The focus of this section is to introduce the reader to the modelling of multiphase flow in general, liquid-gas flows in particular and the prediction of pressure drop and void specifically. The main objective is to provide the reader with the basic fundamentals that are needed to formulate the balance equations for multiphase flow as well as the associated constitutive relations and then employ them in the practical prediction of pressure drop in a closed conduit as the first example of their usefulness. As in other sections this is a diverse subject and more detailed discussions of these topics are provided by a number of individuals; e.g., Collier, 1981; Wallis, 1979; Bergles et al., 1981; Hsu and Graham, 1977; and Ishii, 1974.
Once the concept of flow patterns for multiphase flow has been introduced, one must develop the governing equations which account for the conservation of mass and energy and the transfer of momentum. Consider the case of steam-water flow in a vertical tube with heat addition at the wall boundary (Figure 3.1). In reality the phases are not exactly in thermodynamic and mechanical equilibrium; i.e., if the channel geometry is variable and the flow rate or heat addition rate is changing rapidly with time or space, then the velocity, temperature and pressure of the two-phases are not necessarily the same at a given spatial position in the channel. For certain conditions one may be able to model the multiphase flow and assume that some or all of these potential variables may be equal between the phases. To cover a broad range of applications a number of models have been developed. We consider three general types of multiphase flow models, the homogeneous flow model, separated flow model and two fluid model. Table 3.1 gives a brief summary of important dimensionless groups that can be used to determine model applicability.