in open field. Therefore, in this paper, the turbine is firstly supposed to operate in open field and then in a bounded test section, in order to analyze the effect of flow confinement and to correlate the turbine performance in open field with experimental results obtained from prototypes tested in small wind tunnels of assigned blockage.

In this paper, an innovative power plant, constituted by a gas turbine in combined-cycle fuelled by a synthesis gas (or syngas), produced in a local biomass gasifier, is analyzed. The plant is integrated with an external combustion system, fed by cellulosic biomass, connected to a heat exchanger able to increase the air temperature, as in a regenerative cycle. The combustion products pass through a primary heat exchanger placed in the external combustion system, heating the compressed air, which flows into the principal combustion chamber, where a defined quantity of syngas, coming from the gasifier, reacts with the compressed air in a combustion process. The expanded gas, at the turbine exit, before going back into the external combustor, passes through a Heat Recovery Steam Generator (HRSG1) transferring heat to the bottoming Rankine cycle. The superheated steam undergoes an expansion in a steam turbine providing electrical energy. The syngas used in the combustion chamber is produced by a gasification process, based on a Fast Internally Circulating Fluidized-Bed (FICFB). Heat is transferred from the hot syngas (coming from the gasifier) to water, through a second Heat Recovery Steam Generator (HRSG2), producing steam, which is introduced in the gasifier, reacting with the pomace biomass in order to produce the syngas; since the produced quantity of steam is not sufficient for the gasification process, a further quantity of steam is produced in an auxiliary boiler fed by diesel oil, or in different ways, as described in the paper. This kind of plant is especially interesting for regions, like Italian Apulia, where there is a wide culture diffusion for the use of biomass, particularly from olive products, where there are available technologies for use of pruning, virgin and exhausted pomace, and where there are the market conditions for the commercialization of these resources and the incentives available for their energy development. Finally, the overall plant performance is calculated, shown and discussed.

In industrial process plants, often there is the need to reduce the pressure of the operating flow. Generally this is performed by means of valves which expand the flow without any work done. The same operation could be performed by replacing these valves with turbines, with the advantage of energy recovery, hence improving the overall efficiency of the system. In this work, a simple and rapid method is shown in order to design a single stage, straight bladed, axial impulse turbine for enthalpy recovery. Assigned the desired flow rate and the minimum power output, the turbine design is performed according to a one-dimensional study into which loss effects are considered by means of appropriate coefficients. From the one-dimensional analysis the heights, the pitch angle, the inlet and outlet angles of both rotor and stator blades are obtained. Actually, the rotor and stator blade profiles are defined by means of several analytical functions. The blade design is then validated by means of CFD simulations. The definition of loss coefficients and blade geometrical parameters is clearly an iterative process, which needs to be repeated until convergence is reached. Furthermore, by means of fully 3D simulations, the effect of the rotor-stator distance is investigated in order to maximize the turbine performance.

The focus of this paper is on the part load performance of a small scale (100kWe) combined heat and power (CHP) plant fired by natural gas and solid biomass to serve a residential energy demand. The plant is based on a modified regenerative micro gas turbine (MGT), where compressed air exiting from recuperator is externally heated by the hot gases produced in a biomass furnace; then the air is conveyed to combustion chamber where a conventional internal combustion with natural gas takes place, reaching the maximum cycle temperature allowed by the turbine blades. The hot gas expands in the turbine and then feeds the recuperator, while the biomass combustion flue gases are used for pre-heating the combustion air that feeds the furnace. The part load efficiency is examined considering a single shaft layout of the gas turbine and variable speed regulation. In this layout, the turbine shaft is connected to a high speed electric generator and a frequency converter is used to adjust the frequency of the produced electric power. The results show that the variable rotational speed operation allows high the part load efficiency, mainly due to maximum cycle temperature that can be kept about constant. Different biomass/natural gas energy input ratios are also modelled, in order to assess the trade-offs between: (i) lower energy conversion efficiency and higher investment cost when increasing the biomass input rate; (ii) higher primary energy savings and revenues from feed-in tariff available for biomass electricity fed into the grid. The strategies of base load (BL), heat driven (HD) and electricity driven (ED) plant operation are compared, for an aggregate of residential end-users in cold, average and mild climate conditions.

The aim of this paper is to numerically investigate the performance of a cross-flow water turbine of the Darrieus type for very low head hydropower applications. The interest for this kind of vertical axis turbine relies on its versatility. For instance, in the field of renewable energy, this kind of turbine may be considered for different applications, such as: tidal power, run-of-the-river hydroelectricity, wave energy conversion. Until now, low head hydropower, with heads less than 2 meters, has remained scarcely developed due to the relatively low energy density, which makes the cost of generation higher than traditional hydropower applications. However, in the spirit of distributed generation, the use of low head hydropower can be reconsidered, having the advantage of lower electricity transmission losses due to the localization near the consuming area. Nonetheless, it is fundamental to improve the turbine performance and to decrease the equipment costs for achievement of “environmental friendly” solutions and maximization of the “cost-advantage”. In the present work, the commercial CFD code Fluent is used to perform 2D simulations, solving the incompressible Unsteady Reynolds-Averaged Navier-Stokes (U-RANS) equations discretized by means of a finite volume approach. The implicit segregated version of the solver is employed. The pressure-velocity coupling is achieved by means of the SIMPLE algorithm. The convective terms are discretized using a second order accurate upwind scheme, and pressure and viscous terms are discretized by a second-order-accurate centered scheme. A second order implicit time formulation is also used. Turbulence closure is provided by the realizable k − epsilon turbulence model. The model has been validated, comparing numerical results with available experimental data.