Analysis of the ignition induced by shock wave focusing equipped with conical and hemispherical reflectors

Abstract

The interaction of a plane shock wave in air with concave profiles has been used in the past to understand the nature of shock wave focusing. The current study examines the complex two-dimensional flow field resulting from the interaction of a plane shock wave entering a symmetrical cavity with a curved end wall. The development of reflection patterns of the incident shock wave at the profile wall and the process of gas dynamic focus are of particular interest. In this study, numerical and experimental studies are performed to understand the ignition behavior in a stoichiometric methane-oxygen-argon mixture due to shock wave reflection from a variety of shapes, including planar, 60° and 90° conical and hemispherical reflectors. The numerical simulation reveals the complex two-dimensional flow field when the shock wave collides head-on with different reflectors and the propagation behavior of the reflected waves in the process of focusing. The instantaneous maximum temperature and pressure as a function of shock wave intensity in four reflectors are given and analyzed. The experimental results show that two ignition modes, namely, weak ignition and strong ignition, occur in those reflectors when the incident shock wave velocity (Vi) is greater than a critical value, and the separatrix between the weak and strong ignition for each reflector is demonstrated to be highly dependent on Vi. The critical values of Vi for the 60° conical, 90° conical and hemispherical reflectors are 740, 775 and 780 m/s, respectively. The ignition delay time (tign) of the combustible mixture under shock wave focusing for various reflectors is provided. The results show that in the weak ignition mode, tign is shorter in the hemispherical reflector than in the others, and therefore, its Pign (the maximum pressure after ignition) is higher. However, the result is completely opposite in the strong ignition mode, which is mainly because the ignition temperature of hot spots in the hemispherical reflectors is much lower than those in the conical reflectors. This study provides new simulation and experimental evidence of the ignition behavior in various reflectors, supporting the effect of enhancing shock wave focusing on the ignition performance.