Detonation Transition Limit at an Abrupt Area Change Using a Reflecting Board

Abstract

To realize quick initiation of detonation in the combustion chamber of a pulse detonation engine operating in the air-breathing mode, in which the combustible gas is a fuel-air mixture, the authors have proposed a new pulse detonation engine initiator using a near the exit of a predetonator tube. In this study, we clarify the transition limit of this new initiator by examining the detonation cell size at the predetonator exit and the mechanism that gives this transition limit. The combustible mixtures are stoichiometric hydrogen-oxygen mixtures diluted with nitrogen or argon. The main results obtained in this study are as follows. When the incident detonation wave interacts with the reflecting board before it completely disappears due to the rarefaction wave from the predetonator exit, the number of cells between the exit and the board defines the transition limit from the planar to cylindrical detonation waves. Even when the cylindrical detonation does not occur, the reflecting board converts a planar detonation wave into a torus-shape pressure wave. This pressure wave encompasses the combustible gas in the detonation chamber and concentrate on the axis, causing a detonation bubble behind the board. The necessary minimum diameter of the predetonator with a reflecting board is expressed by D c = 6.3λ.