The Computational Fluid Dynamics Simulation Of The Vacuum Membrane Distillation Process Of Brine

The research of penetration theory in porous membrane started from the study on the average pore size and gas mean free path of molecules. It is called viscous flow when the aperture is much larger than the mean free path of gas molecules, on the contrary, Knudsen flow. For the vacuum membrane distillation (VMD) process, the feed liquid temperature is300K～350K, while the pressure on the cold side is2.5kPa-7.0kPa, so the free path of gas molecule is0.2μm-3.0μm. The pore size of typical membrane used in VMD is0.1μm-0.5μm.So the flow of VMD process is called mixed flow or transition flow, which lies between the Knudsen flow and viscous flow.Computational Fluid Dynamics (CFD) uses computer as a tool to study the fluid flow in different processes and the effects on processes of mass transfer, heat transfer and other transformation, using the numerical method of motion equations. CFD was used as a tool, and it was applied to simulate VMD process in this work.VMD was considered as the multiphase flow (liquid and gas phase) process inside the porous media in the simulation, and membrane module was simplified to the two-dimensional structure with a single membrane silk. The aqueous NaCl solution was seemed as an incompressible and steady fluid. The membrane surface was defined as porous medium, with the hypothesis that all particles are in local thermodynamic balance during the processes of mass transformation, heat transformation and phase transition processes, internal resistance and viscous resistance coefficient were given.The simulation was based on the mixture model, and standard k-ε model was employed to describe turbulent fluid flow, because it was widely used in the industrial cases. The vapor volume fractions were simulated with different feed temperature. The numerical simulation of the two-dimensional model of VMD for aqueous NaCl solution was established under the steady state using FLUENT, which is one of the software of CFD, with which the change of the temperature, the pressure, the vapor and the liquid volume fraction were calculated under the feed temperature60℃. The effects of feed flow rate on VMD permeate flux were studied when the feed flow rates were30L/h,35L/h,40L/h,45L/h,50L/h, and the temperatures were318.15K,323.15K,328.15K,333.15K,338.15K,343.15K, separately.Finally, the three-dimensional model was established to simulate the VMD process of aqueous NaCl solution. Considering the wall thickness of membrane silk, the temperature, pressure, vapor and liquid volume fraction along the whole membrane module were studied. The parameters of VMD process on membrane module cross-section and on the membrane silk boundary layer were also studied in the simulation.