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The study of thermoacoustic combustion instabilities has an important role for safety operation in modern gas turbines equipped with lean premixed dry low emission combustion systems. Gas turbine manufacturers often adopt simulation tools based on low order models for predicting the phenomenon of humming. These simulation codes provide fast responses and good physical insight, but only one-dimensional or two-dimensional simplified schemes can be generally examined. The finite element method can overcome such limitations, because it allows to examine three-dimensional geometries and to search the complex eigenfrequencies of the system. Large Eddy Simulation (LES) techniques are proposed in order to investigate the instability phenomenon, matching pressure fluctuations with turbulent combustion phenomena to study thermoacoustic combustion oscillations, even if they require large numerical resources. The finite element approach solves numerically the Helmholtz equation problem converted in a complex eigenvalue problem in the frequency domain. Complex eigenvalues of the system allow us to identify the complex eigenfrequencies of the combustion system analyzed, so that we can have a valid indication of the frequencies at which thermoacoustic instabilities are expected and of the growth rate of the pressure oscillations at the onset of instability. Through the collaboration among Ansaldo Energia, University of Genoa and Polytechnic University of Bari, a quantitative comparison between a low order model, called LOMTI, and the three-dimensional finite element method has been examined, in order to exploit the advantages of both the methodologies.

Chemical Reactor Modelling approach has been applied to evaluate exhaust emissions of the newly designed ARI100 (Patent Pending) recuperated micro gas turbine combustor developed by Ansaldo Ricerche SpA. The development of the chemical reactor network has been performed based on CFD reacting flow analysis, obtained with a global 2-step reaction mechanism, applying boundary conditions concerning the combustion chamber at atmospheric pressure, with 100% of thermal load and fuelled with natural gas. The network consists of 11 ideal reactors: 6 perfectly stirred reactors, and 5 plug flow reactors, including also 13 mixers and 12 splitters. Simulations have been conducted using two detailed reaction mechanisms: GRI Mech 3.0 and Miller & Bowman reaction mechanisms. Exhaust emissions have been evaluated at several operating conditions, obtained at different pressure, and considering different fuel gases, as natural gas and a high H2 content SYNGAS fuel. Furthermore, emissions at different thermal loads have been investigated when natural gas at atmospheric pressure is fuelled. Simulation results have been compared with those obtained from combustion experimental campaign. CO and NOx emissions predicted with CRM approach closely match experimental results at representative operating conditions. Ongoing efforts to improve the proposed reactors network should allow extending the range of applicability to those operating conditions whose simulation results are not completely satisfying. Given the small computational effort required, and the accuracy in predicting combustor experimental exhaust emissions, both CO and NOx, the CRM approach turnout to be an efficient way to reasonably evaluate exhaust emissions of a micro gas turbine combustor.

In the framework of the non-standard fuel combustion research in micro-small turbomachinery, a newly designed micro gas turbine combustor for a 100-kWe power plant in CHP configuration is under development at the Ansaldo Ricerche facilities. Combustor design starts from a single silo chamber shape with two fuel lines, and is associated with a radial swirler flame stabiliser. Lean premix technique is adopted to control both flame temperature and NO x production. Combustor design process envisages two major steps, i.e. diagnostics-focussed design for methane only and experimentally validated design optimisation with suitable burner adaptation to non-standard fuels. The former step is over, as the first prototype design is ready for experimental testing. Step two is now beginning with a preliminary analysis of the burner adaptation to non-standard fuels. The present paper focuses on the first step of the combustor development. In particular, main design criteria for both burner and liner cooling system development are presented. Besides, design process control invoked both 2D and 3D CFD analysis. Two turbulence models, FLUENT standard k-ε model and Reynolds Stress Model (RSM), are refereed and the results compared. Here both a detailed analysis of CFD results and a preliminary analysis of main chemical kinetic phenomena are 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 κ-τ type as a function of the position within the chamber. The intensity coefficient, κ, is assumed to be proportional to the reaction rate of methane in a two-step mechanism. The time delay τ 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 κ and τ. 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.