Characterization of nucleate pool boiling from smooth and rough surfaces
Abstract
The effects of surface roughness on nucleate pool boiling heat transfer and bubble ebullition characteristics are studied using fluids with different wetting characteristics and surfaces with different roughness and surface structure. Heat transfer characteristics are described for pool boiling of deionized water and perfluorinated dielectric liquid HFE-7300 from novel porous copper evaporator surfaces with and without carbon nanotubes. A hybrid surface composed of sintered copper particles and interstitial carbon nanotube arrays exhibited pool boiling heat transfer coefficients for HFE-7300 improved by 23% and increased critical heat flux by more than 40% relative to a plain flat copper surface. Heat transfer measurements and quantitative measurements of bubble ebullition characteristics are obtained for pool boiling of Fluorinert FC-77 from a smooth and a rough aluminum surface and for pool boiling of Fluorinert FC-72 from ITO coated glass surfaces with widely varying roughness. Extant correlations for bubble departure diameter and ebullition frequency are compared to the experimental data for FC-77. Bubble diameter at departure was shown to increase with increasing wall superheat, but the surface roughness was also shown to have an influence. Bubble departure frequency, which in general also increased with heat flux, was not well predicted by any of the correlations considered from the literature. Nucleation site density and bubble terminal rise velocity were well predicted by correlations. New image processing techniques are developed and demonstrated for an automated bubble measurement routine applied to high-speed video images of boiling of FC-72 from narrow ITO heaters. A new model is proposed for the relevant characteristic length scale of interaction between bubbles and the boiling surface. The large amount of bubble diameter data, growth and ebullition histories, and void distribution data resulting from the automated measurements in this work is used to identify and statistically assess useful forms of correlations between heat transfer data, bubble measurement data, and surface roughness measures based on the new length scale model. Boiling heat transfer coefficients were found to decrease linearly with surface roughness as defined according to the proposed length scale model. Average bubble departure diameters were found to decrease with heat flux and with surface roughness. Average bubble ebullition and growth periods were found to decrease with heat flux and to increase with bubble departure diameter. Bubble growth was found to vary as the one-third power of time, while the maximum value of void fraction above the boiling surface was found to vary as the two-thirds power of heat flux.
Degree
Ph.D.
Advisors
Garimella, Purdue University.
Subject Area
Mechanical engineering|Computer science
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