Transport phenomena in single-phase and two-phase micro-channel heat sinks

Weilin Qu, Purdue University

Abstract

Various transport phenomena in micro-channel heat sinks were investigated in this thesis work. Included are single-phase pressure drop and heat transfer, incipient boiling heat flux, two-phase flow instability, flow patterns, flow boiling heat transfer, flow boiling pressure drop, and critical heat flux (CHF). A micro-channel heat sink experimental facility was designed and constructed. The micro-channel heat sink was fabricated from oxygen-free copper and contained twenty-one parallel rectangular 231 x 713 μm (215 x 821 μ m for critical heat flux experiments) micro-channels. Deionized water was the working liquid. Single-phase pressure drop and heat transfer were studied experimentally and numerically, with excellent agreement. Experiments were performed to measure incipient boiling heat flux. A mechanistic model based on bubble departure criterion was developed, whose predictions showed good agreement with the experimental data. Two types of two-phase hydrodynamic instability were identified: severe pressure drop oscillation and mild parallel channel instability. The former was eliminated by throttling a control valve upstream of the heat sink. Moderate to high heat fluxes produced mostly annular flow. Experiments were also conducted on flow boiling heat transfer and pressure drop. Previous heat transfer correlations were assessed and deemed inaccurate. Previous two-phase pressure drop correlations were also examined, and a new correlation was proposed which shows better accuracy than prior correlations. An annular flow model was developed, incorporating unique features of two-phase micro-channel flow. Good agreement was achieved between the model predictions and experimental data for both flow boiling heat transfer and pressure drop. Experimental critical heat flux (CHF) was found to be independent of inlet temperature, but CHF increased with increasing mass velocity. A new CHF correlation was proposed for two-phase micro-channel heat sink that shows excellent accuracy in predicting existing heat sink data. Based on the fundamental understanding of the various transport phenomena associated with two-phase micro-channel flow, a comprehensive methodology was developed for optimizing the design of a two-phase micro-channel heat sink. The proposed optimization methodology yields an acceptable design region encompassing all possible micro-channel dimensions corresponding to a prescribed coolant flow rate or pressure drop.

Degree

Ph.D.

Advisors

Mudawar, Purdue University.

Subject Area

Mechanical engineering

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