The Gas/Liquid Multi-Phase Formulation in the CRAFT CFD® code treats two different classes of multi-phase problems. The first category deals with heterogeneous gas/liquid mixtures where the liquid is a bulk continuum fluid. Problems in this class include gas/liquid jets and bulk liquid flyout. For this class of problems, the formulation we discuss here entails the solution of a unified set of conservation equations for the generalized gas/liquid mixture. The interface between the gas and liquid phases is captured as part of the solution procedure, permitting its shape to evolve in a transient fashion as it interacts with the gas. The conservation equations are cast in strong conservation form using a compressible, density based formulation. An upwind flux difference scheme is utilized to integrate the equations which captures shock waves as well as contact and gas/liquid interfaces. Pressure waves can propagate across inter-phase boundaries in this scheme with minimal numerical diffusion, and with speeds consistent with the thermodynamics of the multi-phase composition. Both the gas and liquid components can be represented using generalized compressible equations of state.
The second class of is that of a generalized continuum fluid which contains a dense dispersed phase (particles or droplets) which is in non-equilibrium. Problems in this category encompass a wide range of applications dense spray applications that result from a bulk liquid breakup. Specialized sub-models for non-equilibrium vaporization as well secondary breakup of droplets are available. Furthermore volumetric effects due to high volume fraction of the droplet phase are accounted for rigorously. This system of equations retains its hyperbolic characteristics with a dispersive wave system which is consistent with transient multi-phase physics.
|
