An Optical Study on Flame Structure and Soot Formation During Jet Wall Impinging Process in a Constant Volume Combustion Chamber

Abstract

Jet/wall impingement has a strong influence on the fuel-air mixing and combustion process for compressed-ignition engines. However, this phenomenon has not been fully understood yet because of the complex physical process involved and the limitations of experimental diagnostics. In this study, the flame structure and soot formation during jet wall impinging process under a wide of operating conditions has been investigated within a constant volume combustion chamber using a single-hole injector and a flat wall configuration. A high-speed Schlieren imaging and OH* chemiluminescence techniques were applied simultaneously to record the spray geometry, ignition delay and OH* distribution. In addition, a diffused background-illumination extinction imaging technique was also employed to measure the soot formation. All the measurements were performed for both free jet and jet/wall impingement events under identical conditions. It was found the ignition delay of the jet wall impingement was shorter than that of free jet cases because of the heat transfer between the jet and cold wall. However, the effects of wall interaction on flame lift-off length were much less sensitive compared to that of free jets. The results also revealed that OH flame of low reacting cases behaves as a continuous entire cloud, but the spray center for high reacting cases was occupied by soot. Within current studies, the soot production after wall impingement was reduced significantly compared to that of free jet cases at the same position, which could be mainly caused by the cooling effects on the flame from the cold wall.

Keywords: jet wall impingement, Flame structure, soot, optical diagnostics