Study of spacer structure on the enhancement of heat and mass transfer in direct contact membrane distillation modules

Membrane distillation technology, which combines the membrane process with evaporation, has many benefits, including operation at atmospheric pressure, simple configuration, usage of low-grade heat sources, etc. In the current work, a new type of curved spacers was invented for the direction contact membrane distillation. A three dimensional numerical method was established, which coupled the transmembrane mass transfer with fluid flow in the channel by adding user-defined functions. Different distances between the spacer and the top wall, defined as d, were studied. 4 straight spacers (Sd0, Sd1.5, Sd3,Sd4.5) and 4 curved spacer (Cd0, Cd1.5,Cd3,Cd4.5) were investigated in detail. The curved spacers performed better in mixing and enhancing permeate flux than the straight spacers. The averaged performance enhancement factor (αc) was introduced to evaluate the performance enhancement of the curved spacers compared to the straight spacers. When d was 1.5 mm, αc was 1.33, which was the maximal among the studied cases. The curved spacer performed better in terms of heat transfer, but at a considerable cost in terms of pressure drop. The comprehensive evaluation index (Pec) was proposed to evaluate the overall performance of heat transfer and pressure drop. Pec varied with Reynolds number (Re) and spacer configurations. When Re was in 200 - 550, the maximal value of Pec was achieved by Cd1.5 spacer. When Re was in 500 - 920, the maximal value of Pec was achieved by Cd3 spacer.