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Please use this identifier to cite or link to this item: http://hdl.handle.net/1860/3894

Title: Biotemplated nanostructured surfaces for enhanced two-phase heat transfer
Authors: King, Stephen M.
Keywords: Mechanical engineering;Film boiling;Nanostructured materials
Issue Date: Jun-2012
Abstract: The Leidenfrost phenomenon is the stable film boiling of a liquid droplet in proximity to a surface whose temperature is much higher than the boiling point of the liquid; caused by rapid vaporization of the droplet before it can come in contact with the surface. This scenario makes conduction through insulating vapor the only means of evaporating the droplet. It is desirable to offset or delay the onset of this occasion of poor heat transfer, the Leidenfrost point, in situations where droplet-to-surface contact is paramount; namely, nuclear reactor safety. Increasing surface hydrophilicity through the implementation of micro, nano, and hierarchical structures has been shown to increase phase-change heat transfer across a surface. In this work, attempts to delay the Leidenfrost phenomenon by tailoring surface wettability with various microstructures conformally coated with novel Tobacco mosaic virus-templated nickel nanostructures are made. Because of a solution based fabrication process, Tobacco mosaic virus nanostructures are potentially more integratable into real-world systems than traditionally fabricated nanostructures. Micropost arrays are patterned in such a way to determine whether the time limiting mechanism in Leidenfrost droplet evaporation is heat transfer or mass transfer dependent. In all, seven patterns (28 samples) with varying thermal conductivities, two-dimensional permeabilities, and heights are: fabricated from SU-8 (photolithography) and silicon (deep reactive-ion etching); tested to determine each surfaces’ Leidenfrost point, i.e. the longest droplet evaporation time, through the use of a high-speed camera in concert with a stopwatch; and compared to the performance of flat and nanostructured only surfaces. SU-8 hierarchical posts outperform silicon hierarchical posts, increasing the Leidenfrost point by 175°C (20μm tall) and 250°C (40μm tall) over flat samples, with silicon hierarchies showing only modest improvement, < 100°C. Increase in two-dimensional permeability shows no extractable trend in Leidenfrost point; however, contact angle and post height appear to have less of an effect on the Leidenfrost point as two-dimensional permeability increases. Leidenfrost temperatures decrease with increasing effective thermal conductivity and there also appears to be a trend of increasing Leidenfrost point with increasing thermal resistance of post arrays. A model derived from the Navier-Stokes equations has been developed which predicts mass transfer as the limiting mechanism in Leidenfrost droplet evaporation. While this model does not accurately predict the values of evaporation times, it does predict the overall trend of decreasing evaporation time with increasing surface porosity.
Description: Thesis (M.S., Mechanical engineering)--Drexel University, 2012.
URI: http://hdl.handle.net/1860/3894
Appears in Collections:Drexel Theses and Dissertations

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