Fluid flow and heat and mass transfer in a hollow fiber membrane bundle for air humidification (MBH), which is the key process of a membrane desalination system, are investigated. The MBH is like a cross-flow shell-and-tube heat exchanger where heated seawater flows inside the fibers and the process air stream flows across the fiber bundle. Fluid flow and heat and mass transfer in the bundle are studied by considering the air side turbulence which exists even when the Reynolds numbers are as low as 50. Air side, water side and membrane side transfer equations are solved together in a conjugated way. Two turbulence models namely the standard k-epsilon model (STD k-epsilon) and the low-Reynolds-number k-epsilon model (Low Re k-epsilon) are adopted for the prediction of turbulent kinetic energy and its dissipation rate. In order to validate the numerical results and to compare the performance of the different turbulence models, a membrane-based air humidification-dehumidification desalination (MHDD) system is designed and constructed to aid the investigation. Effects of module packing fractions and Reynolds numbers as well as the transverse and longitudinal pitches of the fibers on the mean Nusselt, Sherwood numbers and friction factors for air flow and water flow are analyzed. Correlations for performance analysis are regressed.