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Author: Allen Patrick Love
Publisher:
ISBN:
Category :
Languages : en
Pages : 190
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Author: Allen Patrick Love
Publisher:
ISBN:
Category :
Languages : en
Pages : 190
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Book Description
Author: Yih-Yun Hsu
Publisher:
ISBN:
Category : Ebullition
Languages : en
Pages : 54
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Book Description
The ebullition cycle of nucleate pool boiling from a heating strip was studied through the use of high-speed motion pictures of schlieren and shadowgraph images. The effects of bubble agitation on thermal layer and neighboring bubbles were observed. An over-all model of ebullition based upon experimental observation was proposed that included an analytical method for predicting bubble growth rate and an analysis of the factors that influence the magnitude of the waiting period (time interval between bubbles at a particular site). Thermal-layer thickness, cavity size, and area of influence of a growing bubble are considered. A film supplement is made available. (Author).
Author: Sumita Basu
Publisher:
ISBN:
Category :
Languages : en
Pages : 185
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Author: Barak Gale
Publisher:
ISBN:
Category : Condensers (Vapors and gases)
Languages : en
Pages : 112
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Author: Jorge Padilla
Publisher:
ISBN:
Category :
Languages : en
Pages : 167
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This dissertation summarizes results of an experimental exploration of heat transfer during vaporization of a water droplet deposited on a nanostructured surface at a temperature approaching and exceeding the Leidenfrost point for the surface and at lower surface temperatures 10-40 degrees C above the saturated temperature of the water droplet at approximately 101 kPa. The results of these experiments were compared to those performed on bare smooth copper and aluminum surfaces in this and other studies. The nanostructured surfaces were composed of a vast array of zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average surface roughness of approximately 0.06 micrometer. Various nanostructured surface array geometries were produced on the copper substrate by performing the hydrothermal synthesis for 4, 10 and 24 hours. The individual nanostructures were randomly-oriented and, depending on hydrothermal synthesis time, had a mean diameter of about 500-700 nm, a mean length of 1.7-3.3 micrometers, and porosities of approximately 0.04-0.58. Surface wetting was characterized by macroscopic measurements of contact angle based on the droplet profile and calculations based on measurements of liquid film spread area. Scanning electron microscope imaging was used to document the nanoscale features of the surface before and after the experiments. The nanostructured surfaces grown by hydrothermal synthesis for 4 and 24 hours exhibited contact angles of approximately 10, whereas the surfaces grown for 10 hours were superhydrophilic, exhibiting contact angles typically less than 3 degrees. In single droplet deposition experiments at 101 kPa, a high-speed video camera was used to document the droplet-surface interaction. Distilled and degassed water droplets ranging in size from 2.5-4.0 mm were deposited onto the surface from heights ranging from approximately 0.2-8.1 cm, such that Weber numbers spanned a range of approximately 0-99. Heat transfer coefficients were determined from thermal measurements in the test apparatus. All experiments were conducted inside an ISO Class 5 clean room enclosure. It was observed that when a liquid water droplet impinged upon the ZnO nanostructured at surface temperatures less than 140 degrees C, the nominally spherical droplet spread into a thin film over the surface. The film thickness depended on many parameters but in general it measured approximately 100-400 micrometers. As a result, it was found that the droplet evaporated by film evaporation without initiating nucleate boiling. At wall superheat levels of 10-20 degrees C, it was found in some cases that the heat transfer coefficients were nearly 4 times greater than for those of nucleate boiling at the same superheat level. For these conditions, no bubble nucleation was observed visually, and, nevertheless, extremely high heat transfer coefficients resulting from rapid evaporation of the thin liquid film formed by the spreading droplet were observed. At high wall superheat levels, the vaporization process exhibited Leidenfrost droplet vaporization. The extreme wetting of the nanostructured surfaces resulted in high Leidenfrost transition temperatures in the range of 310-376 degrees C, among the highest in the literature, exceeding those exhibited by bare metal surfaces by 100 degrees C or more. The Leidenfrost transition was detected from a recording of the acoustic signal generated from each experiment during the deposition and subsequent evaporation process. It was defined as the first point for which there is no disturbance to the acoustical signal in the form of a sizzling sound beyond the initial violent popping generated during the droplet deposition. The results document a trend of increasing Leidenfrost temperature with decreasing contact angle, which is consistent with earlier studies. The results of this study are compared with earlier work in this area and the implications for applications are discussed.
Author:
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ISBN:
Category : Mechanics, Applied
Languages : en
Pages : 620
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Author: G. B. Petrazhitskiĭ
Publisher:
ISBN:
Category : Atomization
Languages : en
Pages : 28
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Author:
Publisher:
ISBN:
Category : High pressure (Science)
Languages : en
Pages : 434
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Author: National Aeronautics and Space Adm Nasa
Publisher: Independently Published
ISBN: 9781724028174
Category : Science
Languages : en
Pages : 28
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Book Description
The objective of this effort is to better understand the physics of evaporation, condensation, and fluid flow as they affect the heat transfer processes in a constrained vapor bubble heat exchanger (CVBHX). This CVBHX consists of a small enclosed container with a square cross section (inside dimensions. 3 x 3 x 40 mm) partially filled with a liquid. The major portion of the liquid is in the corners, which act as arteries. When a temperature difference is applied to the ends of the CVBHX, evaporation occurs at the hot end and condensation at the cold end resulting in a very effective heat transfer device with great potential in space applications. Liquid is returned by capillary flow in the corners. A complete description of the system and the results obtained to date are given in the papers listed. Wayner, Peter C., Jr. and Plawsky, J. L. Glenn Research Center
Author: Van P. Carey
Publisher: CRC Press
ISBN: 1351434861
Category : Science
Languages : en
Pages : 732
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Book Description
Liquid-Vapor Phase-Change Phenomena presents the basic thermophysics and transport principles that underlie the mechanisms of condensation and vaporization processes. The text has been thoroughly updated to reflect recent innovations in research and to strengthen the fundamental focus of the first edition. Starting with an integrated presentation of the nonequilibrium thermodynamics and interfacial phenomena associated with vaporization and condensation, coverage follows of the heat transfer and fluid flow mechanisms in such processes. The second edition includes significant new material on the nanoscale and microscale thermophysics of boiling and condensation phenomena and the use of advanced computational tools to create new models of phase-change events. The importance of basic phenomena to a wide variety of applications is emphasized and illustrated throughout using examples and problems. Suitable for senior undergraduate and first-year graduate students in mechanical or chemical engineering, the book can also be a helpful reference for practicing engineers or scientists studying the fundamental physics of nucleation, boiling and condensation.
