Experimental study into the effects of stability between multiple injections on the internal flow and near field spray dynamics of a diesel nozzle

To improve the control accuracy and injection stability in multiple injection processes, the coupling effects of needle valve wobbling and residual bubbles on the nozzle internal flow and near field in diesel engines were researched. And the high-speed microscopic imaging technology was employed to study the dynamics behaviors of a real-size tapered hole nozzle under different multiple injection strategies. Research results indicated that during the pre-injection and post-injection process, the difference in throttling effect caused by the wobbling of the needle valve is the main reason for the instability of the fuel injection quantities. As the pulse widths increases, the actual injection duration and the maximum needle valve lift increased, so that the throttling position changes from the passage between the needle valve and the needle seat to the nozzle holes, and two kinds of string cavitation are gradually formed. String cavitation starts at the needle valve apex and propagates outward along the nozzle wall in a sinusoidal wave trend. During the main injection stage, the spray cone angle showed a boot-shaped trend, forming a development stage, a transformation stage and a stable stage. When the needle valve rises and falls, there is air suction in the nozzle, and there is a secondary disturbance phenomenon. During the injection, the spray pattern is affected by the combined influences of the axial momentum and radial momentum of the injected fuel, resulting in a difference between the secondary disturbance and the primary breakup. When the secondary disturbance occurs, the subsequent high-speed fuel will hit the spray mass with a slow speed, which forms a vortex at both ends to develop radially, and expands the range of the spray tip.