2. Modelling Strategies and Two-Phase Flow Models

The general methods of solution of thermal-hydraulic problems are recalled first and then we show how these are complicated by the presence of multiphase flows, before entering into the descriptions of the various approaches commonly used. The special features of multiphase or two-phase flows are pointed out, in particular the existence of a number of flow regimes. The various two-phase flow and boiling heat transfer variables of interest, such as the pressure gradient, the void fraction, the heat transfer coefficient, etc., will depend on the particular flow regime. In principle, one should model each flow regime separately; when flow-regime-specific models are used, one can “mechanistically” take into consideration the particularities of each regime. The alternative approach often used is to largely ignore the flow regimes and derive methods (most often empirical correlations) covering all flow regimes continuously. The complete formulation of the two-phase-flow problem, which would have required the description of the evolution in time of the fields (pressure, velocity, temperature, etc.) for each phase, together with a prediction of the geometry of the interfaces, is generally impractical. The often chaotic flow fields must be treated in terms of statistical, average properties. There are two general approaches, the two-fluid, or more generally the multi-fluid approach and the mixture formulation. A simple presentation of the two-fluid approach is given. The basis of the method is to write conservation equations for each phase and to include in these equations terms which represent the interaction between the phases. The closure laws required to complete this formulation are listed and examples of implementation difficulties are given. The phase conservation equations may be summed up to yield mixture conservation equations, a particular case is the homogeneous flow model. The relatively new developments that rely on computational fluid mechanics methods to analyse and simulate multi- and two-phase flows are introduced to the reader.