Multiscale Mechanistic Approach to Enhance Pool Boiling Performance for High Heat Flux Applications

The advent of cloud computing and the complex packaging architecture of next generation electronic devices drives methods for advanced thermal management solutions. Convection based single-phase cooling systems are inefficient due to their large pressure drops, fluid temperature differences and costs, and are incapable of meeting the cooling requirements in the high power density components and systems. Alternatively, phase-change cooling techniques are attractive due to their ability to remove large amounts of heat while maintaining uniform fluid temperatures. Pool boiling heat transfer mechanism centers on the nucleation, growth and departure of a bubble from the heat transfer surface in a stagnant pool of liquid. The pool boiling performance is quantified by the Critical Heat Flux (CHF) and Heat Transfer Coefficients (HTC) which dictate the operating ranges and efficiency of the heat transfer process. In this work, three novel geometries are introduced to modify the nucleation characteristics, liquid pathways and contact line motion on the prime heater surface for a simultaneous increase in CHF and HTC. First, sintered microchannels and nucleating region with feeder channels (NRFC) were developed through the mechanistic concept of separate liquid-vapor pathways and enhanced macroconvection heat transfer. A maximum CHF of 420 W/cm2 at a wall superheat of 1.7 °C with a HTC of 2900 MW/m2°C was achieved with the sintered-channels configuration, while the NRFC reached a CHF of 394 W/cm2 with a HTC of 713 kW/m2°C. Second, the scale effect of liquid wettability, roughness and microlayer evaporation was exploited to facilitate capillary wicking in graphene through interlaced porous copper particles. A CHF of 220 W/cm2 with a HTC of 155 kW/m2°C was achieved using an electrodeposition coating technique. Third, the chemical heterogeneity on nanoscale coatings was shown to increase the contribution from transient conduction mechanisms. A maximum CHF of 226 W/cm2 with a HTC of 107 kW/m2°C was achieved. The enhancement techniques developed here provide a mechanistic tool at the microscale and nanoscale to increase the boiling CHF and HTC

Critical Heat Flux for Flow Boiling of Water at Low Pressure on Smooth and Micro-Structured Zircaloy Tube Surfaces (KIT Scientific Reports ; 7627)

Flow boiling of a liquid is characterized by a high heat transfer from the heated surface into the fluid. However, the nucleate boiling regime is limited by the occurrence of the critical heat flux (CHF) leading to a possible thermal damage of the heated surface. The influence of tube surfaces structured with elevations in nano- and microscale, micro-channels, and porous or oxidized layers on CHF was experimentally investigated in a low pressure steam/water test facility

Experimental investigation of pool boiling performance with ethanol and fc-87 on open microchannel surfaces

The growing trend in miniaturization of electronics has generated a need for efficient thermal management of these devices. Boiling has the ability to dissipate large quantity of heat while maintaining a small temperature difference. Vapor chamber with pool boiling offers an effective way to provide cooling and maintaining temperature uniformity. The objective of the current work is to investigate pool boiling performance of ethanol and FC - 87 on microchannel surfaces. Ethanol is an attractive working fluid due to its better heat transfer performance and higher heat of vaporization compared to refrigerants, and lower boiling point compared to water. The saturation temperature of ethanol can be further reduced to temperatures suitable for electronics cooling by lowering the system pressure. Fluorocarbons are considered to be ideal fluids for electronics cooling due to their low normal boiling point, dielectric and inert nature. FC - 87 is selected for the current work. Ethanol is tested at four different absolute pressures, 101.3 kPa, 66.7 kPa, 33.3 kPa and 16.7 kPa using different microchannel surface configurations. Heat dissipation in excess of 900 kW/m2 was obtained while maintaining the wall surface below 85 °C at 33 kPa. Flammability, toxicity and temperature overshoot issues need to be addressed before practical implementation of ethanol-based cooling systems in electronics cooling application. FC - 87 with microchannel yields average performance when compared to literature. Effect of surface area is identified as the key reason for performance enhancement. A new finned structure is developed, which gave a heat flux value 1.25 MW/m2 at 40 °C wall superheat for FC - 87 at atmospheric conditions

Characterization of pool boiling heat transfer from porous-coating-enhanced surfaces

