MODELLING OF GASOLINE SPRAY IMPACT AGAINST HOT SURFACES AND TRANSIENT HEAT TRANSFER EFFECTS ON PHASE TRANSITION

摘要

In gasoline direct injection (GDI) engines, the dynamics of the gasoline spray and the possible spray-wall interaction are key factors affecting the equivalence ratio distribution of the air-fuel mixture at spark timing, hence the development of combustion and the emission of pollutants at the exhaust. Gasoline droplets impact may lead to rebound with consequent secondary atomization, or to the deposition in the liquid phase over walls as wallfilm. This last slowly evaporates with respect to free droplets, leading to local enrichment of the mixture, hence to routs towards increased unburned hydrocarbons and particulate matter emissions. Especially in the so-called wall-guided mixture formation mode, complex phenomena characterise mixture formation, namely a turbulent multi-phase system where heat transfer involves a gaseous phase (made of air and gasoline vapour), the liquid phase (droplets not yet evaporated and wallfilm) and the solid wall. Therefore, a proper numerical prediction based on a 3D CFD modelling of in-cylinder phenomena necessarily derives from the correct simulation of the wall cooling effect due to the subtraction of the latent heat of vaporization of gasoline needed for secondary evaporation and of the conductive heat transfer within the solid. Indeed, the heat transfer mechanism influences the dynamics of the spray impinging over the heated wall, with a consequent direct effect on the mixing interaction between gasoline and air. A proper sub-model is here specifically implemented to reproduce a basic experiment relevant to a simple configuration within a confined vessel, where a multi-hole spray for GDI applications is directed toward a heated wall. The sub-model solves the strongly coupled heat and mass transfer problem and allows achieving a correct description of the liquid and vapour phases dynamics after impact. The validation of the developed 3D CFD model is performed on the ground of the collected experimental measurements deriving from a combined use of the schlieren and the Mie scattering optical techniques.