Buoyancy effects in strongly pulsed turbulent diffusion flames

Abstract

The effects of buoyancy in pulsed turbulent jet diffusion flames were investigated by conducting experiments in both microgravity and normal gravity. In all cases the flames were fully modulated; that is, the fuel flow was completely shut off between pulses. Unheated ethylene fuel was injected using a 2-mm-diameter nozzle into a combustor with an oxidizer coflow at ambient pressure. Microgravity conditions ( 10 − 4 g ) were achieved for 2.2 s in drop tower tests. Flames with short injection times and high duty cycle exhibit a marked increase in the ensemble-averaged flame length due to the removal of buoyancy. For other injection conditions, including steady state injection, the flame length is not strongly impacted by buoyancy. The significant increases in flame length with injection duty cycle are consistent with the duty cycle near the flame tip of microgravity flames exceeding that of normal gravity flames. The celerity of isolated compact flame puffs is approximately 40% less in microgravity than in normal gravity. An analytical argument indicates that duty cycle near the flame tip can significantly exceed that at injection due to the combination of a puff growth and the decrease in the celerity of the flame puffs with downstream distance. This effect is predicted to be significantly greater in the absence of buoyancy and for shorter injection times, in qualitative agreement with the experiments. The cycle-averaged centerline temperatures were generally higher in the microgravity flames than in normal gravity, especially at the flame tip where the difference was as much as 200 K.