Modeling heat transfer in spray impingement under direct-injection engine conditions

A spray-wall heat transfer model based on a newly published spray-wall interaction model was developed and implemented in the KIVA-3V engine computational fluid dynamics code to simulate the heat transfer related to wall films under spray impingement with application to direct-injection engines. The adopted spray-wall interaction model accounts for various wall impingement regimes including rebounding drops, drops sliding along the surface, and those that become wall films. In order to estimate the film formation accurately, a new correlation, which is developed on the basis of diesel spray impingement measurements instead of single-drop impingement results, is introduced to calculate the splashed mass ratio when spray droplets impinge on a wall. In the proposed wall-film heat transfer model, heat exchange between the ambient gas and the film, heat exchange between the film and the wall, heat exchange between the rebounding droplets and the film, and heat exchange due to vaporization are taken into consideration. Correlations from spray impingement measurements are used to calculate the heat flux instead of invoking boundary layer assumptions, so that the impingement-induced enhancement to the heat transfer can be effectively reflected. The heat transfer predictions are compared with studies in which the wall heat flux was measured experimentally. These studies include evaporating sprays directed on to cool surfaces and heat transfer from a hot surface to a cooler impinging spray. The comparisons show that a great improvement is achieved by the proposed model, both in estimations of the film thickness and in predictions of the spray-wall heat transfer. In most validations the estimations of the proposed model and the experimental results are in good agreement. It is thus suggested that the proposed model provides a useful alternative for prediction of the spray-wall heat transfer, especially under conditions relevant to modern direct-injection diesel and gasoline engines.