Computational fluid dynamics modelling of air-gap membrane distillation: Spacer-filled and solar-assisted modules

Air-gap membrane distillation (AGMD) is a novel method of water purification and promises to reduce heat requirements. However, AGMD is characterized by low water permeate flux and a significant downstream performance reduction including temperature, concentration polarisation and membrane fouling. These challenges are difficult to explore both experimentally and numerically. To date, computational fluid dynamics (CFD) of AGMD focuses on temperature polarisation without considering solute transport. In addition, they lacked an accurate calculation of water flux affecting the distributed flow properties, especially close to the membrane. A 2D comprehensive study using CFD simulation of the AGMD was developed to determine the effectiveness of solar absorbers and spacer filaments on these challenges. A precise logarithmic function of vapour pressure was used to model the mass transfer within the membrane. The simulation was in excellent agreement with previously published experimental results. Results showed that using solar absorbers can slightly increase the water flux and decrease both the temperature and concentration polarisation effects. Additionally, the results were more sensitive to the air-gap thickness compared to using solar absorbers. Results also proved that cylindrical detached spacers provided higher water flux when compared to semicircular and rectangular attached spacers. The proposed spacer-filled module improved the AGMD performance and resulted in the uniform water flux from the inlet to the outlet. The water flux increased by 15 %, and the downstream performance variation of the developed module was <3 % throughout the module, compared to 21 % for the module with no spacer. This is a very encouraging development for low-energy water purification systems.