Large-Eddy Simulation on the Effect of Droplet Size Distribution on Mixing of Passive Scalar in a Spray

In this work simulation results of a round spray jet are presented using the combination of Large-Eddy Simulation (LES) and Lagrangian Particle Tracking (LPT). The simulation setup serves as a synthetic model of non-atomizing spray particles taken from the Rosin-Rammler size distribution that enter a chamber filled with gas through an inlet hole with diameter D. At the inlet gas velocity and droplet velocities are specified in addition to the initial size distribution of droplets. The Reynolds number as referred to the gas inflow velocity and jet diameter is Re=10000. The setup is advantageous for understanding the details of diesel sprays since it avoids near-nozzle spray modeling and thereof the corresponding error which is especially important in LES. Here, the implicit LES is applied so that the compressible Navier-Stokes equations are solved directly with a numerical algorithm in a fine mesh without a subgrid scale model. In this approach the subgrid scale diffusion becomes ‘implicitly’ modeled by the ever-present numerical diffusion of low order numerical schemes typical in engineering computations and the approach is assumed thereby to give a more clear picture of the two-way particle-turbulence interaction. Two-way momentum coupling between the phases is considered to be adequate for the relatively dilute droplet loadings that are smaller than 0.002. Results with two given mass loading ratios are presented varying the Sauter mean diameter of the distribution so that the Stokes number of the mean particle sizes is St > 2. The results show that in these setups the droplets enhance mixing in comparison to a single phase jet. It is demonstrated that the spray dispersion pattern and spray profile depend on the Sauter mean diameter of the distribution. Spatial structure of the spray is visualized and accumulation of the particles in certain regions of the flow is visualized. The results are explained in physical terms related to the flow conditions and the droplet sizes.