Tuning the Limits of Heat Transfer by Micro/Nano Structures



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Heat localization approach has promised a new route of solar steam generation with higher efficiency than the current bulk heating approaches. In this approach, the material structure localizes the absorbed solar energy, forms a hot spot, and wicks the fluid to the hot spot for steam generation. Non-equilibrium nature of this approach minimizes energy losses leading to its superior performance to the equilibrium approaches. However, so far, the generated steam is only in the ambient pressure, not suitable for high-pressure applications. In the first chapter, we report development of a flexible artificially networked material structure highly efficient for ambient and high pressure steam generation with integrity for large-scale and various geometry implementation. The structure generates steam in the temperature range of 100-156 oC and pressure of 100-525 kPa under the solar irradiation. This material structure promises a robust and highly efficient approach for solar steam generation.

In the last two chapters two different structures are developed for suppression of Leidenfrost phenomenon. Thermal management of high temperature systems is limited by the existence of the Leidenfrost point (LFP), at which the formation of a continuous vapor film between a hot solid and a cooling medium diminishes the heat transfer rate. This limit results in a bottleneck for the advancement of the wide spectrum of systems including electronics/photonics, reactors, and spacecraft. In the second chapter, we report new multi-scale decoupled hierarchical structures to completely eliminate the Leidenfrost phenomenon. This structure allows to independently tune the involved forces and to

suppress LFP. Once a cooling medium contacts these surfaces, by re-routing the path of vapor flow, cooling medium remains attached to the hot solid substrates even at high temperatures (up to 570 oC) for heat dissipation through liquid-vapor phase change with no existence of Leidenfrost phenomenon. These new surfaces offer unprecedented heat dissipation capacity at high temperatures (two orders of magnitude higher than the other state-of-the-art surfaces).

Decoupled hierarchical structure, allows the suppression of LFP completely. However, heat dissipation by the structure at low superheat region was inferior to other surfaces and the structure required an extensive micro/nano fabrication procedure. In the third chapter, we present a metallic surface structure with no LFP and high heat dissipation capacity at all temperature ranges. The surface features nucleate boiling phenomenon independent of the temperature with an approximate heat transfer coefficient of 20 kWm-2 K-1. This new surface is developed in a one-step process with no micro/nano fabrication. We envision that this metallic surface provides a unique platform for high heat dissipation in power generation, photonics/electronics, and aviation systems.



Microfabrication, Heat transfer