Discrepancy between the measurements (pressure drop; VOF)
I am modeling air-water multiphase flow using the VOF model (2D) for my thesis. My major task is to compare the experimental data obtained by my teacher, with the results obtained by me using Fluent (i.e. pressure drop, flow structures and volume fractions). The hydralic installation is assembled from a static mixer, a long pipe (about 2,5m) and a phase separator. The mixer is basically a cylinder + cone, with a partition in the middle of the cylinder part. The whole assembly is about 3,25 m long. The water and air are "mixed" in the static mixer, then the mixture flows through the long pipe, and the separator. In the middel of the pipe, there are 2 monitors (1 m of each other) of total pressure.
Now the difficult part... Even if I get volume fractions and flow structures similar to those from experimental data, I can't get correct pressure values. I'm getting the pressure drop abotut 250-500 Pa, when it should be around 1100 Pa. I have one additional monitor to measure the gauge total pressure in the static mixer. It should be around 3250 Pa, and Fluent reports 1720 Pa.
Here's my case setup:
1. Solver settings: 2D, dp, rke, vof(explicit), unsteady, NITA. Discretization controls set to second order upwind.
air- density/viscosity set to constant (1,195 kg/m3 ; 1,8225e-05 kg/ms)
water - density/viscosity set to constant ( 997.65 kg/m3 ; 9.587e-04 kg/ms)
3. Operating condition:
101325 Pa, the location of reference pressure is near the upper outlet of the separator, so there will be allways 100% of air,and the pressure will be equal to atmospheric pressure.
4. Boundary conditions:
a)water inlet : mass-flow-nlet (17,8 kg/s), the diameter of the duct is 0,0216m
b)air inlet: mass-flow-nlet (0,0075 kg/s), the diameter of the duct is 0,0216m
Separator: 2x pressure-outlet (one for air, and one for water) with gauge pressure set to 0 Pa. The duct diameter of the outlet is 0,046 m.
The mesh is fairly dens and has a decent quality.
The diameter of the long pipe, where the pressure monitors are, is 0,0211m
I tried many variants (e.g. different boundry conditions, ideal incompressible ideal gas for air density), but no success.
My workflow: first I set a mass-flow-inlet for the water, and after a few seconds (thousands of time-steps), when the water mas flow rate and pressures are stabilized, I let the air in.
In the begining when only water flows through the instalation the pressure drop is still far to small (about 230 Pa) compared to calculated values (~500-600 Pa; Darcy-Weisbach equation).
Please, if anyone has any idea how to fix this problem, or if anybody konws what is the cause of the discrepancy between the measurements, I would be extremely grateful.
Here's how the hydraulic installation looks:
(powietrze is air ; woda is water)
Sorry for my english.
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