Heat and mass transfer in direct contact membrane distillation: Spatial–temporal analysis of temperature polarization using CFD

Membrane distillation has emerged as a promising thermally driven desalination and wastewater treatment technology that enables the utilisation of low-grade thermal energy. However, its performance is constrained by temperature polarization, which reduces energy efficiency, increases thermal losses, and limits freshwater yield in Direct Contact Membrane Distillation (DCMD) systems. Accurate characterization of temperature polarization is essential for improving efficiency, reducing waste heat, and advancing the sustainability of membrane-based desalination processes. This study investigates the spatial and temporal variations of the temperature polarization coefficient (TPC) in a counter-current flat-sheet DCMD configuration using steady-state and transient 3D Computational Fluid Dynamics (CFD) simulations. Rather than representing TPC as a module-averaged value, this study resolves its spatial distribution across the membrane, revealing values ranging from 0.38 to 0.67, with lower values near the channel walls and higher values near the inlet and outlet regions. A refined TPC metric is proposed in which the conventional inlet bulk temperatures are replaced with cross-sectionally averaged local temperatures, enabling the coefficient to represent the local thermal driving force and improving its sensitivity to spatial and temporal thermal gradients. Unsteady simulations capture dynamic fluctuations in TPC under varying inlet temperature and velocity profiles. The refined metric yields TPC values 16% higher on average than conventional metric and reduces spatial variation by 17%, representing a more uniform distribution, better capturing thermal behaviour under transient inlet conditions. These results underscore the importance of understanding spatial and temporal variation of TPC as a diagnostic performance indicator for improving the efficiency of DCMD systems.