Response characteristics of methane–air and propane–air premixed flames to sinusoidal oscillations in equivalence ratio (Effects of Lewis number and preferential diffusion)

Flames propagate in mixture flows with inhomogeneous concentration distributions in practical combustors, such as gasoline direct-injection engines and gas turbine engines. Clarifying the effect of fuel-type differences on the response characteristics of a flame propagating through a nonuniform concentration field will help develop such combustors. This study experimentally investigates the response characteristics (flame position xf, velocity gradient g, CH radical emission intensity CH, and burning velocity Su) of lean methane–air and propane–air wall-stagnation-flow premixed flames to sinusoidal oscillations in the equivalence ratio φ at oscillation frequencies f of 5–50 Hz. Through experiments, MATLAB analysis, particle image velocimetry, and high-speed photography, we find the following: As f increases, the responses of xf, g (flame stretch rate), CH, and Su to the fluctuating φ at the burner outlet are delayed, especially in the case of the dynamic propane flame, which has a small fuel diffusion coefficient. Regardless of fuel type, the frequency characteristics of Su are similar to those of g, and as f increases, the minimum CH of the dynamic flame becomes lower than that of the steady flame and has a local minimum value. However, because of the Lewis number effect, the maximum CH of the dynamic methane flame (CHd,max) exceeds that of the steady flame (CHs,max) and assumes a maximum value with an increase in f. For the dynamic propane flame, even as f increases, CHd,max does not exceed CHs,max and monotonically decreases. Finally, due to preferential diffusion, the combustion intensity of the dynamic methane flame becomes stronger than that of the dynamic propane flame when the nondimensional frequency (2πf/g) exceeds approximately unity.