An SEM Surface Study of Nucleate Pool Boiling Heat Transfer to Saturated Liquid Nitrogen Reduced Pressures from 0.1 to 0.9 PDF Download
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Author: David Virgil Porchey
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 284
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Book Description
"This study constituted an examination of both nucleate boiling and maximum heat flux characteristics of liquid nitrogen boiling from characterized surfaces. Several textures of gold plated copper surfaces were studied in addition to silver and silver oxide surfaces. Data were taken over a wide range of reduced pressures. All surfaces were examined using a Scanning Electron Microscope (SEM). Data and micrograph comparison revealed that both surface chemistry and surface roughness affect the nucleate boiling characteristics of liquid nitrogen. These effects are separated from each other showing that nucleate boiling is a function of both surface chemistry and topography. Surface roughness was found to have a significant effect on the maximum heat flux with rougher surfaces yielding higher heat flux values. The temperature difference at burnout was found to be a function of both surface chemistry and roughness. Some existing correlations for both nucleate boiling and maximum heat fluxes were examined and shown to inadequately predict the data, primarily because of inability to account for surface roughness and/or surface-fluid interaction as affected by surface chemistry. A maximum temperature difference correlation tested predicted burnout temperature differences well. The necessity to age a surface in the fluid to be boiled was found not to be a result of adsorbed gases, but probably a result of impurities acquired on the surface that must be removed. Attempts to observe hysteresis is were unsuccessful. Hysteresis was found not to be a function of adsorbed inert gases. Data indicated that one mechanism of heat transfer by nucleate boiling will not suffice for all pressures. An existing nucleate boiling correlation was modified to allow for a mechanism change and predicted the data better than other correlations tested"--Abstract, pages ii-iii.
Author: David Virgil Porchey
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 284
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Book Description
"This study constituted an examination of both nucleate boiling and maximum heat flux characteristics of liquid nitrogen boiling from characterized surfaces. Several textures of gold plated copper surfaces were studied in addition to silver and silver oxide surfaces. Data were taken over a wide range of reduced pressures. All surfaces were examined using a Scanning Electron Microscope (SEM). Data and micrograph comparison revealed that both surface chemistry and surface roughness affect the nucleate boiling characteristics of liquid nitrogen. These effects are separated from each other showing that nucleate boiling is a function of both surface chemistry and topography. Surface roughness was found to have a significant effect on the maximum heat flux with rougher surfaces yielding higher heat flux values. The temperature difference at burnout was found to be a function of both surface chemistry and roughness. Some existing correlations for both nucleate boiling and maximum heat fluxes were examined and shown to inadequately predict the data, primarily because of inability to account for surface roughness and/or surface-fluid interaction as affected by surface chemistry. A maximum temperature difference correlation tested predicted burnout temperature differences well. The necessity to age a surface in the fluid to be boiled was found not to be a result of adsorbed gases, but probably a result of impurities acquired on the surface that must be removed. Attempts to observe hysteresis is were unsuccessful. Hysteresis was found not to be a function of adsorbed inert gases. Data indicated that one mechanism of heat transfer by nucleate boiling will not suffice for all pressures. An existing nucleate boiling correlation was modified to allow for a mechanism change and predicted the data better than other correlations tested"--Abstract, pages ii-iii.
Author: Calvin B. Cobb
Publisher:
ISBN:
Category :
Languages : en
Pages : 152
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Author: Philip George Kosky
Publisher:
ISBN:
Category :
Languages : en
Pages : 162
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Author:
Publisher:
ISBN:
Category :
Languages : en
Pages : 308
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Author: Bruce David Marcus
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 444
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Author: R. C. Noyes
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 42
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Author: Shailesh Malla
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 124
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Book Description
The present work consists of two major studies. The first study investigates the effects of surface energy or wettability on nucleate pool boiling and the second study investigates the thin-film evaporative cooling for near junction thermal management. For the first study, effects of surface energy or wettability on critical heat flux (CHF) and boiling heat transfer (BHT) of smooth heated surfaces was studied in saturated pool boiling of water at 1 atm. For this purpose hydrophilic and hydrophobic surfaces were created on one side of 1cm x 1cm double-side polished silicon substrates. A resistive heating layer was applied on the opposite side of each substrate. The surface energies of the created surfaces were characterized by measuring the static contact angles of water sessile drops. To provide a wide range of surface energies, surfaces were made of Teflon (hydrophobic), bare silicon (hydrophilic) and aluminum oxide (most hydrophilic). The measured contact angles on these surfaces were ~108, ~57 and ~13 degrees respectively. The results of pool boiling tests on these surfaces clearly illustrate the connection between surface