Mass imbalance in Volume of Fluid

sumit.sh · August 17, 2023, 04:34

I am trying to simulate evaporation in a plate heat exchanger using the Volume of Fluid model and the LEE model for phase change. As the LEE model is empirical, we must specify the evaporation and condensation frequencies. When the frequencies are low (default 0.1), the solution and mass imbalance converge. However, on increasing the frequencies to higher orders, I encounter reversed flow, and the mass imbalance takes a severe hit and keeps oscillating. I have tried improving the mesh quality near the outlet and extending the outlet sufficiently, but nothing seems to work. Following are the boundary conditions and setup

General:
Steady-state, pressure-based solver, gravity

Multiphase:
VOF, implicit, 2 phase, Surface tension modelling, Evap/Cond (Tsat)

Boundary Conditions:
Operating Pressure (Psat)
Inlet: Mass Flux (G_liq and G_vap at Tsat)
Outlet: Pressure outlet (Psat), vapor Backflow volume fraction = 0
Wall_Thermal: heat flux
Wall_Momentum: No slip, contact angle for wall adhesion 1° (highly wetting fluid)

I start by solving the single-phase flow and then use the flow profile for the multiphase simulation. I keep the URFs low initially and increase them gradually.

Any suggestions will be highly appreciated. Thanks in advance.
-SS

Abdullah hassan · April 24, 2024, 03:49

Hi Sumit. Sh,

Did you find the solution?
I am also stuck at the same position and nothing is working for me either.

August 17, 2023, 04:34	Mass imbalance in Volume of Fluid	#1
sumit.sh New Member Sumit Sharma Join Date: Jun 2013 Posts: 4 Rep Power: 13	I am trying to simulate evaporation in a plate heat exchanger using the Volume of Fluid model and the LEE model for phase change. As the LEE model is empirical, we must specify the evaporation and condensation frequencies. When the frequencies are low (default 0.1), the solution and mass imbalance converge. However, on increasing the frequencies to higher orders, I encounter reversed flow, and the mass imbalance takes a severe hit and keeps oscillating. I have tried improving the mesh quality near the outlet and extending the outlet sufficiently, but nothing seems to work. Following are the boundary conditions and setup General: Steady-state, pressure-based solver, gravity Multiphase: VOF, implicit, 2 phase, Surface tension modelling, Evap/Cond (Tsat) Boundary Conditions: Operating Pressure (Psat) Inlet: Mass Flux (G_liq and G_vap at Tsat) Outlet: Pressure outlet (Psat), vapor Backflow volume fraction = 0 Wall_Thermal: heat flux Wall_Momentum: No slip, contact angle for wall adhesion 1° (highly wetting fluid) I start by solving the single-phase flow and then use the flow profile for the multiphase simulation. I keep the URFs low initially and increase them gradually. Any suggestions will be highly appreciated. Thanks in advance. -SS

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April 24, 2024, 03:49		#2
Abdullah hassan New Member Abdullah hassan Join Date: Oct 2023 Posts: 9 Rep Power: 2	Hi Sumit. Sh, Did you find the solution? I am also stuck at the same position and nothing is working for me either.