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Air-water flow (VoF). Need help :(

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Old   October 18, 2009, 22:58
Default Air-water flow (VoF). Need help :(
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Luke
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Hello.

My task is to describe the hydrodynamic phenomena (presurre drop and regimes) of air - water two phase flow in a pipe, using Fluent's VoF. The phases are injected into a T-junction i.e. water from the side (constant flux of 3,5 l/min) and air from above (2 l/min). The inlet diameter for both phases is 0,015 m. The problem is, that in the experiment which I'm trying to confirm, the was plug flow regime was observed, and I'm getting stratified smooth flow every time. I've tried different settings, but non of them worked. Also there is a problem with pressure. In the experiment the total presurre recorded on the manometers was rising if a bubble of air has reached the monitor, and in my case Fluent reports that the total pressure is decreasing. When I print the contours of total pressure, there are areas near the upper wall of the pipe, where the value of pressure is negative (please, see the image).



My case setup:
3D, pbns, transient, garvity on, (VoF explicit; Implicit Body Force), sk-w + Low Re Corrections [btw. does someone know, why it's the only turbulent model, that gives the correct (compared to the value predicted from Darcy- Weissbach equation) value of pressure drop (on the distance of 0,65 m; pipe diameter 0,016m -> 64Pa ), when I try to obtain a steady-state solution before I let the air into the model?], Air(constant 1,197 kg/m3; 1,82e-05 kg/m-s); Water(997.7 kg/m3; 0,0009267 kg/m-s), water as primary phase, surface tension 0,072396 N/m , BC: air inlet (velocity_inlet 0,189 m/s, TI 8%), water_inlet (Velocity_inlet 0,33 m/s, TI 5%), symmetry, outlet (pressure_outlet 0Pa, backflow parameters: 0,046 m, TI 4%, 1 for air in backflow volume fraction), Operating Conditions (101325 Pa set where there allways will be air, Variable density - the density of air).
Solution Methods: PISO (skew -neghbor coupling unchecked); Gradient - Last Squares Cell Based; Pressure - PRESTO!; Momentum - Quick; VF - Geo-Reconstruct; other -default.
Solution Controls:URF for pressure - 0,5 and momentum - 0,3, other -default.
Time step variable i.e. min.1e-06 max 1e-04, Courant Number = 1.

As I said, before I let the air in, I calculate the solution with only water flowing through the system, patching the end of the instalation with air, and waiting till the total pressure is steady. then I let the air in, changing the air BC from wall to velocity_inlet.
I've tried different meshes from very dilute to very dense. I've also tried to refine the mesh in the areas, where the air bubble should tear off, but it didn't help either.
If someone knows how to deal with this problem, I will be eternally grateful (I have only 2 months to solve this and I had started in Nov 2008. It's for my master thesis.) Please help.

Best regards

Luke

P.S. Sorry for my english.
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Old   October 19, 2009, 02:28
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Hi... as I understood you do not get the same results from the calculation as the experiment. First of all, I am not sure that volume flux is good to use [l/s]. Try to use mass flux [kg/s] and be sure the real temperature is the same as pointed in the calculation. Of coarse, I could only suppose what the problem is as I can not "touch" the model.
The second thing is: do not change the BC for air inlet (from wall to velocity inlet) - define form the beginning that this is for example mass flow rate , but for the the steady calculation the value will be "0.00" and then - change to whatever.
The third thing is.... in the experiment, how did you measure temperature of water and air? Did you consider energy transfer? Does the water and air has equal temperature value?
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Old   October 19, 2009, 07:23
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Hello Philipov. Thank You for Your replay and kind help.

I'm not using energy equation, because I assume that the flow is isotermic (the temperature of air and water was 22 C in the experiment). Also, why to use mass flow inlet if the air can be treated as an incompressible fluid (if the velocity il below 0,3 Ma) and water is incompressible, so velocity_inlet boundry condition can be used in both cases.

Best regards.

Luke
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Old   October 19, 2009, 08:20
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The rule that low velocities treat the fluid as incompressible is not applicable to multi-phase flows. Water itself can lead to air compression zones.... if you want ... send me the case file and reference values.... I'll try to help....
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Old   October 19, 2009, 09:01
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I will be very grateful! I will upload the case file on RS , but I've got a 1Mb internet connection, so this may take a while. As soon as posible I will post the link to the case. Thank You!

Luke.
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Old   October 19, 2009, 09:10
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You can use svetlin.philipov@gmail.com
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Old   October 19, 2009, 09:59
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Sent.

Thank You.

Luke.
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