Date of Award

2025

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical and Aerospace Engineering

Advisor 1

Fernando Ponta

Advisor 2

Sajjad Bigham

Committee Member 1

Kazuya Tajiri

Committee Member 2

Sunil Mehendale

Abstract

Since the introduction of transistors, integrated circuits, and silicon packages, the pursuit of higher computational power has driven the electronics industry toward continually shrinking chip sizes and increased power densities. As a result, advanced thermal management has become essential, especially as conventional air and single-phase liquid cooling methods approach their operational limits. Air cooling typically supports heat fluxes of 50–100 W/cm², while liquid cooling can manage up to 800 W/cm². However, with the rise of supercomputers and AI systems, next-generation devices now generate heat fluxes exceeding 1,000 W/cm². This trend makes two-phase flow cooling, particularly flow boiling in microchannels, a compelling solution for dissipating extreme heat within limited surface areas. Among two-phase regimes, slug and annular flow boiling are especially beneficial, as the formation of thin evaporating liquid films produces exceptionally high local heat transfer coefficients. These thin films enable superior heat flux performance compared to other boiling regimes, making them ideal for microscale high heat flux applications. Despite these advantages, challenges remain, such as the risk of dry-out zones as vapor slugs or annular regions expand, and the need to optimize liquid film thickness to maximize overall heat transfer efficiency.

This dissertation introduces novel methods to improve slug flow boiling heat transfer with FC-72 in high heat flux rectangular microchannels. It focuses on reducing dry-out by enhancing the wicking effect, which draws liquid into dry zones, and by optimizing liquid film thickness. Both the extension of liquid film length and the optimization of film thickness are extensively examined to maximize heat transfer in vapor slug and annular flow regimes.

A robust mathematical framework was established to develop an advanced evaporation phase change CFD model using Fluent and custom UDF code. The model, validated with experimental data, accurately delineated the liquid-vapor interface and calculated phase change rates based on mesh element volume fractions. This approach enabled dynamic tracking of vapor evolution, and benchmarking confirmed the CFD model’s reliability for further research stages.

A numerical study investigated how channel width and height affect slug and annular flow boiling in rectangular microchannels. Channel geometry strongly influences local heat flux, with four primary heat transfer mechanisms identified: microlayer evaporation, interline evaporation, transient conduction, and micro-convection. Simulations of five geometries revealed that wider channels form larger thin liquid films at the base, resulting in higher average heat fluxes during slug flow boiling. Measurements showed that wider channels support more evaporation, especially at interfaces with pronounced microlayer convection and interline evaporation, whereas increasing channel height reduces centerline heat flux. The analysis found that film thickness at the centerline is stable, but the bubble tip and rear regions become single-phase conduction zones with thicker films. Overall, wider channels outperform taller ones by sustaining higher heat transfer rates due to thinner average liquid films, offering practical insights for designing microchannels in phase change and boiling applications.

Numerical simulations were performed to optimize wicking and heat transfer in textured microchannels during slug flow boiling, using square pillar textures on the channel base. The study explored pillar spacing, pillar height, and base temperature. Increased pillar spacing enhanced wicking and reduced dry-out, but too much spacing weakened capillary forces. Taller pillars enhanced capillary action and liquid delivery to dry-out zones, although thicker films reduced heat transfer. Higher base temperatures accelerated evaporation but could shorten the liquid film and limit overall heat transfer. The research identified optimal pillar geometry and base temperature combinations for maximizing heat transfer, with base temperature being the most influential factor and channel height having minimal effect on wicking efficiency.

The results of this dissertation show significant advancement in microchannel slug flow boiling, from smooth to textured channel geometries as examined through detailed CFD simulations. Further research is necessary to identify boundary conditions that could further enhance heat transfer in slug flow boiling within textured channels.

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