Acoustic parametric instability in thermo-diffusively unstable CH4/O2/Xe mixtures

Abstract

Acoustic and parametric instability in downward propagating premixed flames of low Lewis number (∼0.6) mixture of CH4/O2/Xe is studied experimentally and analytically. Seven mixtures of increasing laminar burning velocity (SL) at constant equivalence ratio of 0.7 are considered. Flame front history and pressure measurements are reported. Parametric instability of the flame front due to effect of acoustics is observed for mixtures with SL higher than 27 cm/s. Complex stability diagrams are observed for the mixtures. For mixtures of lower SL, a clear condition of wavenumber and acoustic velocity amplitude for the onset of parametric instability is not found in the stability diagram. Rather a wavenumber, k0B is observed where instability is observed for all the acoustic velocity amplitudes. The experimentally measured wavenumbers at the onset of parametric instability were found to match with k0B. For mixtures of SL> 54 cm/s, wavenumber and acoustic velocity amplitude at the onset of parametric instability were clearly identified in the stability diagram. Experimentally measured wavenumbers were found to lie between critical wavenumber of parametric instability and k0B. This work shows a quantitative agreement between theory and experiments and thus proves the applicability of theoretical analysis for low Lewis number mixtures.