The flow maldistribution and the consequent performance deteriorations in a cross flow hollow fiber membrane module used for air humidification are investigated. The effects of structure-induced flow maldistributions on the deteriorations in humidification efficiencies are studied. As a first step, a CFD code is used to calculate the flow distribution in the exchanger, by treating the hollow fiber membrane core as a porous media. Then a coupled heat and moisture transfer model between the air flow and the water flow is set up. The shell-and-tube core is equivalently converted to a cross-flow parallel-plates heat mass exchanger. Using the CFD predicted flow distributions on the core face, the sensible cooling and humidification efficiencies are calculated with a finite difference scheme. The results indicate that the packing fraction affects the flow maldistribution substantially. The consequent sensible cooling and humidification efficiencies are influenced significantly. Depending on air flow rates, the sensible cooling efficiencies can be deteriorated by 3-30%, and the humidification efficiencies can be deteriorated by 26-58%. (C) 2012 Elsevier B.V. All rights reserved.