Numerical modelling of heat and mass transfer during vapour condensation in porous media: Insights from neutron tomography

Moisture condensation in porous media influences heat and mass transport in natural and engineered systems, with implications for energy, environmental, and material applications. This study presents a new framework for analysing coupled heat and mass transfer during vapour condensation in porous materials. The Darcy-scale model, formulated under local thermal equilibrium, incorporates multiphase flow, phase change, and heat transport through standard capillary and permeability relationships. A finite-volume solver was implemented in OpenFOAM to couple heat and vapour transport via a temperature-dependent vapour saturation relationship. An analytical solution for the immobile case, where the condensed liquid remains stationary, defines an effective Péclet number that quantifies the competition between advective and diffusive transport of both heat and vapour. Together with the capillary number (relevant in mobile regimes), representing the balance between pressure and capillary forces, these parameters help classify the condensation regimes. Simulation results reveal that near the condensation front, water flux is driven by capillarity, while farther behind, pressure gradients from vapour injection dominate. Higher effective Péclet numbers yield sharper condensation fronts, while higher capillary numbers enhance liquid spreading near the front. Validation using time-resolved neutron tomography and prior numerical results confirms the ability of the model to capture key condensation mechanisms. The framework also extends to heterogeneous media with fractures, where condensation concentrates near the fracture and gradually diffuses into the surrounding matrix. • A Darcy-scale model captures vapour condensation in porous media with phase change. • Coupled heat and mass transfer is resolved using a finite-volume solver in OpenFOAM. • Two non-dimensional numbers classify front motion and liquid redistribution regimes. • An analytical solution accurately reproduces key trends of the immobile regime. • The model captures saturation and front evolution observed via neutron imaging.