Effect of String Cavitation and Initial Air Bubbles in Diesel Injector Tapered Nozzles on Subsequent Spray = Effect of String Cavitation and Initial Air Bubbles in Diesel Injector Tapered Nozzles on Subsequent Spray

다국어 초록 (Multilingual Abstract)

Flow inside nozzles and atomization dynamics in vicinity of the nozzle were crucial to diesel spray and combustion. And the tapered nozzle have attracted the researchers’ attention in recent years because of the better flow characteristics without the geometry induced cavitation. This paper presented the experimental and numerical analysis of string cavitation and near - nozzle spray dynamics. The string cavitation flow transient characteristics inside the tapered nozzle with the movement of needle were obtained based on the high speed digital microscopic imaging technique. Simultaneously, the formation and compression of initial air bubbles at the SOI (Start-of-Injection), and the air suction after the EOI (End-of-Injection) were captured clearly. The study indicates that the string cavitation was the direct cause of the spray instability. The various spray structures were observed at different injection pressures and different injection cycles under the same injection pressure. They might contain a clear single mushroom, tail region and intact liquid column, or had a tail in front of the mushroom. Occasionally, it presented as a double-mushroom shape, or did not include a clear mushroom. The difference of spray patterns may be due to the residual air bubbles surviving from the last injection or sucking into the nozzle during the needle opening. In order to explain the various spray patterns, effects of air bubbles distribution on initial spray patterns at the start stages of injections were investigated in a single-hole injector with a combination of the LES (Large Eddy Simulation) method and VOF (Volume of Fluid) model. The study confirmed that various spray patterns were affected by the injection pressure and initial air bubbles.