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Pool Boiling Heat Transfer Enhancement Using Microscale Structures for Thermal Management of High-Power Electronics

  • 도수윤 (Su-Yoon Doh, 아주대학교 일반대학원)

초록/요약

The rapid growth of high-power data centers and AI accelerators is driving chip heat flux and power density beyond the capability of conventional air and single- phase liquid cooling. Pool boiling is an attractive passive alternative because it offers very high heat transfer coefficients (HTC) and critical heat flux (CHF), but its use is limited by early CHF, boiling instability, and the fragility or complexity of many surface treatments. This dissertation develops practical microscale porous surface architectures that simultaneously enhance HTC and CHF, clarifies the associated mechanisms, and demonstrates their applicability to thermal management of high-power electronics. First, micro-thick metallic foam (MMF) surfaces are introduced as three- dimensional microporous structures with pore densities of 90 and 130 pores per inch (PPI) and thicknesses of 100–500 µm. Thinning conventional foams to a few hundred micrometers markedly reduces internal conduction and vapor-venting resistances. Structural characterization and wickability tests show that 130 PPI foams possess finer, more uniform pores and stronger capillary suction than 90 PPI foams. Pool boiling experiments in saturated water at 1 atm reveal that all MMF surfaces greatly outperform a plain copper surface; the 130 PPI, 200 µm MMF provides very low wall superheat at a given heat flux, while a 300 µm MMF thickness yields the highest CHF by balancing liquid storage and vapor removal. Next, a dual-layer microporous structure (DMPS) is proposed by combining a lower sintered or roughened powder surface with an upper MMF layer. The lower layer supplies abundant nucleation sites and favorable wettability, and the MMF layer adds highly permeable pathways for capillary liquid supply and vapor escape. Compared with the plain surface, DMPS increases CHF by up to about 225% while maintaining large HTC enhancement, with the best sample achieving CHF near 2700–2900 kW/m² and HTC exceeding 200 kW/m²·K. Droplet wicking tests and high-speed visualization confirm that DMPS rapidly absorbs liquid, suppresses vapor film formation, and partitions liquid and vapor pathways within the foam. To further understand the role of porosity distribution, a gradient dual-porosity structure (GDPS) is fabricated by stacking MMF layers with different pore densities but a fixed total thickness. A “chimney-type” gradient, with smaller pores near the heater and larger pores above, shows simultaneous HTC and CHF enhancement over single-layer MMF. Comparison with a wicking-based CHF correlation indicates an additional micro-chimney effect that strengthens vapor venting beyond what is expected from capillary wicking alone. Finally, the developed surfaces are implemented in a boiling-driven heat spreader for a 1-inch chip and in an immersion cooling configuration using the dielectric fluid HFE-7200, where they substantially reduce thermal resistance and increase allowable heat load, demonstrating a practical pathway toward high-performance phase-change-based cooling for future high-power electronics.

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목차

Chapter 1. Introduction 1
1.1 Background and motivation 1
1.2 Literature review 5
1.3 Objectives and scope 8
Chapter 2. Theoretical background 15
2.1 Overview of pool boiling mechanisms 15
2.2 Governing factors influencing pool boiling 18
2.2.1 Bubble dynamics and local heat transfer mechanisms 18
2.2.2 Wettability and capillary effects 22
2.2.3 Other factors 22
2.3 Surface modification strategies for boiling enhancement 23
2.3.1 Microscale structuring for active nucleation 24
2.3.2 Porous and reentrant geometries for capillary liquid replenishment 24
2.3.3 Wettability and surface energy modification 25
2.3.4 Nanoscale coatings for surface energy control 25
2.3.5 Hierarchical and hybrid structures 26
Chapter 3. Experimental setup and methodology 29
3.1 Experimental setup and test section 29
3.2 Data reduction and uncertainty analysis 33
3.2.1 Data reduction 33
3.2.2 Uncertainty analysis 35
Chapter 4. Result Ⅰ - Micro-thick metallic foam (MMF) 46
4.1 Fabrication and surface characterization 46
4.1.1 Micro-thick metallic foam (MMF) structure analysis 47
4.1.2 Specimen preparation and bonding method 48
4.1.3 Wickability analysis 50
4.2 Pool boiling performance and visualization 52
4.2.1 Comparison of attachment methods: soldering vs. sintering 52
4.2.2 Effect of geometric parameters on boiling performance 55
4.2.3 Visualization results and boiling mechanism analysis 58
4.2.4 Repeatability and reproducibility of MMFs 61
4.3 Summary of Chapter 4 62
Chapter 5. Result Ⅱ - Dual-layer microporous structure (DMPS) 79
5.1 Concept and design of dual-layer structures 79
5.2 Fabrication and surface characterization 83
5.2.1 Fabrication overview and process 83
5.2.2 Surface and cross-sectional morphology analysis 86
5.3 Pool boiling performance and visualization 87
5.3.1 Pool boiling performance of DMPS surfaces 88
5.3.2 Wickability, visualization, and mechanistic analysis of boiling behavior 93
5.3.3 Long-term stability and repeatability evaluation 98
5.3.4 Comparison with other micro/nano scale and hybrid structured surfaces 100
5.4 Summary of Chapter 5 103
Chapter 6. Result Ⅲ – Gradient dual-porosity structure (GDPS) 124
6.1 Concept of gradient dual-porosity structure (GDPS) 124
6.2 Experimental results and visualization 126
6.2.1 Powder-based GDPS fabrication and results 126
6.2.2 Foam-based GDPS fabrication and results 132
6.3 Mechanistic analysis of boiling on foam-based GDPS 134
6.4 Repeatability and reproducibility of GDPSs 138
6.5 Comparative performance map of microscale porous structures 139
Chapter 7. Applications of boiling-driven cooling systems 153
7.1 Boiling-driven heat spreader (BDHS) 154
7.1.1 Overview of BDHS 154
7.1.2 Configuration, fabrication, and experimental procedure of BDHS 156
7.1.3 Thermal resistance analysis and optimization 157
7.2 Immersion cooling with microporous surfaces 160
7.2.1 Experimental configuration for immersion cooling tests 160
7.2.2 Boiling performance of microporous surfaces in immersion cooling 161
Chapter 8. Conclusions 173
References 178
Appendix 192
요 약 194

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