2.1 Motivations
A significant enhancement in heat transfer has been reported with the use of micro/nanostructures for the last one decade. Still the fundamental mechanisms are not well understood. As such, the present study has three main aspects with regard to general motivations and research contributions. The first aspect is the scalable nanomanufacturability for boiling applications, the second one is the characterizing and analyzing the factors affecting the boiling enhancement, and the third one is the heat transfer enhancement mechanism for robust and long-term reliability applications.
2.1.1 Manufacturability
While structured surfaces have been shown to greatly increase boiling heat transfer, the fabrication technology is limited by several factors such as the fabrication requires a specific base surface, through-mold electroplating, elevated temperature chemical oxidation, expensive small scale microfabrication technology. As such, a scalable manufacturing technique of surface-bound nanostructures for heat transfer devices is required to be developed.
2.1.2 Physical Mechanism of Enhancement on Structured Surfaces
While structured surfaces have been leveraged to substantially increase the pool boiling CHF and HTC, the fundamental understandings of various enhancement mechanisms are still limited till date due to a variety of factors. Boiling is a complex phenomenon due to the non-linear nature of three phase contact line motion. Also, limited visual observation at high heat fluxes till now makes it even harder to understand
the physics behind the variations in heat transfer performance of different boiling surfaces. Additionally, heat transfer enhancement through micro/nanostructures introduces further complexity in physical understanding of boiling fundamentals. Several different CHF and HTC mechanisms are proposed in the literature. These models are based on several assumptions and hypothesis and were subsequently refined and extended to more precise boiling conditions and geometries. Still all these models are limited to specific surface parameters and valid in a specific range of operation heat transfer limits. Hence, this complex boiling heat transfer phenomenon requires specific and individual treatment of factors affecting the CHF enhancement such as roughness factor, nucleation site density, capillary wicking, and controlled liquid and vapor flow field using the structured surfaces.
2.1.3 Robustness and Long-term Reliability
While the micro/nanostructured surfaces have been shown to significantly increase the pool boiling heat transfer, the reliability of these approaches for real-world applications is in question due to a variety of factors. All structured surfaces are inherently susceptible to mechanical failure (breaking of the micro/nanostructures), as well as fouling and clogging over time. During boiling, any and all contaminants within the fluid will inevitably be drawn into the structures. This will lead to clogging and filling of the small micro/nano-scale voids, and thus a loss of enhancement. Similarly, the robustness of extremely thin nanostructured coatings has not yet been demonstrated, leaving the potential for the coatings to be destroyed or altered over time via chemical reactions. Additionally, they are not effective with all working fluids, in particular highly
wetting fluids like FC-72. Hence, it is required to develop robust and reliable surfaces for high-efficiency and high heat flux boiling applications over long lifetimes.
2.2 Objectives and Contributions to the Field
The aim of this work is to develop scalable heat transfer surface fabrication methods and provide fundamental understanding of heat transfer enhancement mechanisms. Based on the overall goal, the specific objectives for the current work are stated below.
2.2.1 Development of Scalable Nanomanufacturing Techniques
The first objective of this dissertation is to develop a nanofabrication process that can be potentially scaled to large scale heat transfer surfaces. This requires the demonstration of a potential scalable nanomanufacturing technique that is compatible with different metallic and nonmetallic heat transfer surfaces. Additionally, the compatibility with in-situ depositions by which existing systems can be retrofitted with nanocoatings is very important for real world applications. Also, the first objective of this dissertation focuses on demonstrating the mitigation of nanocoatings due to fouling by stripping and re-coating which is not easily applicable to the nanofabrication techniques reported in the literature.
2.2.2 Characterization of Individual Effects of Structured Surfaces on Boiling Heat Transfer
The second objective of this dissertation is to decouple and understand the separate effects of enhanced structures on fundamental mechanisms of boiling heat transfer. This requires characterization and analysis of the role of surface wettability,
wickability, and surface roughness on pool boiling critical heat flux and heat transfer coefficient. In addition, the effect of substrate material, structure material, and morphology, the porosity of the structures with varied structure density, surface degradation due to fouling are important in understanding the physical mechanism of boiling performance. This dissertation also focuses on characterizing the effect of length scale of surface structures, and the effect of boiling fluid properties on pool boiling critical heat flux and heat transfer coefficient to investigate the fundamental physics behind heat transfer enhancement.
2.2.3 Demonstration of Novel and Robust Enhancement Mechanisms
To develop planar engineered heat transfer surfaces with no degradable structures or thin films, and naturally resistant to fouling, clogging is the third objective of this dissertation. As such, this section focuses on demonstrating a novel enhancement mechanism which relies on bubble dynamics and fluid properties rather than surface properties. Another objective of this section is to provide fundamental understanding of enhanced bubble dynamics during CHF and HTC enhancement.
2.2.4 Design and Engineering of High Performance Surfaces
The fourth focus of this dissertation is to engineer enhanced surfaces for increased boiling heat transfer. This section characterizes and analyzes the role of decoupled individual factors affecting pool boiling heat transfer enhancement such as structured coatings, variations in wettability, and spatial thermal gradient. Finally, this section focuses on incorporating all these decoupled factors and analyzes the effect of coupling during boiling enhancement.