COMBUSTION-DRIVEN pressure and heat-release oscilla-tions, or thermoacoustic instabilities, are a major problem in low-emission gas-turbine engines [1–4]. These instabilities are naturally excited by feedback loops that couple an acoustic mode of the engine with a process causing variations in the rate of heat release from combustion. The resultant large-amplitude pressure and heatrelease oscillations lead to corresponding oscillations in the mechanical and thermal loads on various engine components, causing premature (and possibly catastrophic) component failure. Other problems, such as flame blowoff and flashback, reduced combustion efficiency, and increased pollutant emissions, also can be caused by thermoacoustic instabilities. Although different control strategies have been implemented with varying degrees of success [4–8], these are generally applied as retrofits when unstable conditions are encountered late in the engine testing process, and they are not robust to configuration changes, fuel changes, wide operation ranges, or unexpected transients. It is therefore highly desirable to better understand the mechanisms coupling the acoustics and heat release in order to better predict, avoid, and control thermoacoustic instabilities. In this work, these mechanisms are analyzed in a model gasturbine combustor based on data from high-repetition-rate laser measurements.

Thermoacoustic coupling occurs when a natural acoustic oscillation in the engine causes, through some mechanism, an oscillation in the heat-release rate. Thermoacoustically unstable states exist when such couplings cause the total amount of energy transferred from unsteady combustion to the acoustic field over a given time to exceed the amount of acoustic energy that is dissipated within the combustor or transmitted through its boundaries D. The