A hydrodynamic CHF model for downward facing boiling on a coated vessel

A hydrodynamic critical heat flux (CHF) model has been developed in this study for downward facing boiling on the outer surface of a hemispherical vessel with micro-porous layer coatings. The CHF model considers the vapor dynamics and the boiling-induced two-phase motions in three separate regions adjacent to the heating surface. These include a micro-porous coating layer, a liquid sublayer, and a two-phase boundary layer. Mathematical formulation of the CHF model has been made, taking into account the flow paths to an evaporation site within the porous coating layer, the behavior of the liquid sublayer, and the fluid motions of the two-phase boundary layer. It is found that the local CHF limit is reached when the liquid supply from the two-phase boundary layer through the liquid sublayer and the micro-porous coating to the heating surface is not sufficient to prevent local dryout of the heating surface. The dryout process consists of two sequential stages. In the initial stage, the vapor slug that forms by the downward facing boiling process moves toward the micro-porous coating layer owing to the depletion of the liquid sublayer. In the final stage, depletion of the capillary-assisted liquid supply occurs in the micro-porous coating layer, resulting in the dryout of the heating surface. This is followed by an abrupt increase in the local surface temperature, marking the occurrence of CHF. Comparison of the hydrodynamic CHF model with experimental data has been made and the results are found to be quite satisfactory.