Phase-Field Simulation of Bubble Evolution and Heat Transfer in Microchannels Under Subcooled and Saturated Flow Boiling

This study numerically investigates the growth and dynamics of a single vapor bubble in a rectangular microchannel under subcooled and saturated inlet conditions using the phase-field method coupled with the Lee phase-change model. Results demonstrate that subcooled flow induces early bubble nucleation, pronounced lateral expansion along the heated wall, and prolonged bubble-wall contact due to stronger condensation at the interface and thinner microlayer formation. Enhanced recirculating vortices and steeper thermal gradients promote vigorous evaporation and increased local heat flux, resulting in faster downstream bubble propagation driven by significant axial pressure gradients. Analysis of temperature gradient and heat flux profiles confirms that subcooled conditions produce higher wall heat flux and more frequent peaks in evaporative flux compared to the saturated case, indicating intensified phase-change activity and thermal transport. Conversely, saturated conditions produce more spherical bubbles with dominant vertical growth, weaker condensation, and symmetrical thermal and pressure fields, leading to slower growth and delayed detachment near the nucleation site. These findings highlight the critical influence of inlet subcooling on bubble morphology, flow structures, heat transfer, and pressure distribution, underscoring the thermal management advantages of subcooled boiling in microchannel applications.