An enlarged transparent model of a six-hole vertical diesel injector has been used to allow visualization of the flow at Reynolds and cavitation numbers matching those of real size injectors operating under normal Diesel engine conditions. The visualization system comprised a CCD camera, high-magnification lenses and a spark light source which allowed high-resolution images to be obtained. The flow conditions examined in terms of flow rates and pressures covered the range from low to full load of the real size injector while the needle lift position corresponded to that of full lift of the first- and second- stage in two-stage injectors. In addition, different values of needle eccentricity were tested in order to examine its effect on the cavitation structures within the injection holes.
From this investigation two different types of cavitation were identified: cavitation originating at the entrance to the injection holes and cavitation strings formed inside the sac volume and linking adjacent holes. In the former case, the flow pattern mainly depended on the cavitation rather than the Reynolds number as well as on the needle eccentricity. Different types of cavitation structure were identified varying from incipient bubbly flow to plug-type cavitation films and fully separated flow. In addition to hole cavitation, cavitation strings were found to form transiently inside the sac volume. As these cavitation strings were convected by the mean flow towards the injection holes, they were interacting transiently with the pre-existing cavitation structures resulting in a rather chaotic hole flow pattern. Since this phenomenon was found not to occur simultaneously in all injection holes, transient differences in the flow exiting the different injection holes were observed.
Overall, the results revealed that the flow development in multi-hole vertical nozzles, despite the axisymmetric geometry, may lead to hole-to-hole flow variations as a result of the transient nature of the cavitating structures formed inside the sac volume and holes.