Browse Publications Technical Papers 2008-01-0137
2008-04-14

LES Simulation of the Internal Flow and Near-Field Spray Structure of an Outward-Opening GDi Injector and Comparison with Imaging Data 2008-01-0137

A combined experimental and computational investigation of the break-up structure of conical sprays of the high-pressure outward-opening gasoline direct-injection (GDi) injectors has been carried out, with the objective to investigate its distinct “jet-string” spray break-up structure. The study probes the relationship of the spray breakup structure to the liquid-phase flow within the injector nozzle - especially at the nozzle exit - obtained through Computational Fluid Dynamic (CFD) simulations.
The shadowgraphy imaging technique is utilized for investigation of the spray morphology, and its dependence on the fuel pressure and the injector nozzle design. The Volume-of-Fluid Large-Eddy-Simulation (VOF-LES) computational method for two-phase flow simulation is employed for analysis of the injector internal flow and its proximate ambient, for the identical injector valve-group geometry.
The experimental results reveal a dependence of the conical spray breakup mechanism on the fuel pressure. The primary breakup mechanism at low fuel pressures is growth of the Kelvin-Helmholtz instability waves, which transitions to a “jet-string” breakup structure, at the nozzle-exit, with increase of the fuel pressure.
The VOF-LES simulation results reveal that the origin of the jet-string primary breakup structure is the irregular detachment of the liquid flow form the injector nozzle surfaces, upstream of the nozzle exit. This engenders “ingestion” of ambient air into the nozzle, which promotes the irregular growth of the Kelvin-Helmholtz instabilities at the liquid - air interface, and cause the visible injector near-field irregular jet-string breakup and atomization structure. The VOF-LES results are in good agreement with the present and published experimental data.

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