Air-cooled proton exchange membrane fuel cells are becoming increasingly attractive as an alternative power source for a large number of applications including material handling, telecom back-up power and more recently unmanned aerial vehicles. A salient advantage is their simplicity of operation because they do not require a secondary coolant loop, which greatly simplifies their control. A recognized problem is the thermal management of the electrochemical reaction has the tendency to overheat the stack at low current densities around 0.4 A/cm² e.g. in case of a Ballard 1020 ACS stack. Our research group has recently found out that, the biggest problem in the thermal management is the heat transfer into the air inside the cathode channels, and we have proposed to place a turbulence grid before the air inlet with the goal to induce a mixing effect in this otherwise laminar flow and thereby improve the heat transfer resulting in better thermal management of the electrochemical reaction. First experiments have resulted in an increase in the limiting current density by 20 % along with an increase in power density by more than 30% and a decrease in hydrogen consumption by more than 20 % at the desired operating point.

The work presented here summarizes a modeling analysis employing computational fluid dynamics with the commercial solver Fluent 18. The goal of this work was to gain additional understanding as to the effect of such a turbulence grid placed before the cathode inlet and ideally to optimize the key parameters such as the detailed grid geometry, the distance between the turbulence grid and the flow channel, and the cathode flow channels themselves. In our model, different types of grids were placed in varying distances before the cathode inlet. Only a single channel on the cathode side is modeled, and the airflow rate was calculated out of the desired stoichiometric flow ratio of 50 and a current density of 0.4 A/cm². Inside the cathode flow channel a heating term that accounts for the waste heat generated by the electrochemical reaction was implemented. The numerical results summarize the distribution of the turbulence intensity induced by the grid, the turbulence dissipation rate, the velocity distribution and, most importantly, the predicted temperature distribution inside the cathode flow channel and the bipolar plate. The single most important result was the predicted maximum temperature inside the fuel cell, and the goal was to improve the heat transfer rate and reduce this temperature so that the fuel cell may be operated at a higher current density. The modeling results will be verified with experiments where the fuel cell performance and the temperature in a pre-determined position will be measured for a variety of turbulence grids printed with the 3D printer.