Nozzle hole film formation and its link to spray characteristics in swirl-pressure atomizers for direct injection gasoline engines

The numerical methodology used to predict the flow inside pressure-swirl atomizers used with gasoline direct injection engines and the subsequent spray development is presented. Validation of the two-phase CFD models used takes place against film thickness measurements obtained from high resolution CCD-based images taken inside the discharge hole of a pressure swirl atomizer modified to incorporate a transparent hole extension. The transient evolution of the film thickness and its mean axial and swirl velocity components as it emerges from the nozzle hole is then used as input to a spray CFD model predicting the development of both non-evaporating and evaporating sprays under a variety of back pressure and temperature conditions. Model predictions are compared with phase Doppler anemometry measurements of the temporal and spatial variation of the droplet size and velocity as well as CCD spray images. The results confirm that accurate estimation of the nozzle flow exit conditions plays a dominant role in the prediction of sprays injected from pressure-swirl atomizers, while the proposed methodology seems to offer significant improvements in the accuracy of the predicted spray structure.