Experimental investigation of the nonlinear flame response to flow disturbances in a gas turbine combustor

This paper describes an experimental study of the linear and nonlinear response of the gas turbine combustion process to imposed pressure oscillations. These data were obtained to improve current understanding of the nonlinear phenomenon in unstable combustors that play an important role in its limit cycle behavior. They were obtained by forcing oscillations in the combustor at discrete frequencies and measuring the resulting pressure and global CH* radical chemiluminescence oscillations. These data suggest that the nonlinear relationship between pressure and heat release oscillations, as opposed to nonlinear gas dynamic processes, play an important role in saturating the amplitude of self excited oscillations in gas turbine combustors. Specifically, it was found that the ratio of the magnitude of the heat release response to pressure perturbations decreased at large amplitudes of driving; i.e., that gain saturation plays a role in the combustors nonlinear dynamics. The role of amplitude dependence of the phase between the pressure and heat release fluctuations is less clear from the data, however, as it was found to be nearly independent and moderately dependent upon the drive amplitude at different driving frequencies. Also observed in these studies was the phenomenon of frequency locking, manifested by the decrease in amplitude of self excited oscillations at a frequency near the driving frequency with increasing amplitude of forcing. Introduction The occurrence of detrimental instabilities in lean, premixed combustors continues to hinder the development of industrial gas turbines [1-3]. These instabilities arise from interactions between oscillatory flow and heat release processes in the combustor, and often lead to large amplitude, organized oscillations of the combustor's flow fields. To prevent the onset of these instabilities or, at least, minimize their detrimental effects (e.g., through active control), an understanding of the processes responsible for initiating and sustaining them is needed. That is, an understanding of the mechanism(s) that are responsible for initiating instabilities is needed in order to provide engineers with insight on how to avoid them in the design stage. An understanding of the nonlinear processes that control the steady state or transient limit cycle oscillations of the unstable system is needed in order to develop capabilities for predicting the amplitude of the oscillations or in the development and optimization of active control methodologies. The objective of this paper is to elucidate the nonlinear processes controlling the limit cycle oscillations in these combustors. Some theoretical and experimental work on this topic has been reported previously which is briefly discussed below. Culick and co-workers have made extensive theoretical contributions to current understanding of nonlinear oscillations in unstable combustors (e.g., see Refs. [4-5]. This work has primarily focused on Copyright © 2001 by J. Harper, C. Johnson, Y. Neumeier, T. Lieuwen, and B.T. Zinn, Published by the American Institute of Aeronautics and Astronautics, Inc. with permission 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.