CAV2001:sessionA9.005 1 Comparison of Cavitation Phenomena in Transparent Scaled-up Single-Hole Diesel Nozzles L. C. Ganippa, G. Bark, S. Andersson and J. Chomiak Chalmers University of Technology, Sweden Abstract The structure and evolution of cavitation in a transparent scaled-up diesel nozzle having a hole inclined at 90, 85, 80 and 0 degree to the nozzle axis has been investigated using high-speed motion pictures, flash photography and stroboscopic visualization. Observations revealed that at the inception stage, cavitation bubbles were not seen at the same locations in all the four nozzles. Cavitation bubbles grew intensively and developed into cloud-like structures. Shedding of the cloud cavitation was observed. When the flow was increased further the cloud-like cavitation bubbles developed into a dense large-scale cavitation cloud extending downstream of the hole. Under this condition the cavitation started mainly as a glassy sheet at the entrance of the hole. Until this stage the spray appeared to be symmetric. When the flow was increased beyond this stage, a sheet of cavitation covered a significant part of the hole on one side, extending to the hole exit. This non-symmetric distribution of cavitation within the hole resulted in a jet, which atomized on the side where more cavitation was distributed and non-atomizing on the side with less cavitation. The distribution of cavitation in the hole was different for different nozzles. 1 Introduction The fuel injection system of a diesel engine plays a crucial role in reducing exhaust emissions by determining the spray formation ignition and combustion. The spray formation and in particular atomization process is highly complex owing to the underlying physical processes in the nozzle internal flow and the environment into which the spray is injected. It is believed that cavitation could be a possible contributor to the spray break-up at the nozzle exit. At low upstream pressures, Bergwerk (1959) observed the nozzle hole to run full except for a cavity at the upstream corner. Increasing the pressure causes the cavities to extend throughout the hole, increasing the ruffles of the spray and causing a sudden transition in the appearance of the jet from ruffled to smooth and glass-like, together with a change in the discharge coefficient under certain limit conditions. The cavity formation was found to be a function of the cavitation number (CN = (P 1 -P 2 )/P 2 , where P 1 and P 2 are the absolute upstream and downstream pressures and the vapour pressure is neglected). Reitz et al. (1982) proposed that, among the other factors such as turbulence, surface instability etc., which contribute to the aerodynamic break-up, cavitation phenomena are a dominant factor, which complement the aerodynamic effects. Hiroyasu et al. (1991) observed that the jet break-up length was shortened due to cavitation fixed at the nozzle entrance and the break-up length increased when the vapour cavity at the hole walls was too strong to be disrupted in the nozzle, which resulted in a non-atomizing smooth jet. Studies have been carried out by Kato et al. (1997) to measure the pressure distribution in the nozzle sac and discharge hole. Their observation revealed that cavitation at the hole inlet is very sensitive to the nozzle sac geometry and the injection hole configuration. Badock et al. (1998) investigated the cavitation phenomena in the spray hole of a real-size diesel injection nozzle. They observed cavitation films inside the spray hole using the light sheet technique and were also able to see the core of the flow, which was covered by cavitation films. Cavitation films between the flow and the nozzle wall, as well as single cavitation bubbles could be observed at different times of the injection process. No foam or small bubbles were noticed in the spray hole. The internal flow structure in a scaled-up plain orifice nozzle was studied by Soteriou et al. (1999) using a laser light sheet. They observed incipient cavitation at three distinct locations viz., (i) in the separated boundary layer, i.e. just down stream of the entrance, (ii) in the main stream flow and (iii) within the attached boundary layer, i.e. all along the walls of the orifice downstream of the separated boundary layer and extending out of the orifice. Plug cavitation was identified in the development process. As the plug cavitation reached the hole exit a sudden increase in spray angle was observed. This plug cavitation was not observed at lower Reynolds numbers. At the inception stage