An experimental investigation on the flow structure and mixture formation of low pressure ratio wall-impinging jets by a natural gas injector

Natural gas direct-injection (DI) with diesel pilot ignition using stratified combustion concept has been considered as a challenging and innovative technology to improve engine efficiency and meet stringent emission limits. Due to stratified mixture usually formed by late injection near the top dead center, the jet pressure ratio (PR) between injection pressure and motored in-cylinder pressure is usually very low, which may lead to poor jet mixing efficiency. In the present study, wall impingement was used to enhance the mixing process of low PR pulsed jet (PR<10). The flow structure and turbulent mixing was studied using acetone-based planar laser-induced fluorescence (PLIF) technique. Interactions among impingement wall, jet and ambient air were discussed based on a series of time evolution and high-definition PLIF images. A comparison study with low PR free jet data showed that jet turbulent mixing can be significantly improved by the wall impingement even though under such low PR. A detailed study of wall jet tip vortex provided insight into the vortex evolution and mixing in the wall jet tip region. Scalar dissipation was used to highlight the jet mixing efficiency under different PR. Moreover, both the jet tip penetration and the impingement height were compared under different PR. The results of this study are important in development and optimization of natural gas direct-injection with diesel pilot ignition engine using stratified combustion concept.