Mixing and Combustion of Rich Fireballs

Abstract

A series of experiments was carried out to investigate the effect of fireball composition on secondary combustion. The fireball was created from a 1.5 liter balloon filled with a propane-oxygen mixture (1< [Greek Phi] <3) and initiated by a detonation. Two initiation locations and two initiator strengths were studied. Two pencil pressure gauges located at 0.6 and 1.2 m and in some experiments, simultaneous high-speed imaging, were used as diagnostics. For [Greek Phi] > 1, the incompletely oxidized products from the primary burn mix with the surrounding air and may be oxidized in a secondary combustion process. The unique feature of the present experiments was a repeatable secondary pressure pulse for sufficiently rich mixtures. The secondary pressure rise was observed repeatably for all initiation configurations. The nature of the secondary pressure pulse is a strong function of the initial equivalence ratio. For [Greek Phi] = 1 and 1.5, no secondary pressure waves are observed. An acoustic analysis of the measured pressure histories has been carried out to infer the rate of volume displacement and the total volume displaced by the secondary combustion. The results of the acoustic analysis are in reasonable agreement with both a simplified thermodynamic model predicting the total volume displacement assuming constant-pressure combustion for the secondary burn and the analysis of the fireball luminosity of the high-speed images. For nearly stoichiometric mixtures, [Greek Phi] = 1 and 1.5, the leading blast wave peak pressures and impulses are comparable with the previously-measured gaseous and high explosive blasts when the energy content of the balloon only is used to formulate Sachs scaling variables. Due to a much slower combustion process than detonation for [Greek Phi] >2 the peak pressure of the leading wave rapidly decreases below the energy-equivalent reference blast values as the equivalence ratio is increased. The Sachs-scaled impulse agrees well with the predictions on the basis of the energy in the balloon alone for 2.75 > [Greek Phi] > 1. One of the key results of the present study has been the documentation of the existence of the secondary pressure wave. The present study has emphasized the acoustic nature of the secondary pressure waves and the origin of these pressure waves due to the processes at the interface between the fireball and the atmosphere. The presence of the secondary pressure peak and the higher impulses indicate that there is the potential for significant enhancement of the blast through secondary combustion.