Analyzing Heat Transfer Mechanisms in Tapered Microgaps and Film Boiling With Computational Modeling Incorporating Nucleation Sites and Subcooled Fluids

Applications such as nuclear reactors, data centers, and microelectronics require an understanding of the hydrodynamic and thermal behavior of liquid and vapor phases on heated surfaces. Phase change and interfacial dynamics enhance heat transfer in evaporation and condensation systems. This study simulates film evaporation, subcooled nucleate boiling, and confined boiling in tapered microgaps using customized Ansys Fluent with user-defined functions (UDFs) for sharp interface tracking and temperature-gradient-driven mass transfer. In film evaporation, the peak local heat transfer coefficient (HTC) reached 2170 W/(m²·K) at 0.5 mm film thickness. The maximum Nusselt number was 5.69 at 0.5 mm and the minimum was 1.20 at 12 mm, reflecting the thermal resistance of the vapor layer. In subcooled boiling, increasing subcooling from 1 to 5 K reduced departure diameter (from 2.3 to 1.6 mm), lengthened thermal film thinning (from 0.8 to 1.2 mm), and increased HTC from 90,000 to 115,000 W/(m²·K). Tapered microgap evaporation revealed self-propelling vapor structures that rewet the surface, suppress the thermal layer, and raise local HTC above 30,000 W/(m²·K) within 1 mm of the interface. Interface shape and motion agreed with high-speed imaging results. In microgravity, reduced buoyancy promotes vapor film buildup and degrades heat transfer. Simulations with modified surfaces and condensation effects, along with tapered microgaps, generated continuous self-propelled bubble departure, disturbing the thermal layer and enhancing mixing. Peak HTC in microgravity exceeded 30,000 W/(m²·K) for single and multiple nucleating bubbles. This work advances understanding of interfacial transport, bubble dynamics, and passive vapor removal, offering design guidance for next-generation cooling systems in terrestrial electronics and spacecraft.