Experimental Diagnostics Of Combustion Instabilities

Abstract

T HE problem of unstable combustion continues to be a critical issue that limits the development of gas-turbine combustors for propulsion and land-based power-generation applications.' To a great extent, unstable combustion is a result of the increased use of premixed combustors, which are inherently more susceptible to unstable combustion than nonpremixed combustors. To develop combustors that are capable of stable operation over their entire operating range, an understanding of the mechanisms that initiate and sustain unstable combustion and their relative importance at different operating conditions is essential. Unstable combustion refers to self-sustained combustion oscillations at or near the acoustic frequency of the combustion chamber, which are the result of the closed-loop coupling between unsteady heat-release and pressure fluctuations. That heat-release fluctuations produce pressure fluctuations is well known and well understood, but the mechanisms whereby pressure fluctuations result in heatrelease fluctuations are not. In general, it is thought that flame-vortex interaction,' feed-system coupling, and spray-flow interactions are the most important instability-driving mechanisms in gas-turbine instabilities. Flame-vortex interaction refers to the interaction between the flame front and vortices that are periodically shed at the entrance to the combustor. As the vortex passes through the flame front, the flame is stretched by the vortex. Depending on the rate at which the flame is stretched and the local equivalence ratio, this interaction can either increase the flame area and hence the rate of heat release, or it can lead to local extinction and as a result decrease the rate of heat release. Feed-system coupling refers to a modulation of the fuel flow rate caused by pressure fluctuations in the combustor and fuel-delivery system. This modulation results in a fluctuating fuel concentration that is convected to the flame front and