Transient Heat Transfer Effects on a Gasoline Spray Impact against Hot Surfaces: Experimental and Numerical Study 2017-24-0107

Gasoline direct injection (GDI) engines are characterized by complex phenomena involving spray dynamics and possible spray-wall interaction. Control of mixture formation is indeed fundamental to achieve the desired equivalence ratio of the mixture, especially at the spark plug location at the time of ignition. Droplet impact on the piston or liner surfaces has also to be considered, as this may lead to gasoline accumulation in the liquid form as wallfilm. Wallfilms more slowly evaporate than free droplets, thus leading to local enrichment of the charge, hence to a route to diffusive flames, increased unburned hydrocarbons formation and particulate matter emissions at the exhaust. Local heat transfer at the wall obviously changes if a wallfilm is present, and the subtraction of the latent heat of vaporization necessary for secondary phase change is also an issue deserving a special attention. In Computational Fluid Dynamics (CFD) studies of engine combustion this effect has rarely been taken into account and a constant temperature has been often set as Dirichlet condition for the solution of the in-cylinder turbulent flow even in the presence of impacting sprays.

Present work aims at giving a contribution to the validation of a 3D CFD model for the simulation of the in-cylinder thermo-fluidynamic processes underlying energy conversion. Results of a numerical model accounting for the conductive heat transfer mechanism within the piston are compared with basic experimental measurements carried out to achieve an insight into the dynamics and the impact over walls of multi-hole sprays for GDI applications through combined optical schlieren and Mie scattering techniques.