Experiments and modeling on droplet motion and atomization of diesel and bio-diesel fuels in a cross-flowed air stream

This paper describes an experimental and numerical modeling on microscopic droplet behaviors and macroscopic atomization characteristics of diesel and bio-diesel fuels under the conditions of air cross-flow. In order to obtain the experimental air-assisted-droplet behavior and breakup characteristics, deformation ratio, droplet trajectory, and droplet size distribution were investigated quantitatively for different droplet breakup mechanisms. In order to assess prediction accuracies of the original spray models contained in the standard CFD code and validate the currently modified and suggested models, the computations were implemented using the KIVA-3V code with a three dimensional computational mesh and compared to the experimental results. Because the original TAB model and droplet drag model tend to predict the unphysical breakup characteristics, to improve the calculation accuracy, the model constants of the droplet motion equation in the TAB model and the droplet drag model were modified and optimized. In addition, a new model for determining the droplet size after breakup is currently suggested, because original model does not consider the frictional flow inside a droplet, resulting in impractical droplet size results. Based on inspections of the measured and calculated results, outcomes of atomization characteristics from modified models give a reasonable droplet deformation rate and also show the appropriate droplet trajectory at the first breakup stage. In the results of droplet breakup and atomization, the modified models give improved prediction accuracy on the droplet size distribution better than the standard models. But there still shows a little discrepancy between calculated and measured droplet size and its distribution pattern. It seems to be caused by the own assumption of the TAB model which is based only on the vibration and breakup mechanism.