Introductory Remarks about Two-Phase Mixtures

The hydrodynamics of gas–liquid mixtures are often very complicated and difficult to rigorously model. A detailed discussion of two-phase flow modeling difficulties and approximation methods will be provided in Chapter 5. For now, we can note that, although the fundamental conservation principles in gas–liquid two-phase flows are the same as those governing single-phase flows, the single-phase conservation equations cannot be easily applied to two-phase situations, primarily because of the discontinuities represented by the gas–liquid interphase and the fact that the interphase is deformable. Furthermore, a wide variety of morphological configurations (flow patterns) are possible in two-phase flow. Despite these inherent complexities, useful analytical, semi-analytical, and purely empirical methods have been developed for the analysis of two-phase flows. This has been done by adapting one of the following methods.

1. Making idealizations and simplifying assumptions. For example, one might idealize a particular flow field as the mixture of equal-size gas bubbles uniformly distributed in a laminar liquid flow, with gas and liquid moving with the same velocity everywhere. Another example is the flow of liquid and gas in a channel, with the liquid forming a layer and flowing underneath an overlying gas layer (a flow pattern called stratified flow), when the liquid and gas are both laminar and their interphase is flat and smooth. It is possible to derive analytical solutions for these idealized flow situations. However, these types of models have limited ranges of applicability, and two-phase flows in practice are often far too complicated for such idealizations.

2. […]