HIS paper presents a comparative analysis of the heat release response of laminar premixed flames to perturbations in acoustic pressure, velocity and equivalence ratio. This work is motivated by combustion instability, which causes significant problems in the operation of premixed combustion systems 1. Unsteady heat release processes in a combustor can result in a coupling with one or more of its acoustic modes, causing high amplitude pressure and velocity oscillations. These oscillations lead to poor system performance and hardware damage. While high frequency (eg, kHz frequency range) oscillations have been problematic in rockets for decades 2-5, lower frequency instabilities (eg,< 100 to 100s of Hz) have been most common in air-breathing systems, such as low NOx combustors. However, in the last few years, a significant number of largely unpublished field occurrences with high frequency instabilities have similarly plagued low NOx gas turbines. These instabilities are extremely problematic because they cause major damage within a matter of a few minutes, rather than over hundreds or thousands of hours, as is more typical with lower or mid-frequency instabilities. These observations motivated this study of high frequency combustion instabilities in premixed systems, in order to understand their potentially unique mechanisms and/or qualitatively different controlling physical processes as compared to lower frequency disturbances.

Modeling combustion instabilities in order to develop rational mitigation approaches requires an understanding of the various mechanisms that cause heat release oscillations in lean premixed combustors 6-17. To understand the different physical mechanisms causing heat release oscillations of a flame subjected to perturbations, consider the instantaneous global heat release rate of a flame, which is given by: