Finite diffusion wall film evaporation model for engine simulations using continuous thermodynamics

The formation of engine pollutants directly relates to the local composition of air/fuel mixture within the combustion chamber. The evaporation of wall film, formed by spray/wall impingement, is much slower than free droplets and has strong effects on engine emission. A finite diffusion wall film evaporation model using continuous thermodynamics is developed for multi-dimensional engine modeling, accounting the multi-component nature of practical fuels, which is essentially a complicated liquid mixture. The vaporization rate is evaluated using the turbulent boundary-layer assumption and a quasi-steady approximation. Third-order polynomials are used to model the fuel composition and the temperature profiles within the liquid phase for accurate predictions of surface properties, which are important for evaluating the mass fraction, moments and heat flux. The governing equations for the film are then reduced to a set of ordinary differential equations and significantly reduce the computational cost while retaining adequate accuracy in the solution. The model is verified against high precision numerical solutions obtained with the finite difference method that solves the governing partial differential equations using a fine grid over the same computational domain. Good agreement is obtained for the evaporation of a film on a flat plate under various conditions. The boiling point of a practical fuel is found to increase steadily with evaporation and may surpass the wall temperature. The evaporation process can therefore be reverted to non-boiling evaporation. This phenomenon is unique to multi-component liquids and not observed during the evaporation process of single component liquids. The presented model, with improved computational efficiency and adequate accuracy, is applicable in multi-dimensional engine simulations. Finally, the model is used for a spray/wall impingement simulation. The results show that the multi-component nature of a practical fuel can have significant implications on the distributions of fuel concentration and composition during the spray/wall interaction.