Enhanced boiling heat transfer on plain and featured surfaces using acoustic actuation

Abstract

Two-phase thermal management based on submerged boiling heat transfer has received considerable attention in recent years because of its potential to achieve high heat fluxes along with its relatively simple hardware and system-level coupling. However, this attractive heat transfer approach has been hampered by the critical heat flux (CHF) limit on the maximum heat transfer, which is caused by the dynamics of the vapor bubbles that form on the heated surface and the subsequent transition to film boiling that produces a large increase in the surface temperature. Recent work at Georgia Tech has exploited low-power ultrasonic acoustic forcing to enhance boiling heat transfer and increase the CHF limit by controlling the formation and evolution of the vapor bubbles and inhibiting the instabilities that lead to film boiling. These effects are investigated with plain and textured (surface-embedded microchannels) boiling heat transfer base surfaces. Even without acoustic actuation, the transfer of makeup fluid to the boiling sites in the presence of surface microchannels passively decreases the surface superheat and increases in the CHF by 218% compared to plain surfaces. Acoustic actuation has a profound effect on the onset and evolution of boiling by inducing interfacial forces that affect the bubbles’ contact line with the surface and lead to bubble detachment. Heat transfer measurements with acoustic actuation demonstrate a significant increase in the CHF as the acoustic field impedes the formation of large vapor columns and inhibits the instability that results in the collapse of these columns into a vapor film. The improvements in the CHF in stagnant bulk fluid exceed 66% for the plain surface (up to 183W/cm2), and 31% for the textured surface (up to 460W/cm2 with a 7°C reduction in surface superheat).