Development of techniques for enhancement and optimization of thermal management technologies has been a highly active area of research in recent decades in response to the rapid emergence of compact, high-power electronic systems. Immersion cooling by boiling is one of the preferred methods for high power density applications, due to its passive nature and high heat transfer coefficients obtained. Pool boiling heat transfer has been extensively studied in recent decades to understand the inherent mechanisms yielding the high heat transfer rates, as well as to further enhance the heat transfer by simple modifications or additions to existing approaches. This thesis aims to provide detailed fundamental analysis of heat transfer enhancement by surface-coating-based boiling enhancement methods and to quantitatively analyze the dependence of heat transfer performance on coating properties. For passive systems, which cannot afford active cooling due to either form factor or other application constraints, two-phase pool boiling heat transfer provides highly effective immersion cooling. Surface enhancement techniques, such as surface coatings, may further augment the cooling efficiency of such systems. The experimental study presented in this thesis analyzes the effects of variation of particle size on the pool boiling performance of FC-72 obtained by free-particle and sintered-coating enhancement techniques. In the free-particle technique, loose copper particles are placed on a heated copper surface, whereas in the sintered-coating technique, copper particles are sintered to the copper surface. The particle coatings provide additional vapor nucleation sites in the cavities formed at particle-surface and particle-particle contact points, thereby enhancing boiling performance over a polished surface. The boiling performance enhancement is studied for particle sizes varying from 45–1000 µm at a constant coating layer thickness-to-particle diameter ratio (δ/d) of approximately 4 for both techniques. High-speed flow visualizations are performed to understand the boiling patterns and bubble departure parameters that provide explanations for the trends observed in the boiling curves. The measured wall superheat is observed to be significantly lower with a sintered coating compared to the free-particle layer for any given particle size and heat flux. Performance trends with respect to particle size, however, are remarkably similar for both enhancement techniques, and an optimum particle size of ~100 µm is identified for both free particles and sintered coatings. The free-particle technique is shown to offer a straightforward method to screen the boiling enhancement trends expected from different particulate layer compositions that are intended to be subsequently fabricated by sintering. From the experimental investigation of pool boiling from coated surfaces, it was observed that for a given surface coating, several additional parameters might affect the heat transfer performance of the surface, and the coating porosity and particle morphology did not vary independent of each other. Hence, to further understand the effects of coating properties, pool boiling heat transfer of FC-72 is studied from coatings with independently varying coating porosities with two different particle morphologies. Surfaces are fabricated with same size particles (90–106 µm) having different morphologies, viz., spherical and irregular, at a constant coating layer thickness-to-particle diameter ratio (d/d) of approximately 4, with porosities varying over a wide range (∼40%–80%). The morphology and size of the particles affect the pore geometry, porosity, permeability, thermal conductivity, and other characteristics of the sintered coating. In turn, these characteristics impact the heat transfer coefficient and critical heat flux (CHF) during boiling. The porous structure formed by sintering is quantitatively characterized using image analysis and numerical simulation based on micro-computed tomography (µ-CT) scans to study the geometric and effective thermophysical properties of the coatings. Critical coating properties affecting the boiling performance metrics are identified, and regression analysis is employed to observe the dependence of these metrics on the coating properties. Coatings with irregular particles or lower porosity are observed to yield higher heat transfer coefficients than those with spherical particles or higher porosities. The relative strength of dependence of the heat transfer coefficient and CHF on the coating porosity, pore diameter distribution, particle diameter distribution, particle sphericity distribution, necking and interfacial areas, permeability, and thermal conductivity of the coatings are determined. The importance of high-fidelity coating characterization to understand the heat transfer behavior of coatings is demonstrated. Plans for future work are outlined based on the current findings. Proposed additional studies include investigation of the single bubble dynamics to further the understanding of bubble behavior that influences the heat transfer performance for free-particle and sintered-coating techniques. The quantitative regression analysis from µ-CT scans may be further extended to include a more rigorous model that employs codependence of critical inputs, to determine a predictive correlation for the boiling heat transfer performance based on coating properties

Development of Highly Functional, Surface Tunable, and Efficient Composite Coatings for Pool Boiling Heat Transfer Enhancement

