Low Energy Membrane Distillation: A Numerical Study on the Role of Conductive Spacers

Thermal conductivity is one of the most influencing DCMD performance parameters. Having larger value in the membrane results in lower thermal efficiency and mass flux. However, increasing the conductivity of the fluid, membrane's surfaces, or by means of higher conductive gap have reported unique performance improvements in DCMD. In this work, computational fluid dynamics (CFD) has been used to evaluate the role of highly conductive spacers on the performance of a direct contact membrane distillation (DCMD) system. The cylindrical shape spacer is considered in this work which placed directly on the membrane surfaces. The numerical model assumes a 2D steady state, conjugated heat, and turbulent Navier-Stokes flow. Mass flux is estimated by integrating a combined Knudsen and Poiseuille flow model. The effect of adding conductive and non-conductive spacers on temperature polarization coefficient, mass flux and thermal efficiency as well as the total transmembrane heat has been investigated at different incoming flow velocities. Results showed that copper spacers showed better performance than polymer spacers, but a significant enhancement was not observed from the no spacer case. The cylindrical shape spacers although created turbulence zone, it brought with it localized circulating zone that lead to deteriorating each of the mass flux and thermal efficiency when operating the flow at medium to high Reynolds number.