Modeling and simulation of turbulent multiphase flows

The atomization of liquid spray in turbulent reacting and non-reacting flows usually occurs in two successive steps, i.e., primary breakup and secondary breakup. In the primary breakup region, the evolution of the interface between the phases is usually complex and very difficult to model. In the secondary breakup region, the average droplet size and volume occupied by the droplets are relatively small but the number of droplets is usually very significant.;In this study, we use different mathematical and numerical models for different regions of the spray. For dense spray simulations, a coupled Lagrangian interface-tracking and Eulerian level set method is developed and implemented. In this method, the interface is identified based on the locations of notional particles and the geometrical information concerning the interface and fluid properties are obtained from the level set function. The level set function maintains a signed distance function via the particle-based Lagrangian re-initialization technique. Numerical simulations of several 'standard interface-moving' problems and two-fluid laminar and turbulent flows are conducted to assess this new hybrid method. The results of our analysis indicate that the hybrid particle-level set method can handle interfaces with complex shape change, and can accurately predict the interface values without any significant mass loss or gain. The results obtained for isotropic two-fluid turbulence via the new particle-level set method are validated by comparison with those obtained by the 'zero Mach number', variable-density method. The two-way interactions between the turbulent velocity field and the interface are studied by the particle-level set method. Extensive analysis of vorticity and energy equations indicates that the destabilization effect of turbulence and stability effect of surface tension on the interface motion and interface's effect on turbulence are strongly dependent on the density ratio and Weber number.;For dilute spray simulations, a robust and efficient Eulerian-Lagrangian-Lagrangian mathematical/numerical LES model is employed. This is based on the filtered mass density function (FMDF) methodology and is applicable to two-phase turbulent reacting flows with two-way mass, momentum and energy coupling between phases included. In the LES/FMDF methodology, the "resolved" carrier gas velocity field is obtained by solving the filtered form of the compressible Navier-Stokes equations with a high-order finite difference scheme. The sugrid species, energy and combustion are modeled with the two-phase scalar FMDF transport equation, which is solved by a Lagrangian Monte Carlo method. The liquid droplet/spray is simulated with a non-equilibrium Lagrangian model and stochastic SGS closures. The two-way coupling is implemented through series of source/sink terms. The two-phase LES/FMDF is employed for systematic analysis of turbulent combustion in the double swirl spray burner and spray-controlled dump combustor for various flows and spray parameters. The effects of fuel type, spray/injection angle, mass loading ratio, droplet size and its distribution, fuel/air composition, wall, and other parameters on the combustion and turbulence are investigated.