Mixing and reaction progress in a confined swirl flame undergoing thermo-acoustic oscillations studied with laser Raman scattering

Abstract

A gas turbine model combustor for partially premixed flames was equipped with an optically accessible combustion chamber and operated with methane and air at atmospheric pressure, with a global equivalence ratio of 0.7 and a thermal power of 25kW. At these conditions the combustor exhibited thermo-acoustic oscillations with a frequency of approximately 400Hz. The flame behavior and its cyclic variations were investigated by laser Raman scattering for the simultaneous determination of the major species concentrations, temperature and mixture fraction. Additional information of the mean flow field and the flame shape was provided by particle image velocimetry and OH∗ chemiluminescence imaging, respectively. Previously published results of phase-correlated mean values of this flame showed that the instability was sustained by the mechanism known as equivalence ratio fluctuations with convective delay. The current paper is focused on the characterization of the thermo-chemical state of the flame during the oscillation cycle. The mixture fraction varies considerably with spatial location and with the phase of the pressure oscillation and strong effects of turbulence–chemistry interaction are prevailing in the region close to burner mouth. Further, the effect of locally rich mixtures and elevated temperature on the local CO concentration level is shown.