Detonation Initiation by Annular Jets and Shock Waves

Abstract

The objective of this research is to experimentally determine the feasibility of initiating detonation in fuel-air mixtures using only the energy in hot, compressed air. The existing 6-inch shock tube at Caltech was used to create hot, high-pressure air behind a reflected shock wave. The hot air created an imploding annular shock wave when it jetted through an annular orifice into a 76-mm-diameter, 1-m-long tube attached to the end of the shock tube. A special test section with an annular opening covered by a diaphragm is attached to the end wall of the shock tube. The test section is filled with a combustible gas mixture and initially isolated from the shock tube by both a sliding valve and a very thin diaphragm. The sliding valve is opened immediately prior to the shock tube operation and the diaphragm is ruptured promptly when the shock wave arrives at the end of the shock tube. The test tube was filed with either stoichimetric ethylene-oxygen or propane-oxygen diluted with nitrogen. Piezoelectric pressure transducers and ionization gauges were used to determine the type of combustion event initiated by the annular jet of hot air. The stagnation conditions in the shock tube and the amount of dilution with nitrogen in the test section were varied to find the critical conditions for the onset of detonation in each test mixture. Less sensitive (high dilution) mixtures required larger stagnation pressures in order to initiate a detonation. We were unable to initiate either ethylene or propane-air mixtures within our facility limits. Extrapolation of the low-dilution data indicates that very high stagnation pressures (> 16 bar) are required to initiate detonation in fuel-air mixtures.