Impact of Liquid Subcooling and Condensation on the Local Heat Transfer Coefficients in Bubble Growth Over a Heated Surface

Abstract Boiling heat transfer, particularly in subcooled conditions, plays a critical role in advanced thermal management systems such as nuclear reactors, data centers, and aerospace cooling modules. This study presents a numerical investigation of nucleate subcooled boiling across five subcooling levels. Ansys Fluent was customized with user-defined functions (UDFs) to resolve the coupled thermal–fluid interactions by directly modeling interfacial mass transfer, enforcing saturation temperature at the interface, and maintaining interface sharpness. The framework demonstrates strong agreement with both experimental and semi-empirical benchmarks, with average errors below 11%. Results show a clear trend of decreasing departure diameter (from 2.3 to 1.6 mm) and increasing horizontal thermal film thinning length (from 0.8 to 1.2 mm) as the subcooling level rises from 1 to 5 K. Furthermore, as subcooling increases from 1 K to 5 K, local heat transfer coefficients rise from 90,000 to 115,000 W/m2·K. Velocity magnitudes near the interface increase due to stronger condensation-induced momentum transfer, with peak values rising from 0.30 m/s at 1 K to 0.82 m/s at 5 K, while the shear stress influence region simultaneously expands from 2.2 mm to 3.4 mm and its magnitude increases from 90 Pa to 320 Pa. These findings provide new insights into the interplay of subcooling, interfacial heat transfer, and fluid motion, offering predictive capability for the design and optimization of next-generation phase-change cooling technologies.