Numerical Investigation of Near Nozzle Flash-Boiling Spray in an Axial-Hole Transparent Nozzle

Understanding and prediction of flash-boiling spray behavior in gasoline direct-injection (GDI) engines remains a challenge. In this study, computational fluid dynamics (CFD) simulations using the homogeneous relaxation model (HRM) for not only internal nozzle flow but also external spray were evaluated using CONVERGE software and compared to experimental data. High-speed extinction imaging experiments were carried out in a real-size axial-hole transparent nozzle installed at the tip of machined GDI injector fueled with n-pentane under various ambient pressure conditions (P_a/P_s = 0.07 - 1.39). The width of the spray during injection was assessed by means of projected liquid volume, but the structure and timing for boil-off of liquid within the sac of the injector were also assessed after the end of injection, including cases with different designed sac volumes. Compared to the experiment, the default HRM model produces a more narrow liquid width, and a longer boil-off period, suggesting that the phase-change process is underestimated. When varying the HRM model constant to smaller values to promote evaporation, some level of improvement in predictions was observed, but the spray radial width and boil-off time were still underestimated. These results suggest the need for a detailed reexamination of the model.