Solid oxide fuel cells with graded electrodes consisting of multiple composite layers yield generally lower polarization resistances than single layer composite electrodes. Optimization of the performance of solid oxide fuel cells with graded electrode composition and/or microstructure requires an evaluation of both the three phase boundary length per unit volume and the effective diffusion coefficient in order to provide insight into how these properties vary over the design space. A numerical methodology for studying the three phase boundary length and effective diffusivity in composite electrode layers with controlled properties is developed. A three dimensional solid model of a sintered composite electrode is generated for which the mean particle diameter, composition, and total porosity may be specified as independent variables. The total three phase boundary length for the modeled electrode is calculated and tomographic methods are used to estimate the fraction of this length over which the electrochemical reactions can theoretically occur. Furthermore, the open porosity of the modeled electrode is identified and the effective diffusion coefficient is extracted from the solution of the concentration of the diffusing species within the open porosity. Selected example electrode models are used to illustrate the application of the methods developed, and the resulting connected three phase boundary length and diffusion coefficients are compared. A significant result is the need for thickness-specific effective diffusivity to be determined, rather than the general volume averaged property, for electrodes with porosity between the upper and lower percolation thresholds. As the demand for current increases, more of the connected three phase boundaries become active, and therefore a greater fraction of the electrode layer is utilized for a given geometry, resulting in a higher apparent effective diffusivity compared to the same electrode geometry operating at a lower current. The methods developed in this work may be used within a macroscopic electrode performance model to investigate optimal designs for solid oxide fuel cell electrodes with stepwise graded composition and/or microstructure. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

A graphite foam was developed at Oak Ridge National Laboratory (ORNL) by Dr. James Klett and license was granted to POCO Graphite, Inc. to manufacture and market the product as PocoFoam™. Unlike many processes currently used to manufacture carbon foams, this process yields a highly graphitic structure and overcomes many limitations, such as oxidation stabilization, that are routinely encountered in the development of carbon foam materials. The structure, thermal properties, electrical resistivity, isotropy, and density uniformity of PocoFoam™ were evaluated. These properties and characteristics of PocoFoam™ are compared with natural and synthetic graphite in order to show that, albeit similar, it is unique. Thermal diffusivity and thermal conductivity were derived from Fourier's energy equation. It was determined that PocoFoam™ has the equivalent thermal conductivity of metals routinely used as heat sinks and that thermal diffusivity is as much as four times greater than pure copper and pure aluminum. SEM and XRD results indicate that PocoFoam™ has a high degree of crystalline alignment and near theoretical d spacing that is more typical of natural flake graphite than synthetic graphite. PocoFoam™ is anisotropic, indicating an isotropy factor of 0.5, and may yield higher thermal conductivity at cryogenic temperatures than is observed in polycrystalline graphite.

The purposes of this study were to determine as a function of temperature the thermal diffusivity and/or thermal conductivity of SiC-Mullite and SiC-SiC, and to explain the observed behavior in terms of changes in temperature, microstructure, composition, and/or orientation. Materials used in the SiC-Mullite study consisted of single crystal SiC whiskers (prepared from rice hulls or by the vapor-liquid-solid process) dispersed within a polycrystalline mullite matrix. During measurement of thermal diffusivity, the samples were heated to l500°C and cooled back to room temperature. No hysteresis occurred. However, both thermal diffusivity and conductivity exhibited maximum values at room temperatures, perpendicular to the hot pressing direction, at high volume percentages of SiC whiskers, and when VLS whiskers were employed. The SiC-SiC samples consisted of a crossweave of polycrystalline SiC fibers that were coated with phenolic resin and surrounded by a chemically-vapor-deposited matrix of SiC. The two types of samples examined were prepared with different amounts of resin. The matrices of the high resin samples were found to be dominated by the presence of char. Samples were cycled to 1000, 1400, and l800°C; hysteresis occurred on some of the cycles. Thermal diffusivity was highest parallel to one set of fibers. These results allow the qualitative tailoring of the heat flow properties of these ceramic composites, for particular applications, and set forth limitations on the use of the SiC-SiC composites at high temperatures. / Master of Science

<p> A diffusion cell of the "shearing type" was used to diminish the effect of convection which is always present when two liquid phases are brought into contact with each other in a diffusion cell. Also a special optical arrangement was used to photograph the refractive index distribution of the system. For those systems with refractive index changing linearly with concentration, the concentration profiles were obtained and diffusion coefficients were calculated at different concentrations. </p> <p> This optical method gave only fair reproducibility- the deviation among diffusivities found for systems investigated varying from 3 to 10%- however, it permitted rapid analysis and on this basis is recommended for situations where speed is essential and high accuracy is not required. </p> / Thesis / Master of Engineering (ME)

