In this study, the effect of water vapor condensation on the flow distribution in the manifold of a PEMFC stack is investigated. A solver is developed in OpenFOAM software and is validated against experimental and numerical data. Due to the condensation, the water vapor concentration in the mixture decreases near the manifold walls. Thus, beginning and last cells receive more oxygen than the middle cells. The condensed water forms small droplets on the manifold walls. The small droplets coalesce into larger droplets. Descending large droplets into the last cell of the 26-cell stack leads to the formation of the slug. This process results in blockage of flow through the last cell, causing a significant increase in the flow non-uniformity. The non-uniformity index increases significantly from 0.046 to 0.94 in the water removal process. When the number of cells increases from 26 to 39, the large droplet detaches from the manifold and sinks into seven middle cells. The flow passing through these cells significantly reduces, resulting in a severe flow maldistribution. A comparison between flow distribution parameters indicates that the root mean square value of the deviation from the mean flow rate is a better criterion than the non-uniformity index to determine the severity of maldistribution.

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