A comprehensive collision model for multi-dimensional engine spray computations.

A new collision model with both improved physical accuracy and reduced numerical dependency has been developed and is used for engine spray calculations. A radius-of-influence of collisional interaction approach was employed in conjunction with dynamic discrete particle resolution improvement to reduce numerical dependency. The collision outcomes considered by the model include bounce, coalescence and, fragmenting and non-fragmenting separation processes, since these are the regimes of most interest in practical spray applications. The improved collision probability and collision outcome predictions were coupled to form a comprehensive collision model, which was used to simulate non-vaporizing diesel sprays produced under conditions relevant to diesel engine common-rail injection systems. Spray penetration results from visualization studies and, drop-size and velocity data from PDPA measurements were used to assess the capability of the model for different injection pressures and various chamber densities. The model gave reasonably good predictions indicating that the present improvements to the collision and spray models have increased the fidelity of spray modeling predictions, and are consequently expected to help improve the accuracy of multi-phase flow simulations. * Corresponding author (achuth.munnannur@cummins.com) Introduction Inter-drop collisions, and especially the resultant coalescence and fragmentation processes, are central to determining drop-distributions and mixing processes in non-dilute spray systems. Modeling of collisions involves two distinct problems. First is the numerical problem of predicting the probability of a collision. The second is the physical problem of predicting the effect of the collision viz., the type of outcome and postcollision characteristics. Typically, spray modeling follows the discrete particle approach [1-3] where groups of drops with identical properties, termed ‘parcels’, are tracked collectively. The spray dynamics are solved by Monte-Carlo techniques and accordingly, the collision modeling also follows a statistical sampling procedure. The major challenge in collision probability predictions is preserving accuracy in calculations without undesirable numerical dependencies, such as sensitivity of the results to gas-phase mesh-size and the computational timestep. The following three non-dimensional parameters have been found to be important in predicting the outcome of inter-drop collisions, at least for low-viscosity liquids (see Fig. 1): Weber number, σ s d ρU We 2