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This study examines the effect of spacers and operating conditions (feed inlet temperature, velocity, and feed concentration) on mass transfer characteristics in direct contact membrane distillation (DCMD) systems. The experimental results showed that velocity of the feed and permeate influenced the mass transfer properties more significantly than the temperature and concentration factors. Among the five investigated spacers, the plastic spacer (PL4) had the most influence on mass transfer characteristics, which was proven through analysing the mass transfer coefficient (k s), boundary layer thickness (δ), shear stress (τ) as well as flow property in DCMD spacer-filled channels. Moreover, the mass transfer correlation for each spacer and a general mass transfer correlation for all spacers were determined. Through comparisons between theoretical and experimental Sherwood numbers, the mass transfer correlation developed by Grober for empty channels could be used to describe the mass transfer characteristic in spacer-filled channels in this study. • Propose a new methodology to describe adequately mass transfer properties in spacer-filled DCMD channels. • Identify the mass transfer correlation for all investigated spacers in direct contact membrane distillation module. • Determine the most appropriate spacer for the best improvement in mass transfer property and permeate flux. [ABSTRACT FROM AUTHOR]