A Detailed Analysis of Thermoacoustic Interaction Mechanisms in a Turbulent Premixed Flame

Abstract

A combined theoretical and experimental analysis of thermoacoustic interaction mechanisms of a lean pre-mixed swirl-stabilized gas turbine burner is presented. A full-scale gas turbine burner has been tested in an atmospheric test rig. The test facility was equipped with loudspeakers to excite the acoustic field and with arrays of microphones to measure the response of the acoustic field to the forcing signal. With this set-up transfer matrices relating the acoustic pressure and velocity on both sides of the flame front have been measured. A laser absorption measurement technique allowed for measurement of the fluctuations of fuel concentration in the mixture. Heat release fluctuations were monitored using a photo-multiplier. The measurement of the acoustic field, heat release and equivalence ratio fluctuations have been measured simultaneously. Special attention has been given to the role of fuel concentration fluctuations in the thermoacoustic interaction mechanism. In order to be able to clearly separate this mechanism from other possible mechanisms, all the experiments have been performed in pre-premixing mode as well. In pre-premixing mode the fuel is injected far upstream of the burner in order to avoid fuel concentration fluctuations at the burner location. This is in contrast with premixing mode where fuel and air are mixed in the burner. An acoustic flame model has been derived. The model includes the well-known interaction mechanism of equivalence ratio fluctuations but also includes a novel mechanism that is caused by fluctuations of vorticity. This latter mechanism relates the turbulent flame speed via turbulence intensity fluctuations to the acoustic field. The idea is that periodic acoustic fluctuations cause periodic changes of the turbulence intensity. The turbulence intensity strongly affects the turbulence flame speed. The fluctuations of the turbulent flame speed result in fluctuations of the heat release. This turbulence intensity fluctuation model has been compared with the measured pre-premix transfer functions and shows an excellent agreement. The measured transfer functions in premix mode have been compared with the model that includes fluctuations of fuel concentration and turbulence intensity. Also in this case a very good agreement is found. Moreover, it has been demonstrated that the phase relation between measured equivalence ratio fluctuation and heat release corresponds to the model.