Extinction of Counterflow Premixed Flames Under Periodic Variation of the Rate of Stretch

Abstract

Abstract The influence of periodic oscillations of the rate of stretch on the extinction of lean methane-air and butane-air premixed flames in stagnation-point flow and counterflow is experimentally investigated. The periodic variation of the rate of stretch is achieved by means of two speakers that are attached to the conventional counterflow burners. It is found that for a given frequency, increasing the amplitude of the velocity oscillations tends to weaken the lean flames such that the critical minimum fuel concentration at extinction increases. The effects of oscillating stretch rates on flame position and extinction characteristics are found to diminish with the frequency. The extinction data are also obtained as a function of the frequency of velocity oscillations for given mean flow through the burners. Higher frequency flow oscillations are found to render more robust premixed flames since larger amplitudes of velocity oscillations are needed to extinguish the flames. For the counterflow methane-air and butane-air premixed flames the temperature profiles between the opposing burners with and without periodic velocity oscillations are determined. Periodic velocity fields arc found to reduce the temperatures throughout the flow field resulting in closer approach of the counterflow flames to the stagnation plane.