Modeling the near field of diesel sprays

A multidimensional model that is widely used for spray computations is employed to study Diesel sprays. The computed results are compared with measured results of the liquid penetration reported in the literature. The computed results imply that the maximum penetration is controlled by the rate of vaporization of the drops, which in turn is controlled to a large extent by the rate of entrainment of the ambient air by the spray. It is shown that inadequate modeling of the physics of outcomes of drop-drop collisions in the near field of sprays and the numerical representation of collisions are major limitations. The droplet interaction model is separated into two parts: first, calculation of a collision rate between particles, and second, calculation of the outcome once a collision has occurred. Current models for predicting the outcome of collisions are based on experiments with water drops at atmospheric conditions. Experimental evidence has recently become available that suggests that the effects of ambient density may be significant. In this work, a one-dimensional drop-drop interaction model is developed and employed to improve the understanding of the physics of collisions. From the results of the study with the one-dimensional model, as well as those of the experiments reported in the literature, a composite model is developed to predict the outcome of drop-drop collisions. This composite model includes bounce, grazing and reflexive separation, coalescence and shattering as possible outcomes. It also includes the effects of ambient density on collision outcome. The composite model predicts a significantly reduced rate of coalescence between drops than the current model. A new numerical approach to compute the collision rate that is capable of producing converged results as the spatial grid is refined is developed. Comparison of the computed results with a combination of the composite model for predicting the outcome of collisions and the new numerical approach to computing the collision rate with experimental data in the literature showed improved agreement with measured results of liquid penetration in sprays. The liquid penetration reflects effects of near-field physics.