On the evaluation of stratification effect on unsteady counterflow flames under mixture composition oscillation

Although an extensive study has been conducted to elucidate the effect of equivalence ratio gradient on flame characteristics, some disagreements regarding the mechanism of the stratification effect exist. This may be attributed to differences in the flame properties used to evaluate the stratification effects. The present study evaluated the magnitude of the stratification effects with various flame properties used previously, and compared their response to mixture composition oscillations. The response of displacement speed was affected by unsteady gas expansion, while the response of consumption speed directly represented the variation in the reaction rate. The response of peak heat release rate provided a better representation of the response of fuel consumption rate than the global heat release rate, because it measures the local characteristics at the reaction zone and does not include heat release variations in the wake of the flame. However, the peak heat release rate overestimates the magnitude of the stratification effect due to additional variation in the heat release from H₂ oxidation reaction, which is caused by increase/decrease in the H₂ diffusion from the burned side.