ID 389

Visual Experimental Investigations of String Cavitation and Residual Bubbles in the Real-size Tapered Diesel Nozzle and Effects on Spray

 

Abstract:

Cavitating flow inside the diesel nozzle is quite important to the formation of air-fuel mixture and fuel atomization. In this paper, visual experiments on string cavitation and residual bubbles inside the real-size tapered diesel nozzle and near-field spray characteristics were performed based on a high pressure common-rail fuel injection system and a high-speed camera. This paper put focus on the tapered nozzle because it suppressed the interferences of the geometry-induced cavitation against the string cavitation. Results indicated the string cavitation inside the nozzle excited the instabilities of spray cone angle and provided a reasonable explanation for two bumps of spray cone angle during the opening and closing stages of needle. Both the length-diameter ratio of the nozzle orifice and the structures of sac volume play an essential role in the formation and intensity of the string cavitation. Comparing the results, the smaller length-diameter ratio of the orifice caused stronger string cavitation and larger spray cone angles. Besides, the string cavitation in the VCO nozzle was more prevailing than that in the Min-sac nozzle. The suction and compression of air bubbles at initial and end stages of injection processes, and the shot-to-shot various spray patterns also were captured clearly. The shot-to-shot various initial spray patterns could be attributed to the different distribution of residual bubbles in the nozzle orifice which was survived from the last injection process or sucked into the nozzle at time of the needle opening stages. The study confirmed that the initial mushrooms and the tails were generated by the interactions of the residual/sucked bubbles with the initial fuel, and the leading mushroom was incurred by the combination of the transverse expansion of the jet and the laminar layer theory.