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Improved numerical methods and physical models have been applied to droplet collision modeling. Contrary to the common practice of using the fixed gas phase mesh to calculate droplet collision incidence, a new algorithm generates a collision mesh independent of the gas phase mesh at each time step. By partitioning collision cells according to the number density of parcels, the algorithm is capable of achieving higher spatial resolution than that of the gas phase mesh. At the same time this method maintains an adequate statistical sample of parcels in the collision cells to ensure statistically accurate results. The continual random rotation of the mesh ensures that parcels that are near each other will, on average, be in the same collision cell. This produces the physically reasonable result that parcels in close proximity should have the opportunity to collide. Another important advantage of the algorithm is that it can be applied to any orifice configuration.

To model sprays from pressure-swirl atomizers, the connection between the injector and the downstream spray must be considered. A new model for pressure-swirl atomizers is presented which assumes little knowledge of the internal details of the injector, but instead uses available observations of external spray characteristics. First, a correlation for the exit velocity at the injector exit is used to define the liquid film thickness. Next, the film must be modeled as it becomes a thin, liquid sheet and breaks up, forming ligaments and droplets. A linearized instability analysis of the breakup of a viscous, liquid sheet is used as part of the spray boundary condition. The spray angle is estimated from spray photographs and patternator data. A mass averaged spray angle is calculated from the patternator data and used in some of the calculations.