The Fluorescence Scanning Thermal Microscope (FSTM v3.0), was designed to create an inexpensive, and easily manufactured, device for measuring the diffusivity of samples with microscopic locational precision. This was accomplished by using a Blu-ray device known as a PHR-803T, referred to in this work as a PHR. The optics in the PHR are nearly identical in function to conventional devices used in thermoreflectance microscopy, making the PHR extremely useful to integrate into the FSTM design. The focus of this thesis is the application of the FSTM as a confocal microscope using 3D printed components and various low-cost devices to operate with comparable sampling accuracy to existing confocal microscopes. The electronics and optical filters were then adapted to enable the measurement of thermal waves, particularly by detecting a linear relationship between phase delay and the spacing between heating and sensing lasers, as predicted by previous work on the FSTM.

An experimental study of turbulent air flow in a pipe is reported in this paper. A determination was made of the mean velocity distribution and longitudinal mean turbulent velocity distribution, both in the turbulent core and boundary layer for four different Reynolds numbers from 7300 to 58300. A traversing mechanism was designed in order to measure the turbulence correlations between two points. The variation of the macro scale length, one of the fundamental quantities in recent statistical turbulence theory across the pipe diameter was calculated for Reynolds number equal to 58300, by integrating the correlation curves. The turbulent momentum diffusivity at the center of a pipe was calculated from the correlation study and the dimension less value was found to be 0.111. Ethylene gas was injected into the center of the pipe, and in order to investigate the turbulent mass diffusivity, the concentration distribution curves of ethylene were measured at different test positions downstream from the injection point, for the same series of Reynolds numbers used in the turbulence measurement. A numerical method for calculating the diffusivity was developed. The values of diffusivity obtained. in these experiments show that the assumptions which were used by most of the authors, that of (turbulent mass diffusivity/turbulent momentum diffusivity) has a value between 1.0 to 1.6 is correct. / Thesis / Master of Engineering (ME)

No description available.

Recent global efforts have focused on developing new lightweight alloys specifically designed for high-pressure die casting (HPDC) processes, aiming to achieve the lightweight of electrified vehicles. HPDC offers a distinct advantage by allowing the production of neat-net-shape automotive components, minimizing the need for further processing. An inherent characteristic of HPDC is its rapid cooling rates, making the understanding and characterization of the thermophysical properties of these newly developed lightweight alloys under high cooling rates a must. These properties have a significant effect on controlling the HPDC process and developing suitable filling and solidification models to simulate the HPDC process intricacies for commercial production adaptation. The thermophysical properties of these alloys are shown to exhibit considerable variability with temperature, particularly under rapid solidification conditions, like in HPDC. Hence, an essential step in developing such alloys is to thoroughly investigate the variation of their thermophysical properties with temperature under high cooling rates. To fulfill such a need, an experimental setup has been developed to allow the solidification of molten metal samples under varying cooling rates using a set of impinging water jets. An inverse heat transfer algorithm has been developed to estimate the thermal conductivity and thermal diffusivity as a function of the temperature of the solidifying samples under high cooling rates. To validate the accuracy of the inverse heat transfer algorithm and the experimental methodology, a set of experiments has been carried out using pure Tin, which is a well-characterized material. Its thermal diffusivity and thermal conductivity are readily available in the literature. The estimated thermal diffusivity and thermal conductivity of Tin have been compared with the published data. The estimated thermal diffusivity and conductivity of the solid phase were in good agreement with the published values. A maximum deviation ranging from +10.1% to -3.47% was observed in the estimated thermal diffusivity. The maximum deviation in the estimated thermal conductivity was between +7.8% and -13.6%. Higher deviations have been observed in the estimated thermal diffusivity and conductivity of the liquid phase with deviations in the range of +33.71% to -4.86% and +0.76% to 26.53%, respectively. The higher deviations observed for the liquid phase might be attributed due to the natural convection that developed in the tested liquid sample. The effect of natural convection was examined using a set of numerical simulations that confirmed the existence of a convection-induced movement within the liquid phase. A sensitivity analysis was carried out to examine the impact of the accuracy of thermocouple positions and the effect of temperature sensing accuracy on the estimated thermal properties. / Thesis / Master of Applied Science (MASc) / An inverse heat transfer algorithm along with an experimental setup has been developed to estimate the temperature-dependant thermophysical properties during solidification of metallic alloys under high cooling rates. To verify the accuracy of the developed algorithm and the experimental setup the estimated thermal conductivity and diffusivity of pure Tin have been compared with data available in the literature. The results showed reasonable agreement.

