Porous Heat Exchanger: Evaporation-Condensation

Evaporation-Condensation Process in a Porous Heat Exchanger

Description

This study investigates the performance of a porous heat exchanger using ANSYS Fluent, focusing on the evaporation-condensation process. The study aims to analyze the complex multiphase flow, heat transfer, and phase change phenomena occurring within the heat exchanger, offering valuable insight into its operation and efficiency.

The heat exchanger geometry, consisting of a series of circular pipes within a rectangular domain, was built in Design Modeler and meshed in ANSYS Meshing, producing a high-resolution grid of 8,777,268 elements to accurately capture the flow and heat transfer phenomena involved.

Methodology

The simulation used a pressure-based solver under transient conditions, enabling time-dependent analysis of the flow and heat transfer processes. The Volume of Fluid (VOF) multiphase model was employed with implicit formulation to capture the interaction between water and vapor phases, with the evaporation-condensation mechanism enabled to simulate the resulting phase change.

Turbulence was resolved using the standard k-epsilon model with standard wall functions, providing a robust approach for capturing the complex flow patterns forming around the pipes, with the energy equation solved to account for heat transfer throughout the domain. The domain itself was modeled as a porous medium, representing the heat exchanger's complex internal structure and enabling a more realistic representation of flow resistance and heat transfer behavior within it.

Conclusion

The mass transfer rate contour reveals significant activity concentrated around the pipes, marking the regions where phase change occurs — red areas indicate evaporation, while blue areas indicate condensation, consistent with the expected behavior of a heat exchanger where the pipes serve as condensing surfaces for incoming vapor.

The static temperature contour shows a clear gradient across the exchanger, with incoming fluid cooling as it flows across the pipes and reaching its lowest temperatures near the pipe surfaces — the temperature distribution that drives condensation and demonstrates the exchanger's effectiveness at transferring thermal energy. Vapor volume fraction plots further reveal high vapor concentration in the bulk flow region and lower concentration near the pipes, directly marking where condensation occurs, while the corresponding water volume fraction contour shows liquid accumulating near the pipe surfaces — particularly below them, consistent with gravity-driven condensate collection.

Velocity magnitude contours show higher flow velocities in the spaces between pipes and lower velocities near the pipe surfaces and in wake regions, a pattern that shapes both heat transfer intensity and condensate accumulation throughout the exchanger. Together, these results confirm the CFD simulation successfully captures the intricate interplay between fluid flow, heat transfer, and phase change within the exchanger, offering valuable insight for optimizing heat transfer efficiency and condensate management in future porous heat exchanger designs.