Modelling the Effect of Modulations of the Injection Velocity on the Structure of Diesel Sprays

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.