(Invited) Bubble Trouble: Simulation of Molten Oxide Electrolysis in Reduced Gravity

Molten Oxide Electrolysis (MOE) is a leading contender for processing extra-terrestrial minerals because it produces oxygen and liquid metal without the use of consumables. During electrolysis a gravitational field stratifies gas, electrolyte, and metal from each other by density. Processing on the moon, with ~1/6g, or Mars, with ~1/3g, will reduce the buoyancy force slowing bubble velocity and lowering convection intensity. Low thermal conductivity, (~1Wm-1K-1) and high viscosity, (~1Pa⋅s) of molten regolith contribute to thermal and chemical transport being dominated by convection, even as convection velocity decreases. We explore MOE system dynamics in Earth’s (1g), Mars’ (1/3g), and the Moon’s (1/6g) gravity through simulations that include composition and temperature dependent material properties and two phase flow. We implement the level set method to track the interface between the bubble laden flow near the anode and the bubble free flow far from the anode. By grouping bubble dynamics into the material properties of the fluid near the anode we relax the requirements for a fine mesh to track individual bubble-fluid interfaces, and are able to use a mesh that is finer than that required for a bubbly flow mixture model. Thus, all physics can be captured on one mesh. This is particularly relevant because bubble sizes for proposed systems are 1/10 to 1/100 of the system scale. While other simulations have been presented and experiments performed in micro and zero gravity for gas evolving electrolysis systems, none have investigated the downward facing electrode, heat transfer, or species diffusion. The focus of this work is to address this gap in the literature through simulation. Here, multiphysics simulation is supported by inspection of dimensionless groups and observations from other electrolysis systems. This work demonstrates that combining heat and mass transfer with temperature and composition dependent material properties is critical to designing MOE and other electrolysis systems for earth and beyond.