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 2 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.

Flame propagation behavior of propane–air premixed combustion in a confined space with two perforated plates at different initial pressures

“…At 2.225 ms, the end‐gas autoignition propagates from the upper right to left with a velocity of about 254 m/s, as marked by the white dot line. Meanwhile, as shown in Figure 4, the peak pressure is 3.72 MPa with pressure oscillation up to 0.701 MPa thus developing deflagration (stochastic location and time) due to the stochastic nature of autoignition 47‐49 47 is the main mechanism of autoignition combustion.…”

“…Meanwhile, as shown in Figure 4, the peak pressure is 3.72 MPa with pressure oscillation up to 0.701 MPa thus developing deflagration (stochastic location and time) due to the stochastic nature of autoignition. [47][48][49] RM instability 47 is the main mechanism of autoignition combustion.…”

Parametric Studies of Deflagration-to-Detonation Transition in a Pre-Chamber/Main-Chamber System

“…As high-speed jet flames accelerate into the main chamber they can be accompanied by the generation of shock waves [3]. Under extreme conditions, deflagration-to-detonation transition (DDT) might occur in the main chamber, which is closely related to the "super knock" phenomenon [4]. While a number of studies have focused on the behaviour of super knock in research engines, no consensus has been reached on the exact mechanism of a super knock event.…”

“…There have been a number of recent studies focusing on the effects of pre-chamber ignition and turbulent jet flame on main-chamber combustion [6][7][8], and the onset of DDT [4,9] in real engines or combustion chambers under engine relevant conditions. Qin et al [6] used direct numerical simulations to study the transient mixing and ignition mechanism of methane/air mixtures in a simplified pre-chamber/main-chamber system.…”

“…It was shown that the mean burning velocity in the main chamber could be significantly elevated as the pre-chamber jet entered the main chamber. Recently, Wei and co-workers [4,9,10] carried out a series of experiments in a confined combustion chamber to study: the flame acceleration after passing through a perforated plate, the subsequent propagation of turbulent jet flame and shock waves, resultant end-gas autoignition, and the stochasticity of detonation initiation. They observed various combustion modes, including normal combustion, oscillating combustion, and end-gas autoignition with and without DDT near the end wall of the combustion chamber by adjusting the jet flame speed, initial pressure, and fuel/air equivalence ratio.…”