The Structure of Cavitation and its Effect on the Spray Pattern in a Single-Hole Diesel Nozzle

The structure and evolution of cavitation in a transparent scaled-up diesel nozzle having a hole perpendicular 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 are dominantly seen in the vortices at the boundary layer shear flow and outside the separation zone. Cavitation bubbles grow intensively in the shear layer and develop into cloud-like coherent structures when viewed from the side of the nozzle. Shedding of the coherent cloud cavitation was observed. When the flow was increased further the cloud like cavitation bubbles developed into a large-scale coherent structure extending downstream of the hole. Under this condition the cavitation starts as a mainly 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. A glossy sheet on the other side of the hole does not extend to the hole exit. This non-symmetric distribution of cavitation within the hole resulted in a jet, which atomized on the side towards the right of the hole and a non-atomizing jet on the left of the hole. Any further increase in the flow resulted in a total sheet cavitation structure within the nozzle hole and the same asymmetric spray pattern.