Ball bearings are commonly used to reduce the friction in rotating mechanical components. The present work reports improved numerical approaches to model bearing thermal conductance in the absence of convection. We start by modeling the thermal pathway across a single ball-to-race pathway for a simplified geometry, an azimuthally symmetric ball in contact with a flat surface (ball-on-flat). A first-principles approach is used to calculate the static lubricant meniscus shape using a custom-developed Python code. We apply the finite element method (FEM) to extract total thermal conductance of the lubricated ball-on-flat system. Using similar methods, we also present a three-dimensional numerical model of the lubricant meniscus in a static angular contact ball bearing section (ball-on-race). By generating thermal conductance correlations for Yovanovich's classic lubricated model, our two-dimensional multiphysics model, and our three-dimensional multiphysics model, we enable comparison of the results of all three models. To compare them, parametric studies are conducted to illustrate the effect of lubricant volume and applied load on the total thermal conductance for each model. The hierarchical methodology reported here improves both the fidelity of tribo-thermo-mechanical modeling and establishes a reference for the accuracy of commonly used geometric approximations for thermal transport in spacecraft ball bearings.

The effect of surface roughness on nucleate boiling heat transfer is not clearly understood. This study is devised to conduct detailed heat transfer and bubble measurements during boiling on a heater surface with controlled roughness. This second of two companion papers presents an analysis of heat transfer and bubble ebullition in nucleate boiling with new measures of surface roughness: area ratio, surface mean normal angle, and maximum idealized surface curvature. An additional length scale of importance, the maximum base diameter of an emergent bubble, is identified. Measurements of bubble departure diameters, growth periods, ebullition periods, and void fraction above the surface are obtained from high-speed videographic visualizations by an automated procedure. Correlations of heat transfer coefficient and bubble ebullition characteristics with different measures of surface roughness are compared in terms of relative uncertainty. The data set of results for pool boiling in the perfluorinated dielectric liquid, FC-72, are found to correlate best with a length-scale filtered value of average roughness Ra, filt. Over a larger database with three different data sets including FC-72, FC-77, and water at atmospheric pressure, the most reliable correlations were obtained with the appropriately filtered area ratio. FC-72 bubble growth curves are well correlated for all test conditions with the normalized relationship D* ∼ (t*)1/3. Finally, the maximum void fraction in the region above the surface is correlated with normalized heat flux for these data and for water as the two-thirds power of heat flux.

The effect of surface roughness on nucleate boiling heat transfer is not clearly understood. This study is devised to conduct detailed heat transfer and bubble measurements during boiling on a heater surface with controlled roughness. This first of two companion papers discusses details of the fabrication, construction, and operation of the experimental facility. Test pieces are fabricated from 50.8mm×50.8mm×3.18mm borosilicate glass squares that are roughened by abrading with diamond compound with particles of known size and then annealed in order to control small-scale roughness features. Heater/sensor devices are fabricated by depositing indium tin oxide (ITO), a transparent electrically conductive material, on top of the roughened glass surfaces. A method for calibrating the ITO devices for use as temperature sensors is developed. Experimental boiling curves are reported for seven surfaces of different roughness tested in the perfluorinated fluorocarbon liquid, FC-72. The measured receding contact angle for FC-72 on the ITO-coated surfaces is reported. The wall superheat during saturated pool boiling at atmospheric pressure was found not to vary consistently with surface average roughness (Ra) values. Qualitative differences between smooth and rough surface boiling visualizations obtained simultaneously from the side and from below the heaters are discussed.

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.

Pool boiling heat transfer is measured with two individual working fluids on copper surfaces enhanced with sintered copper powder and carbon nanotubes. The working fluids are a segregated hydrofluoroether, HFE-7300 (3M Electronic Markets Materials Division, St. Paul, MN), and deionized water. The surfaces considered in the experiments include smooth copper, copper with sintered copper particles, smooth copper with copper-coated carbon nanotubes (CNT), and copper with sintered copper particles and copper-coated carbon nanotubes. Characteristics of the resulting boiling curves are discussed and analyzed. Lower wall superheats resulted from both the sintered particles and the CNT array for both working fluids. For water, there was no additional benefit from the addition of CNTs on the sintered particle substrate. For HFE-7300, however, the hybrid (sintered with CNTs) surface achieved the lowest wall superheat at high heat fluxes. Critical heat flux for HFE-7300 increased by more than 45% for the hybrid surface relative to the smooth copper surface.

Quantitative measurements are obtained from high-speed visualizations of pool boiling at atmospheric pressure from smooth and roughened surfaces, using a perfluorinated hydrocarbon (FC-77) as the working fluid. The boiling surfaces are fabricated from aluminum and prepared by mechanical polishing in the case of the smooth surface, and by electrical discharge machining (EDM) in the case of the roughened surface. The roughness values (Ra) are 0.03 and 5.89 micrometers for the polished and roughened surfaces, respectively. The bubble diameter at departure, bubble departure frequency, bubble terminal velocity, and active nucleation site density are measured from the monochrome movies, which are recorded at 8000 frames per second with a digital CCD camera and magnifying lens. Results are compared to predictions from existing models of bubble nucleation behavior in the literature. Wall superheat, heat flux, and heat transfer coefficient are also reported.

Heat transfer in the thermal entrance region of trapezoidal microchannels is investigated for hydrodynamically fully developed, single-phase, laminar flow with no-slip conditions. Three-dimensional numerical simulations were performed using a finite-volume approach for trapezoidal channels with a wide range of aspect ratios. The sidewall angles of 54.7° and 45° are chosen to correspond to etch-resistant planes in the crystal structure of silicon. Local and average Nusselt numbers are reported as a function of dimensionless length and aspect ratio. The effect of Prandtl number upon the thermal entrance condition is explored. The fully developed friction factors are computed and correlated as a function of channel aspect ratio. Correlations are also developed for the local and average Nusselt numbers in the thermal entrance region as a function of a dimensionless axial length variable.

The effect of surface roughness on pool boiling heat transfer is experimentally explored over a wide range of roughness values in water and Fluorinert™ FC-77, two fluids with different thermal properties and wetting characteristics. The test surfaces ranged from a polished surface (Ra between 0.027 μm and 0.038 μm) to electrical discharge machined (EDM) surfaces with a roughness (Ra) ranging from 1.08 μm to 10.0 μm. Different trends were observed in the heat transfer coefficient with respect to the surface roughness between the two fluids on the same set of surfaces. For FC-77, the heat transfer coefficient was found to continually increase with increasing roughness. For water, on the other hand, EDM surfaces of intermediate roughness displayed similar heat transfer coefficients that were higher than for the polished surface, while the roughest surface showed the highest heat transfer coefficients. The heat transfer coefficients were more strongly influenced by surface roughness with FC-77 than with water. For FC-77, the roughest surface produced 210% higher heat transfer coefficients than the polished surface while for water, a more modest 100% enhancement was measured between the same set of surfaces. Although the results highlight the inadequacy of characterizing nucleate pool boiling data using Ra, the observed effect of roughness was correlated using h∝Ram as has been done in several prior studies. The experimental results were compared with predictions from several widely used correlations in the literature.