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Progress in Diesel spray modelling highly depends on a better knowledge of the instantaneous injection velocity and of the hydraulic section at the exit of each injection hole. Additionally a better identification of the mechanisms which cause fragmentation is needed. This necessitates to begin with a precise computation of the two-phase flow which arises due to the presence of cavitation within the injectors. For that aim, a VOF type interface tracking method has been developed and improved (Segment Lagrangian VOF method) which allows to describe numerically the onset and development of cavitation within Diesel injectors. Furthermore, experiments have been performed for validation purpose, on transparent one-hole injectors for high pressure injection conditions. Two different entrance geometries (straight and rounded) and various upstream and downstream pressure levels have been considered.

Under diesel engine conditions the exit velocity of diesel fuel out of an injection nozzle depends on the temporal increase of the supply pressure. Therefore, heading ‘slow’ fuel elements are caught up by ‘fast’ elements leaving the nozzle at a later time. Similar processes occur in modulated flow. This kind of interaction (‘collision’) of liquid elements produces a free internal stagnation point, which moves along the spray axis. From the free stagnation point the liquid is ejected in radial direction, which results in mushroom shaped structures along the spray. A similar structure is observed at the tip of the spray. The spray structures have been observed in modulated laminar laboratory jets as well as in turbulent diesel injection sprays under atmospheric conditions. A model is presented which describes the propagation speed of the stagnation points and the radial and axial flow velocities out of the stagnation points. It also predicts the net mass flux out of the structures.

For high-speed imaging a newly developed eight-fold CCD camera, which permits framing rates of up to one million pictures per second, was used to obtain pictures of the injected sprays during the operation of a diesel engine. For the particular case studied here the framing rate was set at 50,000 pictures per second. This rate was sufficient to resolve the temporal development of the sprays in the transparent version of the four-cylinder, in-line, 1.9 litre DI production diesel engine of Volkswagen. The advantage of the camera is that it needs no light pulses for illumination, but can operate with a continuous light source. Each of the CCD chips is arranged around a central eight face reflecting pyramid, which splits the light coming from the camera lens to each CCD chip. The chips can be shuttered freely (asynchronously) at programmable inter-frame spacings thus permitting operation with continuous illumination. In this particular case a 30 Watt halogen lamp was used.

The deposition of an impinging diesel spray is studied experimentally to improve the understanding of wall-spray-interactions. Experiments are performed under atmospheric pressure conditions as well as in a pressure chamber filled with nitrogen where the pressure is varied between 0.1 Mpa and 2 MPa. In both cases a light reflection method is used to observe size, shape and time development of the wetting imprints of the spray. The duration of the injection pulse is 2 ms. The mean injection pressure is 47 MPa. Additional experiments under atmospheric pressure conditions are performed without the pressure chamber. Here a shadow-graph optic is added to the apparatus to allow the simultaneous observation of the impinging spray and the wetted area of the wall. It is found that the diameter of the impinging spray increases faster than that of the imprint which suggests that the wall spray separates from the wall.

The flow in nozzles of different sizes is studied by using transparent nozzles of the same size as in diesel injectors and refractive index matching. In a steady flow rig, short exposure video pictures of the flow were made at injection pressures up to 100 MPa. Discharge measurements and a measurement of the flow velocity in the nozzle hole using a modified laser-two-focus-velocimeter completed the picture of the flow. Above an injection pressure threshold that depends on the nozzle geometry and chamber pressure, cavitation appears at the sharp inlet corner of the nozzle. With increasing injection pressure the cavitation reaches the nozzle exit (supercavitation). The discharge coefficient and the spray angle then level off at a value that is almost independent of any further increase of injection pressure. The measured flow velocity is close to the velocity calculated from the pressure drop assuming inviscid incompressible flow.