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