Bruno Schuermans e-mail: bruno.schuermans@power.alstom.com Felix Guethe Douglas Pennell Alstom (Switzerland) Ltd., CH-5405 Baden, Switzerland Daniel Guyot e-mail: daniel.guyot@tu-berlin.de Christian Oliver Paschereit Chair of Fluid Dynamics Hermann-Föttinger-Institute, Technische Universität Berlin, 10623 Berlin, Germany Thermoacoustic Modeling of a Gas Turbine Using Transfer Functions Measured Under Full Engine Pressure Thermoacoustic transfer functions of a full-scale gas turbine burner operating under full engine pressure have been measured. The excitation of the high-pressure test facility was done using a siren that modulated a part of the combustion airflow. Pulsation probes have been used to record the acoustic response of the system to this excitation. In addi- tion, the flame’s luminescence response was measured by multiple photomultiplier probes and a light spectrometer. Three techniques to obtain the thermoacoustic transfer function are proposed and employed: two acoustic-optical techniques and a purely acoustic tech- nique. The first acoustical-optical technique uses one single optical signal capturing the chemiluminescence intensity of the flame as a measure for the heat release in the flame. This technique only works if heat release fluctuations in the flame have only one generic source, e.g., equivalence ratio or mass flow fluctuations. The second acoustic-optical technique makes use of the different response of the flame’s luminescence at different optical wavelengths bands to acoustic excitation. It also works, if the heat release fluc- tuations have two contributions, e.g., equivalence ratio and mass flow fluctuation. For the purely acoustic technique, a new method was developed in order to obtain the flame transfer function, burner transfer function, and flame source term from only three pres- sure transducer signals. The purely acoustic method could be validated by the results obtained from the acoustic-optical techniques. The acoustic and acoustic-optical methods have been compared and a discussion on the benefits and limitations of each is given. The measured transfer functions have been implemented into a nonlinear, three-dimensional, time domain network model of a gas turbine with an annular combustion chamber. The predicted pulsation behavior shows a good agreement with pulsation measurements on a field gas turbine. DOI: 10.1115/1.4000854 1 Introduction Thermoacoustic analysis is an integrated part in Alstom’s gas turbine technology and product development process. Alstom’s approach to thermoacoustic analysis is to use measured flame transfer functions and source terms in a nonlinear, three- dimensional, acoustic network model. The acoustic wave propa- gation through the combustion chamber and plenum are repre- sented in the model via a modal expansion. The required acoustic modes are obtained from a finite element calculation of the de- tailed geometry. This combined experimental and numerical ap- proach to thermoacoustic modeling is discussed in detail in Refs. 1,2. A crucial aspect of this modeling approach is to obtain a correct representation of the interaction between the heat release and the acoustic field. Alstom’s approach is to measure flame transfer functions in a single burner test facility and to fit models to this transfer function data. Using this approach has proven to give very good results in predicting stability behavior and pulsation spectra of multiburner gas turbine combustion systems 3,4. The influence of pressure on transfer functions is particularly important for fuel flexibility gases containing significant portions of higher order hydrocarbons or hydrogen as well as liquid fuels. In order to correctly predict thermoacoustic behaviors for cases where such fuels are used, transfer functions measured at elevated pressure are needed. This paper discusses the measurement of flame transfer func- tions of a full-scale swirl-stabilized EnVironmental EV-type burner at full engine pressure and about using these transfer func- tions to predict stability and pulsation spectra of a gas turbine with an annular combustion chamber. Due to the limited access in an industrial high-pressure test facility, new techniques were developed to obtain transfer func- tions from only three pulsation probes and/or optical sensors. Two different techniques have been used to measure the flame transfer function: the first technique uses pulsation probes and multiple chemiluminescence sensors and the second only uses pulsation probes. Using chemiluminescence signals to obtain the transfer function is not very straight forward, because fluctuations of the fuel to air ratio may be present in this type of burner and hence the chemiluminescence intensity is not necessarily proportional to the heat release. To overcome this problem a technique has been ap- plied that utilizes the chemiluminescence signals of different wavelengths bands. This method has been discussed in detail in Ref. 5 and was validated under atmospheric pressure conditions 5,6. At high-pressure, however, the correct measurement of the flame chemiluminescence is more challenging than at atmospheric conditions This is due to constrains regarding the optical access to the flame as well as an overlapping wavelength bands of flame luminescence. In addition the problem is exasperated by heat ra- diation from the combustor walls and the generally lower prob- ability of formation of the chemical species associated to the flame chemiluminescence under high pressures. Therefore, the er- ror of the acoustical-optical technique was expected to be larger than under atmospheric conditions. Contributed by the International Gas Turbine Institute IGTI of ASME for pub- lication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received May 6, 2009; final manuscript received May 7, 2009; published online August 10, 2010. Editor: Dilip R. Ballal. Journal of Engineering for Gas Turbines and Power NOVEMBER 2010, Vol. 132 / 111503-1 Copyright © 2010 by ASME Downloaded 11 Aug 2010 to 91.199.43.40. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm