On large eddy simulations of reacting two-phase flows

A two-phase subgrid combustion model has been formulated for large-eddy simulations (LES). This approach includes a more fundamental treatment of the effects of the final stages of droplet vaporization, molecular diffusion, chemical reactions and small scale turbulent mixing than other LES closure techniques. In this thesis, the liquid droplets are tracked using the Lagrangian approach up to a pre-specified cut-off size. The phase change of the droplets both larger and smaller than the cut-off size and the subsequent mixing of the evaporated fuel with the oxidizer are modeled within the subgrid using an Eulerian two-phase model. Two-way coupling between the gas-phase and dispersed-phase quantities (both mean and fluctuating) have been extensively studied in isotropic turbulence. Comparisons between the DNS and LES have yielded better closures for the turbulence equations in presence of dispersed phase. Finally, the new model has been implemented into the LES code to simulate temporal and spatial shear layers, which are some of the basic building blocks in understanding the complex flow-phenomena in a realistic combustor. Studies on temporal mixing layers have shown that the droplets of Stokes number of order one disperse the most. These studies also suggest that the droplets generate substantial baroclinic torque and inhibit large scale vortical motions. It is shown here that the new subgrid approach works consistently better for both infinite and finite-rate kinetics in turbulent mixing layer even when the cut-off is increased. In contrast, conventional LES under similar conditions results in significant error when the cut-off size is increased. The spatial shear layers studied also suggest that the droplets rearrange energy from large scale vortical structures to smaller scales and this process is closely related to the transfer of energy from large scales to the small scales as observed in isotropic turbulence. The spatial mixing layer simulations were compared to earlier experiments of Hishida et al. and very good agreement has been obtained. The sensitivity of the new subgrid two-phase approach to droplet cut-off size has also been evaluated in the spatial mixing layers.