Author

Congcong Ren

Date of Award

Fall 2020

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Chen Li

Abstract

Microchannel flow boiling is one of the most desired cooling solutions for high power electronics. Owing to the high latent heat of vaporization, high heat fluxes can be achieved through phase-change heat transfer. However, the enhancements of (critical heat flux) CHF and heat transfer coefficient (HTC) are usually inhibited by the transitional flow patterns, which highly influence the liquid rewetting. In the past two decades, many techniques have been explored to enhance the liquid rewetting in microscale.

In this dissertation, four parallel micro-grooves fabricated on the bottom of five microchannels (W=200 μm, H=250 μm, L=10 mm) were designed to promote the liquid rewetting ability through the enhanced capillary pressure. Experiments were conducted with deionized water flowing through the micro-grooved microchannels at a wide mass flux range. The experimental studies demonstrate that the configuration of micro-grooves is effective in mitigating the local dry-out and then significantly increases CHF without escalating the pressure drop. On the other hand, enhanced thin film evaporation was also observed to increase the heat transfer rate.

Another topic of this dissertation is the transient heat transfer performance of flow boiling in microchannels under dynamic heat loads. New understandings about microchannels with the microgrooves or the multiple micronozzles were developed to study the performance of the devices to dissipate heat that generated instantly. The heat was applied to the system in the form of heat pulse and the transient behaviors of the wall temperature, the wall temperature increase rate and the HTC were investigated in this study. The video was captured synchronously, and different flow patterns were matched with the transient thermal characteristics. The effects of heat pulse amplitude and mass velocity were investigated. The results were compared to that of plain-wall microchannel and significant enhancement was noticed along all the boiling stages. The highest enhancement of the HTC in the stable stage was 225% with mass flux of 380 kg/m2s and heat flux of 160 W/cm2. Different from the flow boiling phenomena with DI-water, the boiling of HFE-7100 expanded to the entire channel instantly. A temperature spike was noticed at the ONB resulted from the growing vapor layer beside the wall after the boiling was initiated, which enabled the identifying of ONB from temperature profile. Apart from the visualization study, the effect of heat pulse amplitude on the ONB time and ONB temperature was determined.

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