Large eddy simulation turbulence modeling of spray flows

Bharadwaj, N. Large Eddy Simulation Turbulence Modeling of Spray Flows. University of Wisconsin-Madison, 2010.

The focus of this thesis is on the development of a two-phase sub-grid interaction model for Large Eddy Simulations (LES) of high injection pressure diesel sprays. The two-phase flow governing equations are numerically solved using a Lagrangian-Eulerian approach in the KIVA-3V code. A non-viscosity dynamic structure LES model is used in which the sub-grid stress tensor is modeled with a non-viscosity tensor coefficient. This is a one equation LES approach in which a transport equation for sub-grid kinetic energy ( ksgs) is also solved besides the basic flow equations. To account for the spray induced turbulence effects, a LES model is developed for the spray source/sink term in the ksgs – transport equation. This model requires the sub-grid gas velocities, which are obtained by using an approximate deconvolution method (ADM) in de-filtering the resolved gas velocities.

The performance of the LES spray source/sink model and the overall ability of LES in predicting two-phase turbulent flow is analyzed in different test flow configurations, such as a particle laden gas jet, a particle laden isotropic turbulent flow, and non-evaporating and evaporating diesel sprays. The LES predicted mean and turbulence velocities for the particle laden jet are shown to compare very well with the experiments. The dependence of the spray source model on particle loading and Stokes number is discussed. The characteristics of sub-grid production, dissipation and spray source terms are discussed and found to be consistent with the previous DNS results. The LES results are shown to correctly predict the effect of the particles on different length scales in the isotropic turbulent flow.

Non-evaporating diesel spray LES results are compared with previous experiments. The comparisons are made for quantities such as the transverse integrated mass (TIM), mass averaged axial velocity profile and spray momentum. The near nozzle contribution from droplets to gas phase sub-grid kinetic energy is found to dominate the sub-grid production and dissipation rates. The spray source term in diesel sprays is found to have a significant effect on the spray evaporation and penetration characteristics. Preliminary results for diesel engine LES with the new spray/sink model are shown.