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

Abstract
The early and late portions of transient fuel injection have proven to be a rich area of research, especially since the end of injection can cause a disproportionate amount of emissions in direct injection internal combustion engines. While simulating the internal flow of fuel injectors, valve opening and closing events are the perennial challenges. A typical adaptive-mesh CFD simulation is extremely computationally expensive, as the small gap between the needle valve and the seat requires very small cells to be resolved properly. Capturing complete closure usually involves a topological change in the computational domain. Furthermore, Internal Combustion Engines(ICE) operating with Gasoline Direct Injection(GDI) principle are susceptible to flash boiling due to the volatile nature of the fuel. The presented work simulates a gasoline direct injector operating under cavitating conditions 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 for both flash boiling and non-flash boiling conditions. Further, this study reveals post- closure dynamics that result in dribble, which is expected to contribute to unburnt hydrocarbon emissions. Flashing versus non-flashing conditions are shown to cause different sac and nozzle behavior after needle closure. In particular, a slowly boiling sac causes spurious injection behavior. Furthermore, a qualitative analysis of the injector tip-wetting phenomena under both flash-boiling and non-flashing conditions are conducted and different wetting mechanisms are identified. The jet expansion mechanism is observed to dominate the wetting process during the main injection period, whereas the sac conditions drive the post-closure wetting phenomena. Additionally, the effect of flash-boiling conditions on 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 approach are validated with the experimental observations. This coupling approach requires running an Eulerian primary atomization model, i.e., the Σ −Y model, to initialize the Lagrangian parcels for the secondary atomization process. Hence, this coupling approach does not depend upon the linearized instability models to simulate the dense spray region.
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