The Moving Boiling-Boundary Model of a Vertical Two-Phase Flow Channel Revisited

Abstract

Vertical flow involving phase changes is a subject of interest in several engineering branches, especially in the nuclear industry. Not only is there a concern about it in the design of boiling water reactors, but also safety evaluation and analysis in pressurised reactors involve a substantial amount of two phase flow calculations. As there is not yet a sound theoretical basis able to solve general problems, several mathematical models—often based on empirical or phenomenological observations—with limited applicability have been introduced to deal with particular situations. One example of these ad-hoc models is the moving boundary nodalisation proposed by Clausse and Lahey, that is reasonably simple and at the same time recovers experimental results with a high degree of accuracy. In this paper, the model is revisited and the equations are developed in detail keeping the intermediate mathematical steps. Alternative formulations that can be conveniently applied to different problems (i.e. single channel, parallel channels, adiabatic risers, non-steady external pressure drops, etc) are proposed, trying to take advantages of modern differential-algebraic equations solving methods. The general results obtained in this work can be extended to build particular models for different applications, including thermalhydraulic representations of nuclear power plants.