Generation and Oxidation of Soot due to Fuel Films Utilizing High Speed Visualization Techniques

For a better understanding of how soot is generated due to fuel films, a constant volume vessel was used together with four visualization techniques due to their high spatial (2D) and time resolution: Schlieren, natural luminosity, diffused back illumination and OH* chemiluminescence. The analysis was performed keeping the injection pressure at 30 bar and changing the plate temperature on which the spray impacts: 80, 120, 160 and 200 °C. The fuel is a mixture of iso-octane, hexane, toluene and 1-methylnaphthalene, which presents similar properties to commercial gasoline. Valuable insights were gained from the results that infer the real nature of the radiation observed during combustion events in gasoline direct injection (GDI) engines due to the presence of a fuel films which are conventionally described as “pool fires”. The results show that the highest quantity of soot is generated between plate temperatures of 80 and 120 °C. The composition of the fuel film and the flow field generated after the passage of the flame front are of paramount importance in the description of the soot generated around the impingement region. The 2D maps of KL-factor (obtained from DBI) confirm the same trend of the natural luminosity observations. The image analysis of KL-factor and OH* reveals that the radiation emitted is due to diffusion flames reacting inside a post combustion environment which consumes the fuel film that has survived to the flame. Four key moments on the soot generation are discussed and some limitations in the application of the OH* chemiluminescence are presented.