Abstract
An experimental study to establish the basic mechanisms of transition from deflagration to spherical detonation downstream of flow obstructions has been performed. The flow obstructions consist of circular and rectangular orfice plates, multiple rectangular orifice plates and circular hole perforated plates. In order to keep the apparatus reasonably small and also to facilitate photographic observations at low initial pressures, acetylene-oxygen mixtures at equimolar composition were used throughout. The principal diagnotics were stereoscopic streak and framing schlieren photography. The apparatus consists of a spherical flame chamber connected to a cylindrical detonation chamber via an orifice plate mounted in the port-hole connecting the two chambers. The mechanism of detonation initiation is found to be due to the intense mixing of the hot combustion products with the unburned gases. Large scale eddies are found to be essential for detonation. These eddies provide the main mechanism of entrainment, and are also partially responsible for providing energy to maintain the intensity of the finer scale turbulence. When wire screens are used in conjunction with the orifice plates, the formation of detonation is facilitated, indicating that the fine scale turbulence which is produced plays a role in rapid mixing of the entrained gases inside the large scale eddies. Under critical conditions, when the initiation results from one single eddy, it appears that shock wave amplification by coherent energy release (SWACER) inside an eddy, where a gradient field of induction time due to entrainment is present, is most probable. This suggests that the minimum size of the required eddies must be at least of the order of the detonation kernel (i.e., transverse wave spacing) of the mixture. The present experimental findings that the obstructions which produce eddies of size of the order of transverse wave spacing lead readily to detonation initiation support this view.
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