A Microscale Study of the Impact of Bubble Growth Dynamics on Surface Heat Flux in Microchannel Flow Boiling Process

In this study, the physics of microscale heat transfer events at the wall-fluid interface during the growth of a moving bubble in a microchannel is analyzed. The study is enabled through development of a novel device that utilizes 53 microscale platinum resistance temperature detectors (RTDs) embedded in a composite substrate made of a high thermal conductivity material coated by a thin layer of a low thermal conductivity material. This sensors arrangement enables resolving the thermal field at the bubble-wall interface with unprecedented spatial and temporal resolutions of 40–65 μm and 50 μs, respectively. To prevent random bubble inception, a 300 nm in diameter cavity is fabricated using a focused ion beam (FIB) at the center of a pulsed function microheater. A detailed analysis of the surface heat transfer events and their relations to time scale of formation and dimensions of bubbles are conducted to decipher the underlying physics of the flow boiling process. Experimental results show that four mechanisms of heat transfer are active as a bubble grows and flows through the channel. These mechanisms of heat transfer are 1) microlayer evaporation, 2) interline evaporation, 3) transient conduction, and 4) micro-convection. The results suggest that the average surface heat flux enhances as the bubble grows in size resulting in expansion of the surface area over which the thin film evaporation mechanism is active. Above a certain bubble size, the average surface heat flux declines due to the formation of a dry region at the bubble-wall interface. Hence, the results indicate that there is an optimal bubble length at which the average surface heat flux is maximum.