A computational study of unsteady laminar premixed methane/air flames with composition oscillations

The effects of composition oscillations on the laminar premixed methane/air flames are investigated using computational simulations of unsteady one-dimensional counterflow flames. The code, OPUS, had been developed based on OPPDIF, solving an unsteady opposed-flow combustion configuration, including detailed chemical kinetics (GRI-mech 3.0) and transport model. The flame response to fuel concentration oscillation was numerically investigated for both lean and rich flame conditions. Methane/air mixtures with periodic equivalence ratio oscillations of 2-200 Hz were issued from the burner exit with 1.0 m/s uniform velocity profile. When the fuel concentration ratio was oscillated, the variation in flame temperature, flame location and the consumption speed did not follow those for the steady state condition and established a limit cycles. In the lean case, flame position oscillation made a clockwise limit cycle, while flame location made a counter-clockwise cycle in the rich case. In addition, flame temperature oscillation and consumption speed oscillation made counter-clockwise limit cycles in lean case, while flame temperature and consumption speed oscillation made clockwise cycles in the rich case. The behavior is attributed the effect of the heat transport from downstream burned gases, whose temperature oscillates in response to the imposed equivalence ratio oscillation. Furthermore, the limit cycles were significantly inclined at higher frequencies. The amplitude of the oscillation decreases with increasing frequency of the concentration oscillation at the Strouhal number larger than unity, suggesting that the unity Strouhal number serves as a reasonable criterion for the onset of unsteady flame response.