Hello;
I am conducting a CFD study on two-phase flow in a separator, utilizing refrigerant as the working fluid.
For this study, I have implemented pressure inlet and pressure outlet boundary conditions based on experimental data. The simulation employs the Eulerian method in conjunction with the Volume of Fluid (VOF) and Reynolds Stress Model (RSM). Upon completing the simulation, I observed that the flow rate calculated by Fluent significantly exceeds the experimental data. Additionally, the mass balance is not adequately satisfied.
It is important to note that while a mass flow inlet boundary condition was applied, a reverse flow issue occurred. Attempts to rectify this through geometry extension and mesh refinement have not resolved the concern. Furthermore, the pressure inlet values predicted by Fluent were considerably underestimated compared to the experimental measurements.
I would greatly appreciate any valuable advice or insights you may have regarding these issues.
Fluent is a computer model--an approximation. Neither Fluent or any other computer model is the "last word" on anything, regardless of how much it costs or how many people have used it to do whatever. Experiments (properly conducted and analyzed) are the last word. The more complex the problem (geometry, fluids, phases, etc.), the more you must look to experimental data for direction and confirmation. Far too many on RG simply run a commercial code and think that's enough to get a PhD.
Thank you for your reply. Your ideas to address the concern will be highly appreciated.
When you specify two input pressure parameters, the equation used is the velocity potential, it means, a gradient pressure is stablished with your pressure inputs, from which the velocity distribution profile is generated. This equation is scalar, then some information is lost, but you can use it as initial solution.
The reverse flow is not normal. Check the mass conservation equation explicitly, the fluids phase declarations and the rule for conversion among phases.
Less probable but also important, check for empty volumes in the mesh and the cells quality.
CFD has had some success with gas - solid two phase flows, dispersed mists and annular flows. However, general two-phase flow is little understood being very unstable, and the behaviour at flow discontinuities is even less understood. We use empirical methods normally based on best data available or, better, tests, as pointed out by Dudley J Benton above.
Apart from refining the mesh make sure your time step is lesser than the physical time scale that occur in a multiphase flow, you can refer my small work on multiphase flow for the required time scales - https://iopscience.iop.org/article/10.1088/1873-7005/ad255f/meta
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