Modeling of Wall Film Formed by Impinging Spray Using a Fully Explicit Integration Method

In order to have a better understanding of the mixture air/fuel formation in both PFI-SI and DI diesel engines a wall film model has been implemented in our customized version of KIVA code. The model simulates the dynamics of the wall film generated by an impinging spray by solving the mass, momentum and energy equations of a bi-dimensional liquid flow over a three-dimensional surface under the basic hypothesis of `thin laminar flow'. This assumption has been shown to be satisfactory for film height lower then four hundred micrometers, an upper limit hardly reached for usual film formation both in DI diesel and SI engines. The major phenomena taken into account are: wall film formation by impinging spray; body forces, such as gravity or acceleration of the wall; shear stress at the interface with the gas and no slip condition on the wall; momentum contribution and dynamic pressure generated by the tangential and normal component of the impinging drops; film evaporation by heat exchange with wall and surrounding gas. The model has been implemented following a finite volume approach with a first order upwind differencing scheme and has been integrated with a fully explicit method. Comparisons are made with experimental results: the model well reproduces the dynamics of liquid film in a test case of pulsed injection into a cylindrical pipe and seems to be a useful tool for PFI-SI engines simulation. The model has also been compared with a two-stroke direct injection diesel engine experiment: good agreement is found in the prediction of film thickness and evaporation rate. In DI diesel engines the model also shows the importance of the influence of wall film on engines performances, in particular on soot formation.