Multiphase flow may consist of gas–liquid, gas–solid, liquid–liquid, liquid–solid or gas– liquid–solid systems. For a multiphase system containing very small particles, bubbles or drops that follow the continuous phase closely, reasonable simulation results can be obtained. Systems in which the dispersed phase has a large effect on the continuous phase are more difficult to simulate accurately, and only crude models are available for multiphase systems with a high load of the dispersed phase. At the moment, the quality of the simulations is limited not by the computer speed or memory but by the lack of good models for multiphase flow. However, multiphase flows are very important in engineering since many common processes involve multiphase flow. The mass and heat transfer between the phases are of interest in many applications, e.g. in boiling, heterogeneous catalysis and distillation. For these simulations we must introduce empirical or semi-empirical correlations to describe mass and heat transfer. The mass- and heat-transfer coefficients are usually calculated from the traditional correlations for the Sherwood, Sh, and Nusselt, Nu, numbers. The advantage with CFD is that Sh and Nu can be computed using local flow properties. The mass and heat transfer are also affected by the coalescence and break-up of bubbles and drops. The phenomenon of break-up and coalescence is not included in this book since only very simple models are available for simulation of the effect of turbulence and shear rate on drop or bubble size distributions.
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