Hybrid DEM-VoF Multiphase Model for Microchannels of Electrochemical Devices

In proton exchange membrane (PEM) electrolyzers and in low-temperature PEM fuel cells, the reaction process leads to the formation of two-phase flow in microchannels. Therefore, the secondary phase needs to be removed effectively to ensure high performance and longevity of the device. A deeper understanding of this removal process is needed to optimize the cell design of electrochemical devices. Computational fluid dynamics (CFD) is widely used to investigate multiphase flow. Because different flow regimes ranging from disperse to continuous flow occur in the microchannels of such devices, the selected multiphase model needs to capture different length scales. The volume of fluid (VoF) method requires high mesh resolution to accurately capture the interface between the phases. Consequently, for disperse flow, this method incurs a high computational cost. In contrast, the Euler-Euler model might be computationally more efficient but is only applicable to the disperse phase, so it cannot capture the wide range of length scales present in the system. To bridge this scale gap efficiently, hybrid models have been developed that use different model equations depending on the local flow topology. In literature, the combination of the Euler-Euler model for the disperse phase with a VoF model for large interfaces is the most common. While effective for large-scale processes, these models neglect surface tension in the disperse phase, preventing accurate prediction of the detachment dynamics of droplets or bubbles in microchannels. In this contribution, we propose using a hybrid model that couples a discrete element method (DEM) for the disperse phase and its transition to large structures with a VoF method for large structures. The DEM is an extension of the Euler-Lagrange framework to finite-size particles. This DEM-VoF approach is intended to capture detachment dynamics accurately and, as a result, to enhance prediction of multiphase flow in microchannels. First, we develop the DEM component for modeling detachment dynamics and validate it by comparison to experimental data from literature. Next, we describe the coupling strategy between VoF and DEM and demonstrate the application of this methodology. With this approach, a more physical and efficient simulation for two-phase flow in microchannels of electrochemical devices is possible.