Rapid growth and advancements in high-power electronic devices, IC chips, electric vehicles, and lithium-ion batteries have compelled the development of efficient and novel thermal management solutions. Currently used air and liquid cooling systems are unable to remove the heat efficiently due to significant pressure drops, temperature differences, and limited heat-carrying capacities. In contrast, phase-change cooling techniques can remove the larger amount of heat with higher efficiency while maintaining safer operational temperature ranges. Pool boiling heat transfer is a type of phase-change cooling technique in which vapor bubbles generated on the boiling surface carry away the heat. This pool boiling performance is limited by the maximum heat dissipation capacity, quantified by the Critical Heat Flux (CHF), and efficiency of the boiling surface, quantified by the Heat Transfer Coefficient (HTC). This work emphasizes on improving both CHF and HTC by developing highly surface functional and tunable microporous coatings using sintering and electrodeposition techniques. Initially, graphene nanoplatelets/copper (GNP/Cu)-based composite coatings were developed using a multi-step electrodeposition technique. And 2% GNP/Cu coating rendered the highest reported CHF of 286 W/cm² and HTC of 204 kW/m²-°C with increased bond strength. To further enhance the cohesive and adhesive bond strength of the electrodeposited coatings, a novel multi-step electrodeposition technique was developed and tested on copper-based coatings. This technique dramatically improved the overall functionality, pool boiling performance, and durability of the coatings. Later, a sintering technique was used to develop the coatings using GNP and copper particles. Uniform spreading of GNP over the coatings was obtained via ball milling technique. This technique yielded a CHF of 239 W/cm² and the HTC of 285 kW/m²-°C (~91% and ~438% higher than a plain copper surface, respectively). A novel approach of salt-templated sintering was developed in the final part to attain a better control on porosity and wicking properties of the sintered coatings. This generated interconnected porous networks with a higher nucleating activity, and attained record-breaking CHF of 289 W/cm² and the HTC of 1,314 kW/m²-°C

Fundamentals of microlayer evaporation and its role on boiling heat transfer enhancement

Boiling, a dynamic and multiscale process, is widely used in industrial applications as it can transfer a large amount of heat over a small surface area. It has been studied for over five decades; however, a comprehensive understanding of the process is still lacking. The bubble ebullition cycle (nucleation, growth and departure) happens over a very short time-span (milliseconds) making it challenging to study the near-surface interfacial characteristics of a single bubble, which involves a microlayer. The microlayer is a thin film present at the bubble base and varies from nano-scale to micro-scale in thickness. The dynamics of the microlayer dictates bubble growth and departure, making it of significant importance in understanding the fundamental behavior of the boiling phenomenon, and is the focus of this work. Firstly, a new mechanism of boiling enhancement based on the additional evaporation of the thin film is proposed and validated by fabricating micro-ridges on a surface and testing its boiling performance. A critical height of the ridges is found to exist, determined by the film thickness, below which no enhancement is observed while above which similar enhancements are achieved regardless of the ridge height. An analytical model is developed to determine the critical height from the experimental results. Secondly, the effect of ridge spacing on boiling enhancement is investigated. A ~120% enhancement in the critical heat flux is attained with only 18% increase in surface area due to the presence of ridges. The new enhancement mechanism is determined to be the early evaporation of microlayer, which leads to an increase in the bubble growth rate and departure frequency. Three enhancement regions are mapped based on ridge spacing and height: full enhancement region, partial enhancement region, and no enhancement region. The mechanism of early evaporation of microlayer is further verified by comparison of the bubble growth rate of a laser-created vapor bubble on a ridge-structured surface and on a plain surface. Next, in-situ imaging of microlayer and contact line region is performed in a steady-state vapor bubble created by laser heating. The in-situ measurement of contact angle of vapor bubbles is conducted. For the laser power studied, the contact line readily forms in regular DI water which contains dissolved air, while in degassed water, the microlayer covers the entire bubble base. The vapor bubble contact angle is found to resemble a drop contact angle on the same surface if the three-phase contact line forms; otherwise it is dependent on the curvature of the microlayer and the bubble, and decreases with increasing heating power. The overall heat transfer coefficient and width of the evaporating region in the microlayer are estimated using experimental data and finite-element-method based numerical simulations, thus defining an upper limit to the heat transfer coefficient possible in nucleate boiling and thin-film evaporation. Finally, the contact line and microlayer behavior during bubble formation, growth, and movement is investigated. During bubble formation, the microlayer initially covers the entire bubble base, decreases in thickness as the bubble grows, and eventually forms the three-phase contact line. The surface wettability strongly affects contact line motion. On hydrophobic Trichlorosilane (FOTS) surface, the bubble remains adhered to the surface and does not follow the laser movement; while on hydrophilic SiO2 surface, the bubble moves smoothly following the laser. This difference in behavior is determined to originate from the change in direction of the liquid-vapor surface tension force, which results in a negative net force on FOTS surface inhibiting the bubble from following the laser, while it results in a positive net force on SiO2 surface causing the bubble to move on the surface

