Experimental and Numerical Investigations on Gas Injection-Enhanced Air Gap Membrane Distillation for Water Desalination

Gas-liquid two-phase flow enhanced air gap membrane distillation (AGMD) was investigated experimentally and numerically for water desalination. A carbon molecular sieve tube was used as the separation membrane, and nitrogen gas was selected as the injection gas. A computational fluid dynamic simulation was carried out numerically to re-emerge the AGMD with gas injection using the FLUENT software based on the volume of the fluid model. Experimental results showed that the flow pattern was the bubble flow, plug flow, slug flow, churn flow, and annular flow in sequence as the gas holdup increased. The gas injection-enhanced AGMD can significantly increase the freshwater permeate flux with the maximum of 4.7 times the value without enhancement at the churn flow. Simulation results showed good agreement with the experimentally measured flow patterns and permeate fluxes. The gas-liquid two-phase flow can promote the liquid disturbance within the membrane tube and enhance the momentum, heat, and mass transfer processes of AGMD. The gas injection enhanced the average wall shear stress, Nusselt number, temperature polarization coefficient, and mass transfer coefficient were all greater than those without gas injection. The concentration polarization coefficient with gas injection was smaller than that without gas injection. Nusselt number and mass transfer coefficient correlations were obtained based on the heat convection theory and the Knudsen-molecular diffusion mechanism. The conclusion drawn in this work provides guidance for the gas-liquid twophase flow enhancement of membrane distillation.