energy and CHF. CHF was shown to linearly decrease with contact angle increase, from ~125 W/cm2 on aluminum oxide (most hydrophilic) to nearly one tenth of this value on Teflon (hydrophobic). The most hydrophilic surface also produced increasingly better BHT than plain silicon and Teflon as heat flux increased. However, below ~5 W/cm2 the hydrophobic surface demonstrated better heat transfer due to earlier onset of nucleate boiling, reducing surface superheats by up to ~5 degrees relative to the other two surfaces. Above ~5 W/cm2 the BHT of the hydrophobic surface rapidly deteriorated as superheat increased towards the value at CHF. To further understand the effect of surface energy on pool boiling performance, the growth and departure of bubbles from single nucleating sites on each surface were analyzed from high-speed video recordings. A distinct bubble behavior was observed in the hydrophobic surface where bubble growth and departure period was extremely long compared to plain silicon and aluminum oxide surfaces. This study also investigated the performance of thin-film evaporative cooling for near-junction thermal management. A liquid delivery system capable of delivering water in small volumes ranging 20~75 nl at frequencies of up to 600 Hz was established. On one side of the silicon chip, a resistive heating layer of 2 mm x 2 mm was fabricated to emulate the high heat flux hot-spot, and on the other side a superhydrophilic nanoporous coating (SHNC) was applied over an area of 1 cm x 1 cm. With the aid of the nanoporous coating, delivered droplets spread into thin films of thicknesses less than 10[mu]m. With this system, evaporative tests were conducted in ambient in an effort to maximize dryout heat flux and evaporative heat transfer coefficient. During the tests, heat flux at the hot spot was varied to values above 1000 W/cm2. Water was delivered at either given constant frequency (constant mass flow rate) or at programmed variations of frequency (variable mass flow rate), for a given nanoliter dose volume. Heat flux and hot spot surface temperatures were recorded upon reaching steady state at each applied heat flux increment. Relative to bare silicon surface, dryout heat flux of the SHNC surface was found to increase by ~5 times at 500~600 Hz. Tests were also conducted at various system pressures and temperatures in a micro-gap to emulate the actual embedded thermal management system. The micro-gap was made by positioning a top cover plate 500 [mu]m above the test surface. System temperature did not influence the hotspot temperature. This was due to the formation of near saturation temperature inside the micro-gap for all cases as a result of vapor accumulation. Increase in system pressure increased the hotspot temperature. At 1500 W/cm2, hotspot temperature increased by 6 C and 24 C by increasing the system pressure by 7.32 and 14.7 psi respectively. This was due to increase in saturation point as a result of increase in pressure. On the SHNC surface a mixed mode of heat transfer comprising of thin-film boiling and thin-film evaporation was observed particularly at moderate heat flux (~700 W/cm2). To further enhance the heat transfer coefficient, aluminum microporous coating was developed that increased the number of nucleation sites for thin-film boiling and also maintained the wettability for thin-film evaporation at higher heat fluxes. Test results showed a marginal improvement in dry-out heat flux compared to SHNC, however, significant reduction was achieved in hot-spot temperature at all heat flux levels. A net reduction of ~ 58oC was obtained at ~1600 W/cm2 by using aluminum based microporous coating.
Author: Michael Damon Maynard
Publisher:
ISBN:
Category :
Languages : en
Pages : 86
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Book Description
A system was designed to investigate experimentally the mechanism of nucleate pool boiling heat transfer using commercial grade liquid nitrogen as a working fluid. A circular horizontal flat plate five square centimeters in area was utilized as a boiler surface. The effect of various surface parameters on the characteristic heat flux versus (Tw-Ts) curve at atmospheric conditions was determined. Surfaces used included a highly polished mirror surface, a mirror surface coated with oxide, a mirror surface coated with grease, a roughened surface, and a roughened surface coated with Teflon, all fabricated from commercial electrical tough pitch copper. The final surface used was a Nickel 200 highly polished mirror surface. The data from the copper surface with a mirror finish was in agreement with the data of previous investigations. The effects of roughness, contaminants, hysteresis, and boiler surface material were discussed. A comparison was made on incipient boiling heat fluxes. The results indicate that the surface conditions play a major role in nucleate pool boiling heat transfer to liquid nitrogen.
Author: Kenneth Orion Beatty
Publisher:
ISBN:
Category : Heat engineering
Languages : en
Pages : 412
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Author: Satish R. Parikh
Publisher:
ISBN:
Category : Heat
Languages : en
Pages : 170
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"The object of this investigation was to study the effect of micro-roughness, a thin layer of oil, and infra-red radiations on the cylindrical heat transfer surface of copper, in liquid nitrogen and at atmospheric pressure during nucleate boiling. An attempt was made to find any possible effect of orientation of the heat transfer element on nucleate boiling. A number of different runs were taken with different degrees of roughness, and different orientation positions. Nucleate boiling has been found to be nearly independent of the heat transfer surface roughness and of orientation while strongly dependent on the chemical nature of the surface. Extremely high temperature differences were obtained in the nucleate boiling region by application of a thin layer of oil to the heat transfer surface. Temperature gradients in both the axial direction and the radial direction were found for the surface of the heat transfer element"--Abstract, leaf iii.
