COMPUTATIONAL STUDY OF INTERNAL FLOW, NEAR NOZZLE AND EXTERNAL SPRAY OF A GDI INJECTOR UNDER FLASH-BOILING CONDITIONS

The early and late portions of transient fuel injection have proven to be a rich areaof research, especially since the end of injection can cause a disproportionate amountof emissions in direct injection internal combustion engines. While simulating theinternal flow of fuel injectors, valve opening and closing events are the perennialchallenges. A typical adaptive-mesh CFD simulation is extremely computationallyexpensive, as the small gap between the needle valve and the seat requires verysmall cells to be resolved properly. Capturing complete closure usually involves atopological change in the computational domain. Furthermore, Internal CombustionEngines(ICE) operating with Gasoline Direct Injection(GDI) principle are susceptibleto flash boiling due to the volatile nature of the fuel.The presented work simulates a gasoline direct injector operating under cavitatingconditions by employing a more gradual and easily implemented model of closure that avoids spurious water-hammer effects. The results show cavitation at low valve lift forboth flash boiling and non-flash boiling conditions. Further, this study reveals post-closure dynamics that result in dribble, which is expected to contribute to unburnthydrocarbon emissions. Flashing versus non-flashing conditions are shown to causedifferent sac and nozzle behavior after needle closure. In particular, a slowly boilingsac causes spurious injection behavior.Furthermore, a qualitative analysis of the injector tip-wetting phenomena underboth flash-boiling and non-flashing conditions are conducted and different wettingmechanisms are identified. The jet expansion mechanism is observed to dominate thewetting process during the main injection period, whereas the sac conditions drive thepost-closure wetting phenomena. Additionally, the effect of flash-boiling conditionson the near-nozzle spray during the quasi-steady period of the injection cycle is explored. The exploration captured hole-to-hole variations in the rate of injection (ROI), rate of momentum (ROM), and hydraulic coefficients of injection. Moreover, it also indicates influences of the in-nozzle variations on the near-nozzle spray behaviors.Finally, a novel plume-based coupling approach is developed to couple the Eulerian near nozzle simulations with the Lagrangian spray simulations under both non-flashing and flash-boiling conditions. Predictions from the novel coupling approachare validated with the experimental observations. This coupling approach requiresrunning an Eulerian primary atomization model, i.e., the Σ −Y model, to initializethe Lagrangian parcels for the secondary atomization process. Hence, this couplingapproach does not depend upon the linearized instability models to simulate the dense spray region