Impact of cooling unit configuration on coupled thermal–hydration behavior in a PEM fuel cell short stack using a 3D multiphase non-isothermal model

Effective thermal and water management is essential for proton exchange membrane fuel cell (PEMFC) stacks in automotive applications, where limited space restricts the number of cooling units. This study develops a three-dimensional, steady state, multiphase and non-isothermal model of a five cell PEMFC short stack to clarify how cooling unit configuration and cooling intensity affect coupled thermal water and oxygen transport. Two compact layouts are examined, in which one cooling unit serves either two or three cells, while total heat generation and cooling capacity are kept identical. Without cooling units, the stack operates at about 363 to 365 K, indicating a high risk of membrane dehydration. Compared with this baseline, the configuration with one cooling unit serving two cells lowers the central cell temperature by approximately 16 to 18 K, maintains an intermediate and spatially uniform membrane water content above 10.5 in the central cells while avoiding severe oxygen depletion in the catalyst layers. In contrast, the configuration with one cooling unit serving three cells produces a central hot region and a U -shaped hydration profile, with membrane water content exceeding 12.0 in the end cells but decreasing in the center, the oxygen concentration in the central catalyst layer also decreases to about 2.2 mol·m⁻³. In addition, the thermal benefit of increasing the convective heat transfer coefficient becomes much weaker beyond about 3000 to 4000 W·m⁻²·K⁻¹. These results indicate that the two-cell-per-cooling-unit layout provides more favorable thermal uniformity, membrane hydration, and oxygen transport for compact automotive PEMFC stacks.