Detonation initiation, propagation, and diffraction for pulse detonation engine application

The physics that control the detonation initiation and propagation of a detonation between tubes of different cross-sectional areas has been studied extensively for a practical pulse detonation engine. The use of a spark initiated fuel-air detonation, created within a small-diameter tube or predetonator; to directly initiate a detonation within the main chamber, is considered to be the most practical option for a pulse detonation engine due to system simplicity and low weight. In order to achieve a detonation within a predetonator of practical size and length, an obstacle or series of objects tailored to enhance the acceleration mechanisms of the combustion wave while inducing minimum pressure and momentum losses, were placed in the flow. Simultaneous Schlieren and OH-PLIF images taken throughout the predetonator, show flame acceleration mechanisms present in an obstacle-free tube were enhanced when an effective obstacle was installed. With knowledge of the effect of obstacle parameters on flame acceleration, repeatable ethylene-air detonations were achieved in a round 33.3-mm diameter tube within one meter using a tailored obstacle that was not optimized. Reaching a minimum Chapman-Jouguet or C-J deflagration velocity within the obstacle appears to be a necessary condition for transition to a detonation. Objects were placed in the area expansion to reflect the shock wave of the diffracted detonation upstream over a volume of reactants and the combustion wave, and to force a jet of combustion into the main chamber boundary layer in order to reduce the diffraction effect of the area expansion and initiate a direct main chamber detonation. With the use of the transition obstacle, no direct main chamber detonation was achieved for ethylene-air mixtures and only slight advantages were shown for a 25-degree half-angle 78% blockage ratio cone over a 45-degree cone with the same blockage, the 45-degree cone over a 78% blockage ratio disk, and the 78% blockage ratio disk over no obstacle placed at the expansion exit. There was a weak effect on axial location of the transition obstacle. The second method used to reduce the diffraction effect of the expansion is to propagate an overdriven detonation, resulting from the localized explosion occurring in the last phase of the DDT process, into the area expansion. (Abstract shortened by UMI.)...