A 3D CFD Simulation of an Impacting ECN “Spray G” Accounting for Heat Transfer Effects on Wallfilm Formation

Paper #:
  • 2017-24-0041

  • 2017-09-04
In gasoline direct injection (GDI) engines, dynamics of the possible spray-wall interaction are key factors affecting the air-fuel mixture distribution and equivalence ratio at spark timing, hence influencing the development of combustion and the pollutants formation at the exhaust. Gasoline droplets impact may rebound with consequent secondary atomization or deposit in the liquid phase over walls as a wallfilm. This last slowly evaporate with respect to free droplets, leading to local enrichment of the mixture, hence to increased unburned hydrocarbons and particulate matter emissions. In this scenario, complex phenomena characterize the turbulent multi-phase system where heat transfer involves the gaseous mixture (made of air and gasoline vapour), the liquid phase (droplets not yet evaporated and wallfilm) and the solid wall, especially in the so-called wall-guided mixture formation mode. Therefore, a proper numerical prediction based on a 3D CFD modelling of these 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, this heat transfer influences the dynamics of the spray impinging over the heated wall, with a consequent direct effect on the mixing interaction between fuel and air. A proper sub-model is specifically implemented to solve the strongly coupled heat and mass transfer problem and to achieve a correct description of the liquid and vapour phases dynamics after impact. The discussion is made considering a different wall heat boundary condition with respect to the standard simulations. The validation of the developed 3D CFD model is performed by reproducing the experiments performed in a simple configuration within a confined vessel, thanks to a detailed experimental insight of the impact over walls of a multi-hole spray for GDI applications. The collected experimental measurements derive from a combined use of the schlieren and Mie scattering optical techniques. Moreover, quantitative numerical information about spray footprint on the wall is compared with LIF measurements of literature, allowing the validation of the wallfilm mass evaluation.
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