Toroidal Imploding Detonation Wave Initiator for Pulse Detonation Engines

Abstract

Imploding toroidal detonation waves were used to initiate detonations in propane-air and ethylene-air mixtures inside of a tube. The imploding wave was generated by an initiator consisting of an array of channels filled with acetylene-oxygen gas and ignited with a single spark. The initiator was designed as a low-drag initiator tube for use with pulse detonation engines. To detonate hydrocarbon-air mixtures, the initiator was overfilled so that some acetylene oxygen spilled into the tube. The overfill amount required to detonate propane air was less than 2% of the volume of the 1-m-long, 76-mm-diam tube. The energy necessary to create an implosion strong enough to detonate propane-air mixtures was estimated to be 13% more than that used by a typical initiator tube, although the initiator was also estimated to use less oxygen. Images and pressure traces show a regular, repeatable imploding wave that generates focal pressures in excess of 6 times the Chapman-Jouguet pressure. A theoretical analysis of the imploding toroidal wave performed using Whitham's method was found to agree well with experimental data and showed that, unlike imploding cylindrical and spherical geometries, imploding toroids initially experience a period of diffraction before wave focusing occurs. A nonreacting numerical simulation was used to assist in the interpretation of the experimental data.

Detonation Initiation in a Tube via Imploding Toroidal Shock Waves

“…Increasing the diameter of the detonation tube to the values used by Li and Kailasanath [22] could enhance the initiation process by allowing the imploding shock wave to achieve higher values of compression during the implosion process. However, an increased tube diameter would also reduce the amount of confinement, moving potential reflecting surfaces away from the implosion focus, which has been shown to be detrimental to the success of the implosion process [2]. Thus, it is not clear what net effect changing the diameter would have on the initiation process.…”

“…The current study is intended to serve as an extension of earlier initiation work using imploding detonation waves [2,17] and is motivated by recent numerical simulations that proposed to further refine this initiation technique through use of an imploding toroidal shock wave (instead of an imploding detonation wave) driven by jets of air or fuel. Li and Kailasanath [22] found that detonations could be initiated in a 14-cm-diam tube filled with stoichiometric ethylene-air using an imploding shock wave created from the injection of a converging annular jet of fuel or air at the outer diameter of the tube.…”

“…The uncertainty associated with this technique can be found by determining the velocity variations that would result from the transducer detecting the wave at its leading and trailing edges, as was done in earlier work [2] %velocity error 2d L 100%…”

“…In this study, we briefly review those concerned with the development of the shock implosion facility. More complete reviews on earlier imploding shock and imploding detonation wave works are available elsewhere [1,2].…”

“…In an attempt to improve on this concept, a system was developed [2,18,19] to detonate propane-air mixtures inside a detonation tube using an imploding toroidal detonation wave propagated into the propane-air mixture from the tube walls. To generate the imploding wave, the toroidal initiator used a single spark plug and a small amount of acetylene-oxygen gas.…”