Pressure-driven membrane filtration processes such as microfiltration and ultrafiltration are still hindered by concentration polarization and membrane fouling. Generally in these filtration processes, concentration polarization causes decline of permeate flux and rejection, and fouling leads to permeate flux decline with the increase of rejection. The use of high shear stress for cross flow filtrations has long been considered one of the most efficient methods for overcoming these problems. However, circumferential fluid motion of the hollow fiber membrane surface is also important to avoid formation of a high concentration layer on the surface. In this study, ultrafiltration of humic acid aqueous solution using a polyethersulfone hollow fiber membrane was selected as a model case, and a membrane module with a helical baffle installed around the membrane was used. With the insertion of the baffle, normalized permeate flux and rejection became higher than those without the baffle at the wide range of the feed flow rate. In order to identify the cause of the improvement, CFD simulation was conducted for different baffle geometries. Swirling flow motion generated by the helical baffle around the membrane became more dominant with the lower aperture ratio of the cross sectional area, and there existed the optimum value for the swirling flow generation in terms of the variation of the helical baffle pitch length. The intensity of this fluid motion was characterized by Swirl number and it was found out that high separation performance was obtained at the high Swirl number. (C) 2017 Elsevier B.V. All rights reserved.