Development of nanowire structures on 2d and 3d substrates for pool boiling heat transfer enhancement

Boiling is a common mechanism for liquid-vapor phase transition and is widely exploited in power generation, refrigeration and many other systems. The efficacy of boiling heat transfer is characterized by two parameters: (a) heat transfer coefficient (HTC) or the thermal conductance; (b) the critical heat flux (CHF). Increasing the CHF and the HTC has significant impacts on system-level energy efficiency, safety and cost. As the surface modification at nano-scale has proven to be an effective approach to improve pool boiling heat transfer, the enhancement due to combination of nanomaterials with micro-scale structures on boiling heat transfer is an area of current interest. In this study, metallic- and semiconductor- material based nanowire structures were fabricated and studied for boiling enhancement. A new technique is developed to directly grow Cu nanowire (CuNW) on Si substrate with electro-chemical deposition, and to produce height-controlled hydrophilic nanowired surfaces. Using a two-step electroless etching process, silicon nanowire (SiNW) have been selectively fabricated on top, bottom, and sidewall surfaces of silicon microchannels. An array of the SiNW coated microchannels functioned as a heat sink and was investigated for its pool boiling performance with water. This microchannel heat sink yielded superior boiling performance compared to a sample substrate with only microchannels and a plain substrate with nanowires. The enhancement was associated with the area covered by SiNWs. The sidewalls with SiNWs greatly affected bubble dynamics, resulting in a significant performance enhancement. The maximum heat flux of the microchannel with SiNW on all surfaces was improved by 150% over the microchannel-only heat sink and by more than 400% over a plain silicon substrate. These results provide a viable solution to meet the demands for dissipating a high heat transfer rate in a compact space, with additional insight gained into the boiling mechanism for the microchannel heat sinks with nanostructures

Surface engineering for phase change heat transfer: A review

Owing to advances in micro- and nanofabrication methods over the last two decades, the degree of sophistication with which solid surfaces can be engineered today has caused a resurgence of interest in the topic of engineering surfaces for phase change heat transfer. This review aims at bridging the gap between the material sciences and heat transfer communities. It makes the argument that optimum surfaces need to address the specificities of phase change heat transfer in the way that a key matches its lock. This calls for the design and fabrication of adaptive surfaces with multiscale textures and non-uniform wettability. Among numerous challenges to meet the rising global energy demand in a sustainable manner, improving phase change heat transfer has been at the forefront of engineering research for decades. The high heat transfer rates associated with phase change heat transfer are essential to energy and industry applications; but phase change is also inherently associated with poor thermodynamic efficiency at low heat flux, and violent instabilities at high heat flux. Engineers have tried since the 1930s to fabricate solid surfaces that improve phase change heat transfer. The development of micro and nanotechnologies has made feasible the high-resolution control of surface texture and chemistry over length scales ranging from molecular levels to centimeters. This paper reviews the fabrication techniques available for metallic and silicon-based surfaces, considering sintered and polymeric coatings. The influence of such surfaces in multiphase processes of high practical interest, e.g., boiling, condensation, freezing, and the associated physical phenomena are reviewed. The case is made that while engineers are in principle able to manufacture surfaces with optimum nucleation or thermofluid transport characteristics, more theoretical and experimental efforts are needed to guide the design and cost-effective fabrication of surfaces that not only satisfy the existing technological needs, but also catalyze new discoverie