The purpose of this investigation was to develop an apparatus to measure the thermal conductivity of polymer melts, and to use the apparatus to measure the thermal conductivity of selected melts as a function of melt temperature. The steady-state, coaxial cylinder method with guard heaters was used and the annular gap was 0.075 inch. The polymer was melted in a cylindrical melt chamber, then metered to the thermal conductivity measuring apparatus. Cartridge heaters provided heat input and temperature measurements were made with calibrated, differential, iron-constantan thermocouples. The thermal conductivity of polyethylene, polystyrene and nylon melts tested increased with increased temperature. The thermal conductivity of the polypropylene sample was temperature independent. Complexity of molecular structure lowered melt thermal conductivity. Radiation losses were accounted for and convection was determined to be absent. The results were shown to be within a 3 percent experimental measurement error. Meaningful confidence limits cannot be calculated because of the limited number of data points. / Master of Science

Continuous, liquid, thermal diffusion is a useful separation method which heretofore has been utilized primarily on a laboratory scale for accomplishing the separation of isotopes, azeotropes, and hydrocarbons of a particular series. Although the thermal diffusion effect, wherein a gradient of temperature in a body of fluid gives rise to a gradient of concentration, was discovered in 1856, it did not become commercially feasible until 1939 when Clusius and Dickel developed the thermogravitational procedure. In this method, the temperature gradient is applied in a horizontal direction to a vertical layer of liquid, and the convection currents set up by the concentration gradient thus established move the fluids of different concentrations in opposite directions; streams of different concentrations are continuously drawn off from the top and from the bottom of the apparatus. The important commercial application has been the separation of uranium isotopes contained in liquid uranium hexafluoride. This work was part of the Atomic Energy program; the results have not been published. One major oil company has carried out a detailed research and development program on the separation of petroleum fractions by liquid thermal diffusion. This investigation was concerned with developing formulae, methods of calculation, and empirical data necessary for designing commercial equipment utilizing liquid thermal diffusion. Vertical, concentric-tube, thermal diffusion columns were constructed which could be operated with a continuous feed and draw-off of liquid from the diffusion annulus. Steam or hot water was used as the heating medium and tap water was used for cooling purposes. The columns were assembled with a copper center tube through which the cooling water flowed, and a concentric glass tube which formed the hot wall of the diffusion annulus. This set of tubes was jacketed with a pyrex pipe to form a steam or hot water reservoir about the hot wall of the diffusion annulus. Aqueous solutions of sugar were used as the test liquid. Separation effects were observed in solutions where the initial concentration was between 0.5 and 2.0 gram mols of sugar per liter. All tests were made under steady-state conditions and differences observed between the concentration of the bottom product and that of the top product varied from 0 to 28.3 weight per cent sugar. Curves, together with specific and generalized formulae, were presented to indicate the effect of each of seven variables on the separation ratio. The feed concentration was varied between 15 and 55 weight per cent sugar, and the separation ration reached a maximum between 26.6 and 44.2 weight per cent sugar. Heating fluid temperatures varying from 77 to 124 degrees centigrade were employed and the separation ratio increased linearly with the heating fluid temperature. Annulus width varying from 3.1 to 19. 7 millimeters were studied and the separation ration reached a maximum for widths between 13.0 and 16. 7 millimeters. The separation ratio decreased as the feed rate was increased from 3.0 to 248 milliliters per hour; however, it increased as the ratio of the bottom draw-off rate to the feed rate was increased from 0.047 to 0.916. The separation ratio reached a maximum when the feed port was located near the middle of the column. Maximum separations were obtained with a column length of only 24.1 centimeters; however, for other column lengths from 54.2 to 114.3 centimeters, the separation ratio increased slightly as the column length was increased. In addition to the desired temperature gradient across the liquid in the diffusion annulus, large temperature drops occurred in the glass wall of the diffusion annulus (19 to 23 degrees centigrade), and in the water film inside the copper, center tube (8 to 16 degrees centigrade). Certain fundamental factors relative to continuous, liquid, thermal diffusion have been developed in this investigation. Further research should provide similar data for other liquid systems and for other column designs. This will enable the chemical engineer to design commercial apparatus for specific applications. / Ph. D.