Detonation initiation by shock focusing at elevated pressure conditions in a pulse detonation combustor

Abstract

This work contains experimental investigations on the correlation of the detonation initiation process via a shock-focusing device with various initial pressures and mass flow rates. A pulse detonation combustor is operated with stoichiometric hydrogen--air--oxygen mixtures in single cycle operation. A rotationally symmetric shock-focusing geometry evokes the onset of a detonation by the focusing of the reflected leading shock wave, while a blockage plate at the rear end of the test rig is applied to induce an elevated initial pressure. The results show that the reactivity has a major influence on the success rate of detonation initiation. However, measurements with different blockage plates suggest that the mass flow rate has to be considered as well when predicting the success rate. Three main statements can be drawn from the results. (1) An increase in the mean flow velocity induces higher velocity fluctuations which result in a stronger leading shock ahead of the accelerating deflagration front. (2) An increase in the initial static pressure reduces the critical shock strength that must be exceeded to ensure successful detonation initiation by shock focusing. (3) Since the initial pressure is directly linked to the mass flow rate, these contrary trends can cancel each other out, which could be observed for 40% vol. of oxygen in the oxidizer. High-speed images were taken, which confirm that the detonation is initiated in the center of the converging--diverging nozzle due to focusing of the leading shock.

Numerical study on detonation initiation process in the chamber with characteristic structures of the dual-mode scramjet combustor

“…Han et al 9 researched the detonation initiation and deflagration to detonation transition (DDT) process of hot jet in supersonic premixed gas by experiment and numerical simulation, and found that the hot jet detonation was essentially initiated by the reflected shock wave formed by the induced bow shock wave at the upper wall. Habicht et al 10 experimentally investigated the detonation initiation by shock focusing at elevated pressure conditions in a pulse detonation combustor and found that an increase in the initial static pressure reduced the critical shock strength which was exceeded to ensure successful detonation initiation by shock focusing. Bengoechea et al 11 represented a numerical study of detonation initiation by means of a focusing shock wave using a one-step chemical model with variable parameters optimized by the adjoint approach in terms of the induction time.…”