Mechanistic model of wall heat transfer for vertical subcooled boiling flow

Nucleate boiling rate is significantly affected by heat flux partitioning from the heated wall during subcooled boiling flow. Therefore, precisely modeling wall heat flux partitioning is crucial for the realistic prediction of boiling two-phase flows. In this study, a new mechanistic model based on bubble dynamics and associated heat transfer mechanisms is proposed for wall heat flux partitioning under vertical subcooled boiling flow. The proposed model categorizes the wall heat flux into four components: evaporation, nucleation quenching, sliding transient conduction, and forced convection. Unlike previous models, the nucleation quenching and sliding transient conduction components, both triggered by sliding bubbles, are modeled separately owing to their different active times in the new model. Additionally, the proposed model incorporates suppression factors into the existing model for nucleation site density to address boiling suppressions induced by subcooled liquid and static bubble interactions. Evaluation results reveal that the new model, including its closure models, exhibits a good prediction capability in a wide range of thermal-hydraulic conditions covering 298–3527 kg/m²s of mass flux, 100–6000 kW/m² of heat flux, 5–83.3 K of liquid subcooling, and 1–6 bar of pressure.