Flame Structure Dynamics and Buoyancy Effects in Pulsed Turbulent Diffusion Flames

Abstract

The dynamics of large-scale structures in strongly-pulsed, turbulent jet diffusion flames were investigated by conducting experiments in microgravity (2.2 Second Drop Tower) and in normal gravity. The flames were fully-modulated, with the fuel flow completely shut off between injection pulses, resulting in the generation of puff-like flame structures. Unheated ethylene fuel was injected using a 2 mm diameter nozzle into a combustor with an oxidizer co-flow at atmospheric pressure. Diagnostics included highspeed imaging of the visible flame and thermocouple temperature measurements. The celerity of large scale flame structures was determined from the cross-correlation of temperature measurements. The celerity increases markedly as the off-time between pulses is decreased and the degree of interaction between individual flame puffs correspondingly increases. The celerity is also generally higher for shorter injection times, which yield more compact flame puffs. The removal of buoyancy in microgravity results in a decrease in the flame puff celerity in all cases, amounting to as much as 40%. A dimensionless parameter relates the celerity of both buoyant and non-buoyant puffs to the injection time and mean flame length. The mean flame length of the fully-modulated flames was not strongly impacted by buoyancy. For the shortest injection time the increase in flame length due to the removal of buoyancy in microgravity amounted to less than 20%; the steady flame length in microgravity was within 6% of that of the normal-gravity steady flame. The observed increases in the flame puff celerity and the mean flame length with increased flame puff interaction are consistent with a decreased rate of oxidizer entrainment into the individual turbulent flame structures.