Heat pipe (Thermosyphon): VOF Multiphase Model

Heat Pipe (Thermosyphon) CFD Simulation Using VOF Multiphase Model, ANSYS Fluent Training

Description

This project simulates a heat pipe using the multiphase VOF model in ANSYS Fluent. The 3D geometry was designed in Design Modeler as a rectangular domain measuring 10 cm long and 1.2 cm wide, meshed in ANSYS Meshing with a total of 18,000 cells. Given the nature of this problem, a transient solver was used throughout.

Methodology

This study models the heat pipe's operation using the multiphase VOF model, with evaporation and condensation mass transfer explicitly activated to capture the phase-change cycle central to the device's function. A hot wall was set at 400 K and a cold surface at 300 K, and solving the problem in a time-dependent manner revealed water droplets forming at the top of the geometry and moving downward under the combined influence of capillary action and gravity. The saturation property was defined using a piecewise linear input, with turbulence resolved using the Realizable k-epsilon model.

Conclusion

Results include 2D contours of water and vapor volume fraction, mass transfer rate, and temperature. Comparing volume fraction contours at two different time points clearly shows an established, repeating evaporation-condensation cycle, with droplets forming at the top of the tube and migrating downward under gravity and wall capillary effects.

The results also capture the vapor phase's density variation over time, following the ideal gas law as local temperature and pressure conditions shift. The mass transfer rate itself trends toward a stable average value, indicating that evaporation and condensation reach a balanced, self-sustaining equilibrium within the heat pipe — consistent with the expected steady-cycling behavior of a properly functioning thermosyphon.