Flame dynamics of equivalence ratio oscillations in a laminar stagnating lean methane/air premixed flame

This study investigates the effect of fuel concentration oscillation on laminar stagnating premixed flames by both experiment and numerical simulation. The numerical analysis is conducted using ANSYS Fluent 14.5. The equivalence ratio oscillation in the experiments is formed by a novel oscillator with two cylinder piston units that can produce alternating ejections of leaner and richer pre-mixtures. Velocity fluctuation is well suppressed by installing screens on the burner exit. The fuel concentration oscillation between the stagnation plate and the burner exit is visualized and analyzed by acetone ultraviolet light-induced fluorescence in the isothermal condition. The oscillator frequency is varied in the range 2-20 Hz, and the oscillation wavelength is much longer than the flame thickness. The flame oscillates with the fuel concentration, and in the experiment, the amplitude of the flame oscillation attenuates as the frequency of fuel concentration oscillation increases above 5 Hz, which corresponds to a Strouhal number of unity. This indicates that the Strouhal number distinguishes quasi-steadiness for St < 1 and unsteadiness for St > 1. The flame oscillation pattern is a closed loop, which might be attributable to variation of the back support effect on the flame. The numerical results show a similar trend for the flame response to oscillations in fuel concentration. This study finds the flame motion is significantly affected by fuel concentration oscillations, even at low frequencies; in other words, the oscillation wavelength is much longer than the flame thickness, as a result of the back support effect.