Effect Of Diluent Gases On End-Gas Autoignition And Combustion Modes In A Confined Space

Diluent gases are used in practical industrial applications that involve flame propagation and detonation. In this study, we investigated the effect of diluent gases on end-gas autoignition and detonation development in a constant volume combustion chamber. High-speed Schlieren photography was employed to capture the flame-shock wave interaction and detonation evolution. Two reactants (hydrogen and methane) and three diluent gases (argon, nitrogen, and carbon dioxide) were used for the process. In this study, statistical analysis confirmed that the ratio of maximum pressure to equilibrium pressure (P-max/P-equ) can be a criterion to determine the occurrence of end-gas autoignition. In addition, it was observed that the mechanism of end-gas autoignition with detonation development was dominated by the flame propagation velocity and associated shock wave intensity in the present experimental conditions. With an increase in the flame propagation velocity, the combustion mode transitioned from normal combustion to autoignition Mode 1 (the end gas suffers two compressions by shock wave before autoignition), and to autoignition Mode 2 (the end gas suffers one compression by shock wave before autoignition), which was observed for different reactants and diluent gases. The diluent acted as an inhibitor for the chemical reaction and moderated the flame acceleration, and thus, restrained the end-gas autoignition and detonation intensity. CO2 is a better inhibitor of end-gas autoignition with high-pressure oscillation and detonation than N-2 and Ar; CO2 can be employed to prevent knock or super-knock in boosted gasoline engine. Further, we discussed the basis of random location of the autoignition kernel, and the influence of the location on measuring the intensity and destructive capability of the detonation. Moreover, owing to the addition of carbon to the combustion, a blue-white light was observed after the propagation of the shock wave. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.