Alexandre Guion (IJLRDA) - Advances in subgrid models for boiling heat transfer: a numerical study of vapor bubble growth at a wall

The transport of latent heat makes boiling the most efficient mode of heat transfer, allowing power plants to greatly improve their thermal performance. Subcooled boiling heat transfer for example is able to accommodate very high heat fluxes, thus cooling reactor components very efficiently. Predicting and enhancing boiling heat transfer has therefore garnered significant attention, but remain complicated by the need to consider phenomena occurring over multiple scales, from the adsorbed liquid layer at the wall (nm) up to the bubble diameter (mm). Initially, bubble growth is driven by intense evaporation occurring both at the bubble interface and in the microlayer, a thin liquid layer that remains in contact with the wall underneath the bubble. Evaporation of the microlayer directly fuels bubble growth and affects the thermal response of the wall. In the literature, its representation is typically limited to a very short (order of microns) region near the apparent Triple Phase Line (TPL) between the bubble and the wall. However, experimental observations show that the microlayer may actually extend hundreds of microns beyond the TPL region. Guided by this observation, we develop a robust model to predict the time evolution of the extended microlayer, and the associated corresponding evaporation rate and heat flux underneath a bubble in pool boiling conditions. Most recent experimental benchmark from MIT will be presented and used to assess the validity of the model, implemented as a sub-grid source term in the multiphase fluid dynamics code TransAT. The geometry of the liquid-vapor interface is no longer assumed but calculated, using the Level-Set method. Sensitivity to the initial shape of the microlayer will be discussed. Most recent numerical results will be presented and take advantage of the adaptive mesh refinement capability of the Gerris Flow Solver to resolve with unprecedented accuracy early stages of microlayer formation, for a range of contact angles, growth rates, and system pressures relevant to experimental and industrial conditions.