ANALYSIS OF THE CAVITATING FLOW IN REAL SIZE DIESEL INJECTORS WITH FIXED AND MOVING NEEDLE LIFT SIMULATIONS

For Diesel engines, the determination of the exit conditions of the injector nozzle flow is of great importance because it greatly influences the spray development inside the combustion chamber and hence the combustion process. In addition, it is relevant to know the flow conditions to provide the initial conditions for spray models. Since the possibility of experimentally observing and measuring the flow inside real size Diesel injectors is very limited, CFD calculations are generally used to obtain the relevant information. Because of the complexity of moving mesh calculations of real size injectors, the nozzle flow is often studied at full needle lift only or by quasi-steady state fixed needle lift calculations, so that little is known about the transient phase of the needle opening/closing. The aim of this work is to evaluate this methodological approach, comparing predicted results obtained by simulations with fixed and moving mesh needle lift, and to characterize the cavitation process during the whole transient of the injection. The mathematical basis of the model and the numerical methodology followed are detailed in the paper. In fixed simulations, the needle was positioned at six different lifts, while in moving mesh simulations a range of 15μm to full needle lift was covered. By analyzing the acquired flow field images during the injection period, the highly transient nature of the flow was revealed. The moving mesh calculations provided a number of significant flow characteristics concerning the effects of the needle position in time. For instance, the effects of ascending and descending needle movement were noted by differences in the flow at identical needle lifts. Also observed, was the decrease of vapor and turbulence intensity at full lift, and clouds of vapor exiting the nozzle at certain times. Although the steady simulation captured the basic flow structure inside the hole at high and low lifts, it did not capture the time dependent flow characteristics mentioned above. The two provide different nozzle exit results for low needle lifts, while the percentage difference changes depending on the lift. Hence, the calculations with moving mesh boundaries provide information about the transient phase of injection that may not be neglected.