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A 3D CFD Simulation of GDI Sprays Accounting for Heat Transfer Effects on Wallfilm Formation
Abstract:
During gasoline direct injection (GDI) in spark ignition engines, droplets may hit piston or liner surfaces and be rebounded or deposit in the liquid phase as wallfilm. This may determine slower secondary atomization and local enrichments of the mixture, hence be the reason of increased unburned hydrocarbons and particulate matter emissions at the exhaust.
Complex phenomena indeed characterize the in-cylinder turbulent multi-phase system, where heat transfer involves the gaseous mixture (made of air and gasoline vapor), the liquid phase (droplets not yet evaporated and wallfilm) and the solid walls. A reliable 3D CFD modelling of the in-cylinder processes, therefore, necessarily requires also the correct simulation of the cooling effect due to the subtraction of the latent heat of vaporization of gasoline needed for secondary evaporation in the zone where droplets hit the wall. The related conductive heat transfer within the solid is to be taken into account.
In this work, a preliminarily validated spray model is specifically implemented by solving the strongly coupled heat and mass transfer problem describing the liquid and vapor phases thermo-fluidynamics after impact and the wall change of temperature. The discussion is made considering a different boundary condition with respect to standard simulations. Sprays are assumed from to different injectors in order to verify the wallfilm simulation model: the impact over heated walls of the ECN “Spray G” is first discussed, by comparing numerical results with experimental measurements deriving from a combined use of the schlieren and Mie-scattering techniques, then the footprint on the wall of the spray delivered from a 6-hole Bosch injector is related with infrared thermography and LIF measurements taken from the literature.