General Remarks

The design and analysis of systems often require the solution of mass, momentum, and energy conservation equations. This is routinely done for single-phase flow systems, where the familiar Navier–Stokes equations are simplified as far as possible and then solved. The situation for two-phase flow systems is more complicated, however. The solution of the rigorous differential conservation equations is impractical, and a set of tractable conservation equations is needed instead. To derive tractable and at the same time reasonably accurate conservation equations, one needs deep physical insight (to make sensible simplifying assumptions) and mathematical skill. Fortunately, the subject has been investigated for decades, and at this time we have well-tested sets of tractable two-phase conservation equations that have been shown to do well in comparison with experimental data.

Generally speaking, conservation equations can be formulated and solved for multiphase flows in two different ways. In one approach, every phase is treated as a continuum, and all the conservation equations are presented in the Eulerian frame (i.e., a frame that is stationary with respect to the laboratory). This approach is quite general and can be applied to all flow configurations. In another approach, which is applicable when one of the phases is dispersed while the other phase is contiguous (e.g., in dispersed-droplet flow), the contiguous phase (the gas phase in the dispersed-droplet flow example) is treated as a continuum and its conservation equations are formulated and solved in the Eulerian frame.