SUMMARY

Two-phase heat transfer involving boiling and condensation in a liquid pool is widely used to accommodate high heat fluxes. However, coupling this attractive approach to system-level heat transfer is hampered by the rate-limiting steps of vaporization (inhibited by the critical heat flux limit on the maximum heat transfer rate) and condensation (limited by the subcooled liquid temperature and heat transport at the liquid-vapor interface). The performance of thermal systems that utilize two-phase heat transfer can be significantly enhanced by independent augmentation of boiling and condensation using nonintrusive, low-power acoustic actuation at the flow boundary that exploits the acoustic properties mismatch at the liquid-vapor interface. The present investigations focus on the fundamental mechanisms of acoustic enhancement of two-phase heat transfer at long and short actuation wavelengths (order 1 m and 1 mm, respectively). It is shown that surface capillary waves induced by long wavelength actuation enhance condensation by forcing mixing at the interfacial thermal boundary, while short-wavelength actuation enhances boiling by affecting vaporization and advection of vapor bubbles to extend the critical heat flux limit, and enhances direct contact condensation by bulk deformations at vapor-liquid interface that inject subcooled liquid into the vapor volume.