[Hema kothimbare]

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..