Trajectory based droplet collision model for spray modeling

Abstract

This study presents a droplet collision model which takes into account trajectories of colliding droplets. Droplet collision dynamics can be represented more realistically by using a trajectory based model since collisions are calculated by the position and velocity vectors of droplets. Unlike the O'Rourke's model which is used in KIVA code, the presented model is not mesh dependent. The key point of the model is defining the droplet impact parameter. In this study, a new calculation method of impact parameter, which is based on binary droplet collision model, is presented. The impact parameter of two colliding droplet is obtained by following their colliding trajectory. The algorithm was implemented into the KIVA3V Rel2 code and obtained droplet calculations were compared with O'Rourke's collision model. Binary collision of droplet pairs were taken into account and the impact parameter of two colliding droplets was calculated from their position and velocity vectors. The new model represents the physical process of collusion more accurately since it uses colliding droplets in the calculations. ______________________________________________________________________ Keywords: Droplet collision, impact parameter, spray modeling. * Corresponding author: Tel: +90 532 601 33 51 E-mail address: taskiranoz@itu.edu.tr IntroductionThe numerical spray modeling offers very important and valuable information for the calculation of diesel spray combustion. KIVA3V R2 code, which is a Computational Fluid Dynamics (CFD) program used for engine research, describes the spray dynamics and combustion process with related sub-models, such as breakup, collision, evaporation models and etc.Droplet collision is an important physical part of numerical spray calculations. Droplet collisions affect the spray propagation, droplet evaporation and distribution by changing the droplet number and size. In the standard KIVA3V R2 code which uses Discrete Droplet Model (DDM), the collisions may give two physical results; coalescence and grazing [1]. In fact, there are other important physical results of droplet collisions [2]. Qian and Law [3] defined the results of collision of hydrocarbon droplets as bouncing, coalescence, reflexive separation, stretching separation (grazing) and shattering collision. The standard KIVA code is based on the O'Rourke's [4] collision sub-model and the calculations proceeds with the basic steps given below. Droplets groups are represented by the parcels and all droplets in a parcel behave in the same way. The collision calculation is performed for the pair of particles if they are in the same cell. The collision probability Pn, that a larger parcel undergoes "n" collisions with smaller parcel, is assumed to follow a Poisson distribution [1].The standard collision model has the disadvantage of being mesh dependent since there is a requirement that the parcels must be in the same cell to be taken into the collision calculation. A finer mesh would include less droplet parcels and the collision probability depen...

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