Thermo-hydrodynamic behaviors inside a multi-stage-flash evaporator

Abstract

Open channel type multi-stage flash (MSF) evaporator is the most widely used distillation system in practice, usually constructed along with the electric power plant in water deficient areas. The superheated brine stream enters the flash stage with bubble formation. As the two-phase mixture flows downwards, the bubbles grow and rise up due to buoyancy and finally escape through the free surface to the vapor space to be condensed as fresh(distilled) water; and the remaining liquid flows out to the next stage maintained at the lower pressure. As a whole, the system is designed to operate with small thermal potential. In the present study, the open channel two-phase flow behaviors inside the flash stage and the evaporation performance of the system were predicted numerically. In operational bases, the liquid level, size and number density of bubble nuclei embedded in the incoming liquid, system pressure, and the degree of inlet superheat were taken as parameters. In the analysis, two-dimensional, steady turbulent flow was assumed; and the continuous(liquid) phase was handled with Eulerian approach whereas the dispersed phase with Lagrangian approach. A bubble motion equation was solved in conjunction with the appropriate constitutive relations on the drag coefficient and on the virtual mass coefficient, and the bubble growth was considered to be thermally controlled. Standard k-ε model was used in turbulence calculations for the liquid phase. The PSI-Cell method was adopted to count for the interactions between the phases; that is, the phase interactions were considered in the source terms of the balance equations for the liquid phase. The velocity and temperature distributions of the liquid phase, as well as bubble trajectories and sizes, were obtained. Several comparisons were made with the others`` experimental data to confirm the adequacy of the present numerical approach, and the results turned out to be satisfactory. Then the non-equilibrium allowance was ca...