Optimization of microstructured hollow fiber design for membrane distillation applications using CFD modeling

Abstract

This study explores the potential of microstructured hollow fiber designs to enhance process performance in a direct contact membrane distillation (DCMD) system. Hollow fibers with 10 different geometries (wavy- and gear-shaped cross sections) were evaluated. A series of three-dimensional computational fluid dynamic (CFD) simulations were carried out to investigate their capability in terms of depolarizing the buildup of liquid boundary layers, thus improving water productivity. Analyses of heat and mass transfer as well as the flow-field distribution in respective MD modules were obtained. It was found that the enhancement of the heat-transfer coefficients, hf, was up to 4.5-fold for a module with a wavy fiber design 07 and an approximate 5.5-fold hp increase for a gear-shaped fiber design. The average temperature polarization coefficient and mass flux Nm of the gear-shaped fiber module showed an improvement of 57% and 66%, respectively, over the original straight fiber design, followed by the wavy designs 07 and 08. The enhanced module performance was attributed to the improved hydrodynamics through the flow channels of various fiber geometries, which was confirmed by the visualization of flow-field and temperature profiles in CFD. Investigations of the fiber-length effect showed that the gear-shaped fiber modules exhibited the highest flux enhancement of 57–65% with the same length, compared to the modules with original straight and wavy fibers. In addition, the gear-shaped fiber module is very sensitive to feed velocity changes. Therefore, employing a smart microstructured design on the membrane surface would bring in a significant improvement under adverse flow conditions. Moreover, the computed water production and hydraulic energy consumption (HEC) among the modules with various fiber geometries were compared. With 1.9-fold surface area increase per unit volume, the gear-shaped fiber configuration had the highest water production but the lowest HEC, followed by wavy designs 07 and 08.