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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.

The discharge coefficient is an important functional parameter of an injector characterising the nozzle flow, in terms of cavitation and hydraulic flip, which subsequently play a crucial role in the spray formation and development. Thus it is important to have the possibility of measuring instantaneously the value of the discharge coefficient. The method proposed is based on the measurement of force developed during the impingement of the fuel jet on a normal target. In this study the method was verified experimentally and also the variation of a diesel nozzle discharge coefficient over the entire injection time was studied. The impingement results were in good agreement, when compared with the results from mass flow measurements both at high and low injection pressures. Strong variations of the discharge coefficient during the injector needle opening and closing periods were seen.