The precursor shock wave and flame propagation enhancement by CO2 injection in a methane-oxygen mixture

Abstract

Successful initiation of a detonation in a confined combustor with limited length is a formidable task for designing the detonation-based engines, shortening the length for the run-up distance of a detonation greatly reduces the weight of the detonation engine, rendering the detonation propulsion become possible in the advanced aerospace propulsion systems. In this work, we proposed a new method that using a non-reactive gas jet (CO2) to facilitate the formation of the precursor shock wave and the flame propagation speed in a deflagration wave in a stoichiometric methane-oxygen mixture, by changing a series of the injection parameters, i.e., the initial pressure of mixture, the jet delay time, the jet pressure and its position, the effects of those parameters on the deflagration propagation are systematically investigated. The experimental results show that, the development processes of pressure rise are divided into two stages prior to the DDT: 1) slow deflagration stage, which characterized by two discrete shock waves named as the 1st shock wave and 2nd shock wave; 2) Fast deflagration stage, the two discrete waves merge into a strong precursor shock wave, and the velocities of precursor shock and flame achieve to the “quasi-detonation” state. The experimental results also illustrate that, introducing the CO2 jet in the first stage prior to the DDT promotes the formation of precursor shock wave and accelerates the flame speed, which speeds up the transformation of a detonation in a shorter distance. By comparing different jet parameter combinations, it is found that high jet pressure and long jet time increase the local concentration of CO2, which adversely affect the formation of a detonation. Thereby there exists an optimal condition of injection, whose injection pressure ratio is 15, jet delay time is 20 ms and jet position is found to be 0.6 m, those parameters are verified to have prominent detonation-enhancement effects on DDT, the underlying mechanisms for those optimal conditions are explored and explained in this study. This study provided new experimental evidences supporting the theory of turbulence-enhancement effect on the initiation of a detonation.