Effects of the Residual/Sucked Air Bubbles on Diesel Near - Nozzle Spray Structure

Study of the spray formation in vicinity of the nozzle is essential to better understand and predict the physical processes involved in the diesel atomization. The initial spray patterns were found to be different from one injection to another during our visualization experiments, which was carried out based on a long distance microscope with a high speed camera in this work. It was found that the initial spray might contain a clear single mushroom, tail region and intact liquid column, or have a tail in front of the mushroom without changing its direction. Occasionally, it presented as a double-mushroom shape, or did not include a clear mushroom. Our visualization results showed that the various spray structures were observed at different injection pressures and different injection cycles under the same injection pressure. The difference of spray patterns may be due to the residual fuel/air bubbles surviving from the last injection or sucking into the nozzle during the needle opening, turbulent nature of the flow and the unsteady movement of the injector needle, while the reasons are not identified. In order to explain the various spray patterns, effects of the distribution of the simplified air bubbles inside the nozzle on the spray patterns at the start stages of the injection were further investigated with a combination of the LES method and VOF model coupling the cavitating flow inside the orifice. A higher injection pressure was set due to the faster the spray under the larger the injection pressure, and the operating condition was closer to the realistic diesel condition. The study confirmed that the initial mushroom and the tail were generated by the residual/sucked bubbles, and the main mushroom occurred because of the boundary layer theory. The location, size and amount of the residual/sucked air bubbles were responsible for the variation of initial spray structures between different injections. Moreover, the cavitation model was shown to play a role in the development of spray.