Dynamics of Strongly-pulsed Turbulent Diffusion Flame Structures under Buoyant and Non-Buoyant Conditions

摘要

The dynamics of the large-scale structures in strongly-pulsed, turbulent jet diffusion flames were investigated by conducting experiments in microgravity and in normal gravity. Cross-correlation of thermocouple temperature measurements and high-speed imaging of the luminous flame were used to estimate the celerity of the large scale flame structures. Both diagnostics indicate that 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. These trends are apparent both for the case of fixed injection velocity as well as for the case of fixed fueling rate. The removal of buoyancy results in a decrease in the flame puff celerity in all cases, amounting to as much as 22%. The celerity, normalized by the jet-on fraction, correlates well with the inverse of downstream distance scaled with an appropriate injection parameter. This scaling suggests that the changes in celerity due to buoyancy can be partially accounted for by the corresponding changes in the mean flame length. The applicability and limitations of the two measurement techniques for different downstream regions of the strongly pulsed flames are discussed.