Characteristics of laminar stagnating premixed flame with fuel concentration oscillations near lean flammability limit

This study investigates characteristics of laminar stagnating lean premixed flame with fuel concentration oscillation experimentally. The equivalence ratio oscillation is formed by an oscillator with two cylinder piston units that can produce alternating ejection of leaner and richer mixtures. Flame response with fuel concentration oscillation near lean flammability limit is discussed as a function of frequency. The flame position oscillates following the fuel concentration oscillation. Methane and propane are used as fuels. The oscillator frequency varies in 2∼20 Hz and burner exit velocity varies from 0.6 to 0.9 m/s. The amplitude of the flame position oscillation increases with an increase in the frequency in St <1 and attenuates in St > 1 regardless of the fuel and the burner exit velocity. This indicates that the Strouhal number plays a key role for the flame motion with fuel concentration oscillation. It is also interesting to note that the methane/air premixed flame does not extinguish even when the flame position exceeds that at the lean flammability limit of the steady state condition. This suggests that the lean flammability limit is expanded by the fuel concentration oscillation. The extension of the lean flammability limit occurred due to the back support effect which intensify the flame by the heat transport from the burnt gas region. In the case of the propane/air premixed flame, the expansion of the lean flammability limit is less clear although the flame position slightly exceeds that at the flammability limit in the steady state condition. This difference is supposed to be attributed to the difference in the Lewis number.