Visualization of Internal Flow and Spray Combustion with Real Size Diesel Nozzle

Abstract

The internal flow of a diesel mini-sac (MS) and valve-covered orifice (VCO) nozzle and its spray formation have been investigated by flow visualization with a real-size transparent nozzle. The mechanism of cavitation generation in the nozzle has been studied by focusing on the transient flow pattern in the sac with micro PIV and numerical simulation. String-type and film-type cavitations are separately observed in the nozzle hole during the injection stage. In the case of low needle-lift for MS nozzle, the string-type cavitation is generated by a secondary flow in the hole and stretched to the other hole through the vortex core in the sac. With the increase of needle-lift, film-type cavitation that formed at the hole inlet edge becomes dominant. This transition of the cavitation type is related to the change of the flow pattern in the nozzle sac. On the other hand, only the film-type cavitation was observed in the VCO nozzle. Through analyzing the frequency of cavitation generation and spray fluctuation, it is also evident that with the increase of string-type cavitation, spray cone angle tends to be wide.