Numerical modeling of airflow on the cathode-side of a bipolar flow plate : How the formed geometry affects the pressure drop and flow distribution in a hydrogen fuel cell

Sammanfattning: Climate change and rising temperatures is a well-known problem. To tackle global warming a transition from fossil fuels to renewable and reliable energy sources is necessary. Hydrogen, in fuel cells, is proposed to replace diesel and gasoline in the transport sector. Hydrogen is a pure fuel and the fuel cells only emit water and heat as a byproductbyproduct. Combined with electric motors, the hydrogen fuel cell can be 2-3 times more efficient compared to combustion engines fueled by gasoline. The performance of the fuel cell is affected by how the individual parts of the cell are designed. There are some difficulties in manufacturing complex geometries which requires require a forming in more than one step.  The goal isis to investigate, with the help of COMSOL Multiphysics software, how the performance of the fuel cell is affected by the shaped geometry at the cathode side of the flow plate. A numerical model is developeding will be made with varying parameters on the measurement of the cross-sections of the channels where pressure drop and flow distribution for ten different geometries isarewill be investigated. The model iswas built in the COMSOL Multiphysics 6.1 Software and includes a three-dimensional geometry consisting of a gas channel and a gas diffusion layer. The flow is laminar and the gas diffusion layer is set as a porous medium.  The results show that geometries with less sharp edges have lower pressure drops and more uniform flow distribution compared to geometries with sharper edges. The geometry with the sharpest edges has the highest pressure drop of 4.8 Pa/mm and the geometry with rounder edges has the lowest of 3.8 Pa/mm. A relationship between pressure drop and cross-sectional area can be found. With increasing radius and increasing cross-sectional area will the pressure drop decrease. The Reynolds number is higher for sharper geometries since the average velocity in the channels is higher, which also gives a lower friction factor. The length of the top flat becomes less for rounder geometries, which positively affects uniform flow distribution. The geometries with rounder edges have the most uniform distribution at the top of the gas diffusion layer and the sharpest geometry has the least uniform distribution. The deviation from the mean velocity is lower for sharper geometries, mainly because the velocities in the gas diffusion layer are lower. Sensitivity analysis was made over the mass flow rate and mesh, showing that the pressure drop is proportional to the mass flow rate and it becomes higher with less fine mesh.  Less fine mesh also gives lower velocities in the gas diffusion layer. Further studies can be made on how the gas diffusion layer behaves in the fuel cell when adding clamping force to the stack when putting it together and investigate if and how it affects pressure drop and flow distribution. The environmental benefit can be crucial if the performance of the fuel cells improves and motivates the investments which is are needed for, among other things, the infrastructure.