Detonation waves through foam

Abstract

Detonation waves in plain gas and gas trapped in a light water based foam, were compared to find out if there is a connection between the detonation velocity achieved in the foam, to the size of the foam bubbles, and if such a connection can give an indication of the size of the detonation cells in the given gas mixture. Basically two phenomena are investigated a) the basic detonation parameters in the two media, and b) comparison of the deflagration to detonation transition (DDT). The explosive gas contained pure ethylene in oxygen. The concentrations of the fuel ranged between 9 to 44 percent. All mixtures were tested with and without foam. Bubbles of sizes between 1 and 13 mm were generated. The amount of water in the foam was also recorded. The results identify two distinct concentration regions: o A) Mixtures between 16 to 40 percent fuel (equivalence ratios =0.61–2.0), where the DDT process was instantaneous or very close to it. A very modest damping of the detonation velocity was detected reaching in extreme cases up to 8 percent. The variation in the sizes of foam bubbles showed no dramatic changes. B) In mixtures containing between 9 to 16 percent fuel (=0.30–0.61) where changes in DDT time are encountered according to changes in concentration, drastical influence of the foam is found. The presence of foam bubbles starting from certain sizes and upward, shorten the DDT time between 60 to 75 percent and more. In conclusion the bubbles promote the DDT and shorten the DDT time at all concentrations. There is always a critical bubble size at which the DDT time is equivalent to that of plain gas and in larger sizes the DDT time decreases. This critical size of bubbles increases with the decrease of fuel percentage in the mixture.