Experimental investigation of the stochastic nature of end-gas autoignition with detonation development in confined combustion chamber

Abstract

In the present work, end-gas autoignition formation, and the effects of oxygen concentration on the flame/shock waves propagation and pressure oscillation, are investigated in a self-designed constant-volume chamber equipped with a perforated plate. A hydrogen–oxygen–nitrogen mixture with adjustable oxygen to nitrogen ratio is chosen as the test fuel. In an oxygen-enriched condition, the probability of an end-gas autoignition occurrence increases significantly. End-gas autoignition with detonation development is further investigated, with a special emphasis on the stochasticity of the detonation development. In a low-oxygen condition, detonation occurs randomly owing to its stochastic physical behavior. However, when the oxygen concentration increases to 28%, the stochastic factors have a lower impact, and the detonation occurrence is certain. Nevertheless, the pressure and pressure oscillation in the autoignition exhibit random behaviors and are unrelated to the oxygen concentration. The variation tendency of the flame tip velocity remains constant under different oxygen concentrations. However, an increase in the oxygen concentration improves the flame tip velocity, thereby inducing stronger shock waves and promoting autoignition. Based on the start time of the autoignition, two types of autoignition modes were identified: Mode 1 and Mode 2. In Mode 2, the unburnt mixture experiences merely one compression by the shock wave before autoignition and only occurs under high oxygen concentrations of 27–28%. Under equal oxygen concentrations, the pressure and pressure oscillation of Mode 2 are higher than those of Mode 1 owing to the larger amount of unburnt mixture. Finally, the exhaust gas was introduced into the initial unburnt mixture to investigate the effect of an inert gas on the combustion.