Large eddy simulations of turbulent dispersion effects on direct injection sprays 2019-01-0285
In CFD studies of engine sprays, the two-way coupling between liquid droplets and ambient gas phase needs to be accounted for as source terms in the respective governing equations. Accurate estimation of these source terms requires the relative velocity “seen” by the droplet as it moves across the flow, which generally needs to be estimated using a turbulent dispersion model. In this study, a sub-grid scale (SGS) dispersion model is constructed in which the gas velocity affecting the droplet is decomposed into a deterministic part and a stochastic part. The deterministic part is reconstructed using the approximate deconvolution method (ADM), in which the instantaneous large-scale flow field (of the order of the LES cut-off width or larger) can be readily determined. The stochastic part represents the impact of the SGS flow field, which is assumed to be locally homogeneous and isotropic, and governed by a Langevin equation. The model is applied to the Spray H and Spray G conditions defined by the Engine Combustion Network (ECN) group. Simulation results are compared with the available experimental data for spray characteristics such as penetration rates, mixture fraction profile, and droplet and gas phase velocity distributions. Simulations with no SGS dispersion and a RANS type model are also performed for comparison purpose. Results show that the turbulent dispersion has considerable impacts on the spray characteristics. The proposed SGS model also improves the prediction of spray and ignition characteristics at the spray conditions studied in this paper.
Hongjiang Li, Christopher Rutland, Hong Im, Francisco Hernandez Perez
Univ of Wisconsin Madison, King Abdullah Univ of Science & Tech