A bubble dynamics-based model for wall heat flux partitioning during nucleate flow boiling

Many physical mechanisms are responsible for wall heat transfer during nucleate flow boiling, such as evaporation of microlayers, gradual rewetting, transient conduction, and forced convection. The nature of these mechanisms tightly connects with the complex dynamics of nucleating bubbles (e.g., growth, sliding, and merger), leading to considerable challenges of modeling the partitioning of wall heat flux into these mechanisms. In this study, we proposed a mechanistic model for wall heat flux partitioning relying on the coupling of heat transfer mechanisms with relevant bubble dynamics. The heat transfer via evaporation of superheated liquid (including microlayers) and gradual quenching over dry spots during the bubble growth period was determined as the latent heat transported to growing bubbles using bubble energy balance and growth equations. The heat transfer over the areas swept by bubbles while sliding and merger whose thermal effect is counted from after the bubble departure to the instant it changes to forced convection or nucleation was quantified by the conventional transient conduction combining with the bubble growth equation and wall functions. The residual wall heat transfer corresponds to forced convection over the region unoccupied by bubbles and the region it replaces transient conduction during the remaining period of bubbling cycle. These three primary mechanisms mechanistically constitute the present wall heat flux partitioning model that is physically concrete and confirmed to has good predictability against experimental data for nucleate boiling at a variety of flow conditions. (C) 2017 Elsevier Ltd. All rights reserved.