Yueyang Zhao

Date of Award

Summer 2020

Document Type

Open Access Dissertation


Mechanical Engineering

First Advisor

Chen Li


Due to the shrinking size and increasing power of electronics, the heat flux generated by electronic devices has greatly increased and imposed challenges to thermal management technologies. Numerous cooling technologies have been developed to cool high-power electronics. Among them, flow boiling in microchannels or a microgap has been one of the most efficient ways to remove high heat flux, which requires less coolant compared with single-phase cooling techniques. However, the flow boiling instability is the one of the most challenging issues, which lead to the premature critical heat flux (CHF) conditions and deteriorate heat transfer rate. The capillary force generated along the microchannel or microgap can enhance the liquid rewetting process and hence, delay CHF conditions. Moreover, flow boiling heat transfer coefficient (HTC) can also be enhanced because of promoting thin liquid film evaporation.

Superhydrophilic surfaces can greatly enhance the capillary force in the microchannel

or microgap. In this study, a new kind of superhydrophilic nano-structure (CuO) coatings have been developed and applied in the copper microgap. Microgap heat sinks with superhydrophilic nano-structured walls have been systematically characterized. Deionized (DI) water was selected as the coolant. The microgap dimension is 26 mm × 5.5 mm × 0.3 mm (length, width, height). Heat was applied on the bottom surface. Single and two-phase heat transfer has been characterized with flow rates ranging from 151kg/m2s to 757.7kg/m2s on both bare and black CuO coated surfaces. Visualization studies show the high-frequency rewetting processes and twolayer flow were observed in the entire flow channel, which can significantly enhance the thin film evaporation and delay the dry-out process and increase the CHF. The

CHF on the black CuO coated surface can be enhanced nearly 3 times compared with that on the bare surface at all flow rates. The 170% higher HTC has been realized compared with the bare surface. Higher supersaturations prior to surface flooding cause the decreasing of the surface temperature. Surface temperature became more uniform on the black CuO coated surface because of high frequency rewetting process and thin film evaporation. Note that all enhancements have been achieved only with slightly elevated pressure drop.


© 2020, Yueyang Zhao