Modeling of Multicomponent Droplets Collision in Dense Evaporating and Non-Evaporating Sprays

A new coalescence model for droplets with different mixture composition has been developed and is implemented for spray calculations. The coalescence of droplets with different radius, density, and temperature is modeled in a constant volume chamber at high pressure and temperature. Each droplet contains several components, which represent the diesel fuel surrogate, including toluene, decane, dodecane, tetradecane, hexadecane, and octadecane. Considering the rotational kinetic energy and surface energies of a system of two droplets, coalescence efficiency for the two colliding droplets with different densities is formulated. For grazing collision, the effect of density of droplets is also considered. The present coalescence/collision model was implemented into a multi-dimensional CFD code, KIVA-3V, and was applied to model multicomponent fuel spray. Multicomponent species within droplets are assumed to be well mixed and the continuity equation was formulated to track each component of the fuel regardless of the direction of component mass transfer motion, whether evaporation or condensation. Validation studies were carried out for non-evaporating sprays and the results were compared with available experimental data. The model show good predictive capability and is expected to improve the accuracy of multi-phase flow simulations. Corresponding author: chenc@uah.edu ILASS Americas, 25 Annual Conference on Liquid Atomization and Spray Systems, Pittsburgh, PA, May 2013 Introduction The fuel atomization process in diesel and gasoline direct injection (GDI) engines is important for enabling efficient and clean combustion. High pressure injectors (e.g. fuel line pressure of 400 to 1800 bars for diesel engine and 30 to 200 bars for GDI engine) are widely utilized in diesel and GDI engines to accomplish better and rapid air-fuel mixing. The fuel atomization process is determined by a number of parameters, such as fuel property, injector geometry, combustion chamber gas density, fuel injection pressure, etc. Optimization of these parameters could lead to cleaner and more stable combustion. Multi-dimensional computational fluid dynamics (CFD) analysis is widely used for understanding the physics of atomization process. Modeling of atomization process could be divided into two distinct categories: Lagrangian-Droplet Eulerian-Fluid (LDEF) and Eulerian methods. Depending on the injector type, the LDEF approach may not resolve the near nozzle physics, due to the fact that liquid core region may exit in the near nozzle region. On the other hand, in the Eulerian approach, the physics of transitioning from a continuum fluid to discrete parcels, far from the injector, is not well understood. In terms of CFD approach for spray simulation, direct numerical simulation (DNS) solves the governing equations without modeling [4]. However, it is computationally expensive and is not yet practical for whole domain multiphase turbulent flows. LargeEddy Simulation (LES) method resolves the large scale structures, while modeling the effects of small or subgrid scales. This sub-grid modeling is challenging and the computational time can be considerable [4]. A method which is widely used in spray modeling is Reynolds-Averaged Navier-Stokes (RANS). RANS uses modeling over all of the scales. Droplet collisions are an important physical process that determines mixing, evaporation and droplet distribution. It is necessary to solve two problems simultaneously in droplet collision modeling. The first problem involves predicting the probability of collision [2, 3] and the second is the outcome of collision [1]. For predicting the outcome of the collision of droplets with the same physical and thermophysical properties, three dimensionless parameters are important: Weber number, impact parameter and droplet size ratio [1]. Recently, Tang et al. [17] demonstrated the importance of viscous dissipation in predicting the outcome when two unequal size droplets collide. In evaporating multicomponent sprays, colliding droplets have different properties in terms of density, surface tension and viscosity. Several numerical studies have been carried out for predicting the evaporation process of multi-component sprays [e.g. 7, 18]. Most of them implemented O’Rourke and Bracco’s collision methodology [2], which was originally developed to model the collision and coalescence of two identical droplets with the same physical and chemical properties. Thus, the motivation in the present paper is to develop a model that does not have this restriction, thereby making it suitable for application in multicomponent sprays. In the following section, the formulation of the new coalescence/collision is presented. Validation studies using the new model for non-evaporating spray was carried out and the predicted results are compared with available experimental measurements. Finally, results for multicomponent evaporating spray cases are presented with concluding remarks. Coalescence collision formulation We begin by examining the work of BrazierSmith et al. [1]. In fact, O’Rourke and Bracco’s [2] also referred to the work of [1] for determining the coalescence efficiency. In order for droplet coalescence to occur, the distance between two droplets x, with radius r1 and r2 must be less than (r1+r2) as shown in Figure 1. Figure 1. Schematic representation of two colliding droplets. The coalescence efficiency is defined by [1]: