Turbulent Structure Dynamics of Buoyant and Non-Buoyant Pulsed Jet Diffusion Flames

Abstract

The flame structure dynamics of strongly pulsed, turbulent diffusion flames were examined experimentally in a co-flow combustor. High-speed visual imaging and thermocouple measurements were performed to determine celerity, defined as as being the bulk velocity of a given flame puff structure in the large-scale, turbulent flame structures. Tests were conducted in normal gravity and microgravity with a fixed fuel injection velocity with a Reynolds number of 5,000 and also with a constant fueling rate where the Reynolds number ranged from 5,000 to 12,500. The celerity of strongly interacting flame puffs is as much as two times greater than for the case of isolated flame puffs. The amount of decrease in celerity at the visible flame tip due to the removal of buoyancy ranges from 7% to 11% in most cases, to as much as 36% for both fixed jet injection velocity and constant fueling rate. At the same time, the flame length is modestly affected by the removal of positive buoyancy, amounting to a decrease of as much as 20%. These observations hold for both fixed injection velocity and constant fueling rate cases. The observed increases in the flame puff celerity and the mean flame length with decreasing jet-off time, for a given injection time and gravity level, are consistent with a decreased rate of oxidizer entrainment into each flame puff structure due to increased flame puff interactions. A scaling argument accounts for the decrease of the flame puff celerity with downstream distance when both quantities are normalized by the appropriate injection conditions. The celerity, as characterized by the temperature measurement method, appears to be essentially unaffected by buoyancy at any given downstream location when appropriately scaled. The visual tracking method suggests a modest buoyancy effect at a given downstream distance, suggesting a subtle impact of buoyancy on the flame puff structures that does not impact the bulk motion.