CFD prediction of the flashing processes in a MSF desalination chamber

Vapourisation inside a desalination flashing chamber arises from the energetic evaporation with self-boiling of liquid brine flow due to a reduction of pressure, and is a complex, multiphase, thermo-fluid phenomenon. A computational field model for the flashing process inside a flashing chamber is developed around a two-phase VOF formulation. Two different phase-change mechanisms are allowed for, based on the saturation temperature and on the vapour pressure, respectively, that enable the model to compute the phase change regions, also the level and the shape of the free surface. The model is applied to solve for steady multiphase flow inside a flashing chamber without a baffle, and thus to predict the chamber flow details and behaviour. Thermo-fluid performance results are presented in terms of flow patterns and distributions of hydrostatic pressure, temperature, vapour volume fraction and mass transfer inside the flashing chamber. The average outlet temperature and average vapour temperature agree well with the values of a real MSF plant, as does the estimated vapour production rate. Two main mechanisms of phase change produced by the flashing process are captured in the simulation. Vapour bubbles are formed at the entrance to the flashing chamber, and bubble production reduces along its length. At the free surface of the liquid there is also a phase change and mass transfer. Computational field modelling is valuable for a better understanding of the combined heat transfer, mass transfer and the fluid dynamics during the flashing flow evaporation processes. Results can be used to estimate MSF design factors such as non-equilibrium temperature difference and flashing efficiency.