Heat-transfer performance comparison between overlapped and woven spacers for membrane distillation

Sustainable production of fresh water from seawater desalination is a problem of crucial importance nowadays. Recently, some desalination technologies are taking advantage from the coupling with renewable resources. Among emerging technologies, Membrane Distillation (MD) is considered as one of the most promising as it can be easily powered by solar thermal energy or waste-heat. As an emerging technology, efforts are required to optimize MD unit geometry and operating conditions in order to reduce fresh water production specific cost. Temperature polarization phenomenon is a well-known detrimental effect for the MD process. Spacers are traditionally used to enhance mixing and shrink temperature boundary layers yet yielding higher pressure-losses. The present work is devoted to testing the performance of two different two-layer net-spacers: overlapped and woven. Investigations were carried out by both experiments and Computational Fluid Dynamics (CFD) simulations at different Reynolds numbers ranging from creeping to turbulent flow regime. Experiments (intermediate to high Re) were performed via a novel experimental technique making use of thermochromic liquid crystals and digital image analysis. Computational data (low to intermediate Re) were obtained via steady state (low Re) and Direct Numerical Simulations (intermediate Re), adopting the Unit Cell approach. A good agreement between experiments and CFD data was found in the range of superposition. Results showed that woven spacers guarantee a better mixing than overlapped ones especially in the low-intermediate range of Re thus resulting into a higher Nusselt number. On the other hand, the less disturbed flow field induced by the overlapped wires was found to give raise to lower pumping losses.