Unsteady response of hydrogen and methane flames to pressure waves

Recent measurements of the direct response of premixed hydrocarbon flames to acoustic pressure fluctuations have shed doubt on the validity of analytical models that use irreversible one-step chemistry (Wangher et al., 2008) [1], and suggest that more realistic chemical kinetic models are needed to fully describe the unsteady dynamics of premixed flames. In this paper we present experimental results and numerical simulations for planar hydrogen flames which have simpler chemical kinetics than hydrocarbon flames. The simulations employ detailed chemical kinetics, including OH∗ chemiluminescence chemistry, so that the validity of using the emission from the excited OH∗ radical as a marker of the reaction rate can be assessed. By comparing our numerical results with measurements on hydrogen, and with previous measurements on methane, we show that OH∗ chemiluminescence does not always provide a reliable measure of heat release rate in the presence of a pressure driven interaction. Finally, our results are compared to the predictions of an analytical model for two-step chemistry with a chain-branching and a chain-breaking reaction (Clavin and Searby, 2008) [2]. We conclude that multi-step chemistry must be taken into account when evaluating the unsteady response of flames to pressure waves.