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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 fluctuations 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.

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The prediction of the performance of HRSG (Heat Recovery Steam Generator) by means of CFD codes is of great interest, since HRSGs are crucial elements in gas turbine combined cycle power plants, and in CHP (combined heat and power) cycles. The determination of the thermo-fluid dynamic pattern in HRSGs is fundamental in order to improve the energy usage and limit the ineffectiveness due to non-homogeneous flow patterns. In order to reduce the complexity of the simulation of the fluid flow within the HRSG, it is useful modeling heat exchangers as porous media zones with properties estimated using pressure drop correlations for tube banks. Usually, air-side thermo-fluid dynamic characteristics of finned tube heat exchangers are determined from experimental data. The aim of this work is to develop a new procedure, capable to define the main porous-medium non-dimensional parameters (e.g., viscous and inertial loss coefficients; porosity; volumetric heat generation rate; etc...) starting from data obtained by means of accurate three-dimensional simulations of the flow through tube banks. Both finned and bare tube banks will be considered and results presented. The analysis is based on a commercial CFD code, Fluent v.6.2.16. In order to validate the proposed procedure, the simulation of an entire fired HRSG of the horizontal type developed by Ansaldo Caldaie for the ERG plant at Priolo (Italy) has been performed and results have been compared with their data.