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-   -   Mass Imbalance in orifice with cavitation-model (http://www.cfd-online.com/Forums/fluent/116068-mass-imbalance-orifice-cavitation-model.html)

SmallOrifice April 12, 2013 07:08

Mass Imbalance in orifice with cavitation-model
 
Hello,

I am doing my master thesis on studying cavitation in small orifices. The diameter of the inlet and the outlet are 2 mm and the sharp-edged orifice has a diameter of 0.2 mm. The length of my outlet is about 50 mm.

I run my calculation as steady with k-e-rng-model on. My BC are pressure-inlet (100 bar) and pressure outlet (atm).
For a one-phase flow I have a mass imbalance for inlet to outlet less 1 %.
But if I run the simulation with mixture model (water-liquid and water-vapour) and default cavitation model (schnerr-sauer), I get a difference of the order of around 10.
inlet: 12.53 kg/h
outlet: - 335 kg/h
My residuals fall to 1e-04 and don't change over 5000 iterations.

The problem seems to be the condensation of the vapour to the liquid in the outlet. Direct after the orifice the volume fraction of vapour is close to 1. Around 15 mm after the orifice the vapour condensates. (This aspect I also see in pictures I recieve from experiments).
Before the condensation I have the same mass flow as at the inlet, after the condensation I have the much higher values of the outlet.

Do you have any ideas about this? I'd appreciate any help.

Thank you,
Burkard

RodriguezFatz April 12, 2013 10:17

Is this 2d? Can you afford to refine the mesh strongly?
What do you use for meshing?

SmallOrifice April 12, 2013 14:11

Hello Rodriguez,

sorry I totally forgot to write s.th. about this.
Yes, it's 2D and axisymmetric.
I used the mesh generator from the ansys workbench. I have around 800 k cells and the mesh is finer around the orifice. I'll try to refine the grid at the outlet.

Burkard


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