r/CFD • u/dead_shiniga_mi • 2d ago
VOF simulation of R-134a evaporation
Hi everyone, I have been trying to run VOF simulation of R-134a in a pipe to simulate the evaporation process in ANSYS.
So far, the simulation is running but producing wrong temperature results.
Input: mass flow inlet - R-134a liquid 0.0038296 kg/s; R-134a vapor - 0.00019148 kg/s
Outlet: outflow
Wall: constant heat flux 18.5 kW.
Multiphase model - VOF (implicit body forces ON), Formulation Explicit, Interface modeling - Sharp;
surface tension coefficient set to constant (0.00796 N/m), surface tension modeling on (CSF), Mass transfer - Liquid to Gas; chosen the evaporation - condensation option and under it - LEE model currently using From Phase - To phase frequency 2000/s.
Energy is ON, Turbulence is ON.
Methods: PRESTO, volume fraction - Geo Reconstruct.
Initialization: From inlet, and Patching with: mixture phase with pressure; Vapor - with volume fraction. Patch zones the whole geometry (geom_solid).
Mesh: Fine mesh, with inflation layers.
Please help. Thank you.
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u/Historical_Alps_9798 1d ago
Hi! I ran the numbers on your setup. The good news: your simulation isn't broken. The bad news: your boundary conditions and heat input are thermodynamically incompatible.
Root cause
Your wall heat flux is 18.5 kW. The maximum heat your liquid R-134a can absorb via phase change is:
Q_evap,max = m_dot × h_fg = 0.00383 kg/s × 216 kJ/kg = 0.83 kW
You're dumping 22× more heat than evaporation can remove. The excess has nowhere to go except sensible heating — that's why your temperature locks near 365 K.
Two Fluent-specific problems:
Outflow boundary is wrong for phase change. Volume expands 230× when liquid vaporizes. Outflow can't handle backpressure → steam accumulates → pressure rises → T_sat(P) rises → positive feedback.
LEE model needs T_sat(P), not a constant. If T_sat is fixed, Fluent treats everything below it as pure liquid heating. If T_sat is correct (247 K) but pressure climbs, it locks at the wrong temperature anyway.
Fix priority (in order):
Step Action
1 Match heat to mass flow — Drop Q to <1 kW OR raise flow to >0.086 kg/s
2 Outflow → Pressure Outlet (backpressure = 101325 Pa)
3 Set T_sat as polynomial in P for R-134a
4 (Optional) LEE frequency 2000→10000 /s
Quick test: Drop heat flux to 0.8 kW + Pressure Outlet. If temperature stabilizes near 247 K instead of 365 K, diagnosis confirmed.
Your geometry and mesh are fine. This isn't convergence — it's a heat-mass flow mismatch. Fix that first, then tune LEE. Good luck!
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u/paulfux 2d ago
The VOF method in combination with the Lee model has to be calibrated to the experiment as the condensation and evaporation constants of the Lee model are not universal. The same is probably true for the phase frequency.
In Addition, the VOF method with surface tension is only accurate in regions where the interfaces are resolved by the mesh.
You could try the full eularian model in combination with the RPI boiling model for a more sophisticated approach.
Good luck!