Characterization of Dynamics of Unstable Fuel-Rich Flame

The flame dynamics during unstable combustion occurring in a model gas turbine combustor under fuel-rich conditions analogous to idle and sub-idle conditions in an aero-engine is characterized by simultaneous measurement of flame emissions and dynamic pressure fluctuation as well as high-speed imaging. Pressure fluctuation during unstable combustion causes linearly increasing velocity fluctuation at the combustor inlet. The fluctuation level of CH*-band emission which is mainly from soot linearly increases with respect to the combustor inlet velocity fluctuation up to ∼40% of mean velocity while that of OH*-band emission which is from OH* is non-linear. Highspeed imaging shows that the OH*-band emission fluctuation occurs mainly near the dump plane but the CH*-band emission fluctuation occurs downstream of it. When the pressure fluctuation is more than 1% of mean pressure, there exists an almost constant phase delay between emissions from OH*- and CH*-band and dynamic pressure fluctuations and the phase delay satisfies the Rayleigh criterion. In addition, the Rayleigh integral made over the whole flame and one period of oscillation of thermoacoustic instability becomes positive. These may suggest either OH*- or CH*-band emission can be used as a representation of heat release. However, the observations that the mean OH*-band emission intensity increases but the mean CH*-band emission intensity does not as the mean equivalence ratio increases and the fluctuation level of emission in OH*-band increases but that in CH*-band emission does not as the pressure fluctuation level increases strongly suggest that the emission from OH*-band should be considered as a representation of heat release for sooty flames under the employed operating condition in this study.