The high performance requirements imposed on automotive fuel cell designs cause a significantly increased build-up of liquid water compared to older generations, accompanied by very rapid gas flows in the channels of the bipolar plate (BPP). This results in a complicated interaction of flows between droplets, plugs and films. In the end, it causes a significantly increased pressure loss in the cell, which is unfortunate in terms of fuel cell efficiency.
This additional pressure loss has to be taken into account when designing the cell, but it cannot be determined with the usual simple two-phase models in flow simulations. Explicit two-phase models such as the Volume of Fluid (VOF) model, on the other hand, allow for an exact representation of reality. The simulations must sometimes be parallelised on several hundred computer cores due to the extreme computational power required. With a systematic study within the AutoStack-Industrie project funded by the German Federal Ministry of Transport and Digital Infrastructure, ZSW investigated how different model parameters (mesh fineness, temporal resolution) should be selected in order to correctly reproduce reality. The influence of channel shape and wettability on the expected pressure drop is determined based on this.
The image shows two instances of such simulations. Water enters the channel (top left) in the form of droplets from the gas diffusion layer (GDL) and coalesces into larger fast-moving accumulations that lead to a short-term increase in pressure loss, ultimately forming a film.