Volume-Of-Fluid simulations of boiling and electrolysis

In studying nucleate boiling in the microlayer regime and hydrogen production in electrolysis, the geometric Volume-of-Fluid (VOF) method is used with Basilisk's quad/octree-based adaptive mesh refinement (AMR).

For boiling, the conjugate heat transfer between the fluid and solid is resolved by solving the heat conduction in both the fluid and solid simultaneously. At the fluid-solid boundary, the continuity of heat flux is imposed, and the interfacial heat transfer resistance (IHTR) is considered through a temperature contact discontinuity. However, the approximate projection method for AMR causes nonphysical oscillations in phase-change flows, leading to incorrect microlayer dynamics and heat/mass transfer predictions. To address this, a ghost fluid method is adopted, replicating experiments from MIT. This method achieves grid convergence, aligns well with previous and experimental numerical results, and significantly reduces CPU hours by several hundred times. The impact of contact angle on nucleate boiling in the microlayer regime is then explored, showing that while hydrodynamic effects vary, thermal effects remain consistent.

For electrolysis, the two-phase problem is solved using a one-fluid approach, with a single set of Navier-Stokes equations for the entire domain. At the fluid-solid boundary, a constant hydrogen flux is applied and the mass transfer equation can be solved by writing Henry’s formula based on the jump conditions for the concentration of species at the interface. This approach allows for effective tracking of the interface and simulation of bubble growth and detachment processes. The method has been verified for studying bubble detachment mechanisms, deformation, and coalescence in 2D axisymmetric and 3D simulations, demonstrating good predictions of mass transfer and capture dynamic effects. The results indicate that bubble detachment is affected by contact angle and surface tension, while bubble deformation and coalescence also play an important role in early detachment. Future work will involve more full 3D simulations to explore the effects of convection and bubble curtains on bubble detachment.