Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (p. 61-65).Two-phase microchannel heat sinks are of significant interest for thermal management applications, where the latent heat of vaporization offers an efficient method to dissipate large heat fluxes in a compact device. However, a significant challenge for the implementation of microchannel heat sinks is associated with flow instabilities due to insufficient bubble removal, leading to liquid dry-out which severely limits the heat removal efficiency. To address this challenge, we propose to incorporate micro/nanostructures to stabilize and enhance two-phase microchannel flows. Towards this goal, this thesis focuses on fundamental understanding of micro/nanostructures to manipulate liquid and vapor bubble dynamics, and to improve overall microchannel heat transfer performance. We first investigated the role of micro/nanostructure geometry on liquid transport behavior. We designed and fabricated asymmetric nanostructured surfaces where nanopillars are deflected with angles ranging from 7 -52'. Uni-directional liquid spreading was demonstrated where the liquid propagates in a single preferred direction and pins in all others. Through experiments and modeling, we determined that the spreading characteristic is dependent on the degree of nanostructure asymmetry, height-to-spacing ratio of the nanostructures, and intrinsic contact angle. The theory, based on an energy argument, provides excellent agreement with experimental data. This work shows a promising method to manipulate liquid spreading with structured surfaces, which potentially can also be used to manipulate vapor bubble dynamics. We subsequently investigated the effect of micro/nanostructured surface design on vapor bubble dynamics and pool boiling heat transfer. We fabricated micro-, nano-, and hierarchically-structured surfaces with a wide range of well-defined surface roughness factors and measured the heat transfer characteristics. The maximum critical heat flux (CHF) was ~250 W/cm2 with a roughness factor of~-13.3. We also developed a force-balance based model, which shows excellent agreement with the experiments. The results demonstrate the significant effect of surface roughness at capillary length scales on enhancing CHF. This work is an important step towards demonstrating the promising role of surface design for enhanced two-phase heat transfer. Finally, we investigated the heat transfer performance of microstructured surfaces incorporated in microchannel devices with integrated heaters and temperature sensors. We fabricated silicon micropillars with heights of 25 [mu]m, diameters of 5-10 [mu]m and spacings of 5- 10 [mu]m in microchannels of 500 [mu]m x 500 [mu]m. We characterized the performance of the microchannels with a custom closed loop test setup. This thesis provides improved fundamental understanding of the role of micro/nanostructures on liquid spreading and bubble dynamics as well as the practical implementation of such structures in microchannels for enhanced heat transfer. This work serves as an important step towards realizing high flux two-phase microchannel heat sinks for various thermal management applications.by Kuang-Han Chu.Ph.D
