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Highlights:  X-ray computed tomography used to reconstruct three different types of GDL  Models of GDLs analysed using Lattice Boltzmann method  Invasion pattern and saturation of water in the GDLs is controlled by wettability  Liquid water travelled with a stable displacement under hydrophilic angles  Conversely, at hydrophobic contact angles it travelled with capillary fingering

Available online xxxKeywords: Thermal management Water balance Fuel cell vehicle PEM Heat transfer Cooling a b s t r a c tDespite having efficiencies higher than internal combustion engines, heat rejection from fuel cells remains challenging due to lower operating temperatures and reduced exhaust heat flow. This work details a full system simulation which is then used to compare a conventional liquid cooled fuel cell system to two types of evaporatively cooled fuel cell systems. Both steady state and transient operation are considered. Results show the radiator frontal area required to achieve thermal and water balance for an evaporatively cooled system with an aluminium condensing radiator is 27% less than a conventional liquid cooled system at 1.25 A/cm 2 steady state operation. The primary reason for the reduction is higher heat transfer coefficients in the condensing radiator due to phase change. It is also shown that the liquid water separation efficiency has a significant influence on the required radiator frontal area of the evaporatively cooled system.

Available online xxxKeywords: Thermal management Thermal Stack Evaporative Cooling PEM a b s t r a c t Maintaining proton exchange membrane fuel cell (PEMFC) stack operating temperature across transient current profiles presents a significant challenge for fuel cell vehicles.Liquid cooled systems require active control of coolant temperature and flow rate to match heat rejection to heat generation. Evaporative cooling is an alternative to conventional liquid cooling in automotive sized PEMFC stacks. In an evaporatively cooled system, liquid water is injected directly into the cathode flow channels where it evaporates, both cooling and humidifying the stack. This paper uses a validated simulation to explore the inherent temperature regulation abilities of an evaporatively cooled PEMFC stack across a range of current profiles and drive cycles. Results show that throughout the normal operating current range, stack temperature varies by less than ± 2.0 C, this is comparable to liquid cooling but without the need for active temperature control. The introduction of variable operating pressure and cathode stoichiometry using proportional integral control, can further reduce temperature variation to ± 1.0 C and ±1.2 C respectively for step increases in current demand. Variable operating pressure is also shown to improve warm up time and reduce heat loss at low operating loads.