The use of gas permeable membranes for bubbleless aeration is of increasing interest due to the energy savings it affords in wastewater treatment applications. However, flow maldistributions are a major factor in the impedance of mass transfer efficiency. In this study, the effect of module configuration on the hydrodynamic conditions and gas transfer properties of various submerged hollow fibre bundles was investigated. Flow patterns and velocity profiles within fibre bundles were predicted numerically using computational fluid dynamics (CFD) and the model was validated by tracer-response experiments. In addition, the effect of fibre spacing and bundle size on the aeration rate of various modules was evaluated experimentally. Previous studies typically base performance evaluations on the liquid inlet velocity or an average velocity, an approach which neglects the effect of geometric features within modules. The use of validated CFD simulations provides more detailed information for performance assessment. It was shown that specific oxygen transfer rates declines significantly with increasing numbers of fibres in a bundle. However, the same trend was not observed when the fibre spacing is increased. A correlation was proposed for the prediction of the overall mass transfer coefficient utilizing the local velocity values obtained from the validated CFD model. (C) 2016 Elsevier B.V. All rights reserved.