Shock Reflection Detonation Initiation Studies for Pulse Detonation Engines

Abstract

A new technique to initiate a detonation wave via the interaction of a high-speed flame/shock complex and an obstacle is investigated. Experiments were performed in which a flame was ignited at the closed end of a tube partially filled with orifice plates leading up to the obstacle. Two different types of obstacles were tested. The obstacles include assemblies consisting of a circular plate followed by an orifice plate and a cone followed by an orifice plate. The flame accelerates in the orifice plate laden section of the tube to a high velocity producing a shock wave ahead of it. This shock wave interacts with the obstacle by first colliding with the leading central blockage, that is, the circular plate or the cone, and then reflecting off the orifice plate. Shock reflection produces a region of high temperature and pressure that ignites the reactants, which under certain circumstances, can initiate a detonation wave. The experiments were performed with stoichiometric ethylene-oxygen mixtures at atmospheric conditions with nitrogen dilution. The nitrogen dilution was varied, and the maximum nitrogen dilution that resulted in detonation initiation, that is, the critical mixture, was found. For experiments performed with the circular plate reflector obstacle, detonation initiation was observed for mixtures with a nitrogen-oxygen concentration ratio up to 3.98. In experiments with the cone reflector obstacle, detonation initiation occurred for less reactive mixtures, that is, mixtures with a nitrogen-to-oxygen concentration ratio of up to 4.33. Additional experiments were performed where the cone obstacle was moved closer to the Ignition point to find the minimum flame acceleration distance required for detonation initiation. For stoichiometric ethylene-air mixtures, detonation initiation was observed with the cone obstacle located at least 1.36 m from the ignition point. It was found that detonation initiation occurred for the cone reflector when the incident shock wave velocity was above roughly 750 m/s.