Mass Transfer: Heat Pipe, Evaporation and Condensation

This project presents a transient simulation of the evaporation and condensation occurring inside a thermosyphon heat pipe using ANSYS Fluent, with phase-change mass transfer as the central theme. The defining feature of a heat pipe is that it moves heat by repeatedly changing the phase of a working fluid, and capturing that behaviour requires a model able to compute the transfer of mass between liquid and vapour. Heat added at the evaporator produces vapour, while heat removed at the condenser promotes condensation, establishing a continuous phase-change cycle that transports thermal energy efficiently through the device. The simulation resolves the transient evolution of vapour generation, condensate return and the resulting fluid circulation.

The three-dimensional geometry was created in ANSYS DesignModeler and meshed in ANSYS Meshing. The model comprises three sections: the evaporator at the bottom (heat input), the insulated adiabatic middle section, and the condenser at the top (cooling). The domain was discretised with an unstructured mesh of approximately 3,900,000 elements, giving sufficient resolution to represent the phase boundaries accurately while keeping the computational cost manageable.

A three-phase Volume of Fluid (VOF) model was adopted to track the interaction among liquid water, water vapour and air as a non-condensable phase, with VOF providing the sharp interface tracking needed to follow the moving liquid–vapour boundary. The heart of the methodology, however, is the evaporation–condensation mass-transfer mechanism in ANSYS Fluent, which drives phase change based on the local pressure and temperature fields — converting liquid to vapour where the fluid is heated and vapour back to liquid where it is cooled. Turbulence is represented with the standard k-ε model and standard wall functions. A heat-flux boundary condition supplies energy at the evaporator wall, the condenser wall is held at a fixed temperature to promote condensation, and the remaining walls are adiabatic, so that heat transfer occurs only between the active evaporator and condenser regions. The problem is solved transiently to capture the dynamic evolution of the liquid and vapour distributions.

At a transient time of 1.655 s, the results illustrate the coupled evaporation and condensation inside the heat pipe. The liquid volume-fraction contour shows the working fluid concentrated in the lower evaporator region, where the fraction approaches unity, while the upper zones contain little liquid — evidence of vapour formation and its movement toward the condenser. The mass transfer rate contour confirms this directly: positive values in the evaporator mark active vapour generation as the liquid absorbs heat from the wall, while near the condenser the mass transfer decreases as vapour condenses on the cooled surfaces. The temperature contour displays a clear gradient along the pipe, with the evaporator near 323 K and the condenser near 283 K, the difference that sustains continuous phase change and circulation. The velocity-magnitude contour shows enhanced flow near the interface, with vapour driving upward motion through the core and condensate returning slowly downward along the walls.

Together, these transient results reveal the well-developed two-way flow loop characteristic of effective thermosyphon operation — strong evaporation at the heated section and condensation at the cooled section. As a study in mass-transfer modelling, the project demonstrates how a VOF formulation combined with an evaporation–condensation mass-transfer mechanism can reproduce the phase-change cycle that underlies heat-pipe performance, quantifying where and how rapidly mass is exchanged between liquid and vapour throughout the device.