A hollow fiber membrane-based bundle or humidifier (HFMH) is used for air humidification in the process of membrane type seawater desalination, where seawater flows inside the fibers and the process air stream flows across the fiber bundle. The HFMH has a staggered arrangement of cylindrical fiber tubes with equal transverse and longitudinal pitches. The forced convection heat and mass transfer in the cross-flow tube bundle under naturally formed boundary conditions are investigated numerically on a periodic computational cell. A membrane-based air humidification-dehumidification type sea-water desalination (MHDD) system is constructed and used to validate the model. The mathematical model proposed here takes into account of the fiber-to-fiber interactions. The typical velocity vector profiles, temperature contours as well as the concentration contours for both the air stream and saline water stream are plotted to disclose the heat and mass transfer mechanisms. The effects of air Reynolds numbers (ranging from 50 to 300) as well as the module packing fractions (ranging from 0.126 to 0.503) on the fluid flow and heat mass transfer properties of the module are investigated. A set of correlations are also proposed for the prediction of the friction factor, mean Nusselt numbers and Sherwood numbers in air side. These fundamental data is of interest for the design and optimization of the HFMH, a promising membrane type seawater desalination system. (C) 2017 Elsevier Ltd. All rights reserved.