Direct Contact Membrane Distillation (DCMD) and Air-Gap Membrane Distillation (AGMD) have the potential to help reduce water scarcity by providing an efficient and cost-effective method for desalination and water purification. These technologies can be used to treat seawater, brackish water, and other types of contaminated water sources, producing high-quality water suitable for various purposes such as drinking, agriculture, and industrial use. By capturing the flow properties of DCMD and AGMD, researchers and engineers can optimize the operating conditions to achieve maximum performance and efficiency. This can lead to improvements in water productivity, energy efficiency, and cost-effectiveness of these processes. Moreover, understanding the flow properties can help in designing and scaling up these technologies for industrial applications, which can ultimately contribute to addressing the global challenge of water scarcity.

While experimental techniques can provide valuable insights into the flow properties of DCMD and AGMD, computational fluid dynamics (CFD) can be a more efficient and effective approach to study these processes. CFD can provide a comprehensive understanding of the behaviour of the process and can be used to optimize the performance of the process for various applications. To date, CFD studies of DCMD and AGMD use simplified water permeate flux and focus on temperature polarization. However, downstream performance reduction and concentration polarization are other major issues that degrade the MD performance. A logarithmic function of vapour pressure, which is independent of experimentally determined parameters, was used to model water flux across the membrane for the DCMD process. A new equation was derived for AGMD to model water permeate flux passing through the membrane. A 2D computational fluid dynamics model that considers simultaneous mass and heat transfer across the membrane and throughout the channels was developed for both DCMD and AGMD. The coupled conservation of mass, momentum, energy and concentration was solved by considering the conjugate heat and mass transfer. A comprehensive parametric study of DCMD and AGMD was conducted for a wide range of feed and permeate operating conditions. In addition, twenty-two commercial membranes were analysed to obtain a real vision on the influence of membrane characteristics on system performance metrics. Different types of rectangular and semi-circular attached spacers and different types of detached cylindrical spacers to promote turbulence were examined in order to propose the efficient spacer-filled module for both DCMD and AGMD. The effect of solar absorbers including solar membrane and solar plate on the DCMD and AGMD performance were also investigated. It was shown that among the operating conditions, only increasing the Reynolds number improved all the system performance metrics, including higher water flux and lower temperature and concentration polarisation effects. It was proved that unlike AGMD, DCMD suffers from a substantial decrease in trans-membrane water flux along the module. For DCMD, the exit water flux value decreased by 50% in comparison with the inlet value, while the water flux decreased by only 2% for AGMD, using a 1 mm air gap thickness. Cylindrical detached spacers provide a higher water flux than the semicircular and rectangular attached spacers for both DCMD and AGMD. Using the proposed spacer-filled module improves AGMD performance and results 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 less than 3% throughout the module, while it was 21% for the module with no spacer.