Diffraction of detonation from tubes into a large fuel-air explosive cloud

Abstract

This paper reports on a series of field tests performed at the Canadian Defence Research Establishment Suffield and at Raufoss, Norway, to obtain the critical tube diameter d c , for ethylene-air mixtures by investigating the diffraction of detonations from tubes into large plastic bags simulating an unconfined fuel-air cloud. The critical ethylene-air compositions for successful re-establishment of detonation upon emerging from tubes of diameters 0.31 m, 0.45 m, 0.89 m and 1.36 m were determined by monitoring the diffracted detonation wave in the bag. High-speed cinematography of the diffracted wave shows that re-establishment of detonation is via one or more re-ignition centres at sites along the head of the expansion wave which originates at the area change. The characteristic transverse wave spacings, S, associated with the detonations were measured from smoked foils mounted in the tubes. These measurements demonstrate that the empirical relation d c =13 S provides a good correlation between the critical tube diameter and the cell size over a wide range of ethylene-air compositions. However, a better understanding of the coupled chemical-gasdynamic processes within the cell is required in order to clarify the link between cell size and chemical kinetics. In addition to the open tube tests, transmission of detonations through circular orifice holes was investigated in both ethylene-air and acetylene-air mixtures. Based on these tests it was concluded that the critical tube and critical orifice diameters are equal.