# Membrane porous jump problem in VOF

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 August 7, 2009, 01:22 Membrane porous jump problem in VOF #1 New Member   Hema kothimbare Join Date: Jul 2009 Location: Taiwan Posts: 1 Rep Power: 0 Hi, I am Hema, i am doing 2d cross flow microfiltration study using fluent. i have 2d geometry of 57mm x 1mm two rectangle with middle connecting edge (50mm) defined as porous jump. i have defined velocit inlet and pressure outlet at top edge, and pressure outlet for permeate at bottom edge (all 3 mm). these are the conditions Solver: Solver: segregated Formulation: implicit Space: 2D Time: unsteady Gradient option: node-based -Multiphase model: Model: VOF, Number of phases: 2 VOF scheme: geo reconst. Open channel flow: No Body force formulation: implicit body force. -Viscous model: Laminar -Operating conditions: Operating pressure: 101325 Gravity: Y=-9.81 Operating density: 1.225 -Phases: Primary phase: water Secondary phase: air -Boundary conditions: Velocity inlet: mixture - 0.14 m/s (air-0.04 m/s, water-0.1 m/s), air vol fraction 0.285 Pressure-outlet (both): 0kPa (gauge) Wall: no slip conditions membrane - (porous jump- phase permeability- 6.25e-16, membrane thickness- 0.000125m, porous jump coefficient - 0) Solution: Velocity-pressure coupling: PISO Skewness correction: 0 Neighbor correction: 1 Pressure: Body forced weighted Momentum: 2nd order upwind URL: default values except for momentum of 0.5 initialize at inlet Patch - water at top surface and air at bottom surface Time step - 5e-7 Questions- 1) one problem i am facing is very small time step, how i can reduce time step, i found that if i reduce phase permeability- from 6.25e-16 to 6.25e-10 the time step changes to 2e-5. but i want very low permeability. whay i can improve? 2) i also found that the air bubbles passes through membrane surface. but in actual case air do not pass through membrane it should come out through reject. what is the problem? 3) in my experiment i have pressure drop between my velocity inlet and reject pressure outlet as 20kpa. so i also need to know how i can define that in my case under velocity inlet condition? 4) one problem is the how we can reduce the diffusion of air into water. because in simulation we cann't see air and water boundry (air bubbles) very clearly. 5) is voulme fraction of 0.285 defined correct in my case? i calculated vol frac as = ug/(ug+ul) and mix velo=ug + ul , is it correct? please anyone can help..i will be very happy to get answers to any of my questions.. Thanks in advance.. Last edited by Hema kothimbare; August 7, 2009 at 01:26. Reason: need to add some details

 Tags membrane, multiphase, porous jump, velocity inlet, vof