High Thermal Conductivity Values Research Articles

Effect of geometrical and thermophysical characteristics of bed materials on the enhancement of thermal performance of packed bed solar air heaters

In this experimental investigation, a packed bed solar air heater has been designed, fabricated and tested under the local weather conditions of Roorkee, India. Data were obtained from May to June 1992. Tests were conducted to cover a wide range of influencing parameters,including the geometrical and thermophysical characteristics of absorber matrices, mass flow rates and input solar energy fluxes under actual outdoor conditions. The effects of these parameters on the thermal performance have been investigated, and the results have been compared with those of flat plate (plane) collectors. Based on thermal performance, the woven screen of different geometry (having the lowest values of bed thickness to element size ratio, bed porosity and extinction coefficient) and the copper woven screen (having highest value of thermal conductivity) have been found to be the best absorber matrices for packed bed solar air heaters. It is observed that the performance of the collector improves appreciably as a result of packing its duct with blackened absorber matrices, and this improvement is a strong function of the bed and operating parameters.

Thermal diffusivity, heat capacity and thermal conductivity in Al 2O 3Ni composite

Thermal diffusivity, heat capacity, and thermal conductivity of Al 2O 3Ni composite with 5 vol.% of Ni particulate were investigated. The thermal conductivity of the composite increases monotonically with increase of Ni particle size, and approaches a limiting value for Ni particles of 2 μm or larger. The reduced conductivity at smaller particle sizes is due primarily to the increased interfacial resistance related to the increase of Al 2O 3/Ni contact area, even though metal Ni has higher value of thermal conductivity than that of Al 2O 3. The mode of heat conduction in the composite with the smallest Ni particulate, i.e. D = 0.1 μm, is a result of both phonons and interfacial defects, and this leads of the composite characterizing with a lower temperature-dependent thermal conductivity. The Kapitza radius of the composite was experimentally determined as approximately 0.6–0.7 μm, at which the Ni conductivity is balanced by interfacial resistance, resulting in composite conductivity = matrix conductivity. This suggests that a minimum Ni particle size of ∼ 1.4 μm is essentially required for the improvement of the overall thermal conductivity in Al 2O 3Ni composite.

Potential high-strength high thermal conductivity metal-matrix composites based on diamond fibres

Abstract Thermal conductivity data reported for chemical vapour deposited (CVD) diamond films have been used to calculate the effective thermal conductivity of continuous diamond-fibre-reinforced metal-matrix composites. It is concluded that the very high thermal conductivity values (about 2000 W m −1 K −1 ) reported for CVD diamond may be difficult to achieve in practical composites. However, the combination of properties such as thermal conductivity, elastic modulus and density predicted for diamond-fibre composites are far superior to those offered by alternative materials. With hollow fibres the ability to combine conductive and convective cooling increases the design options in thermal management systems.

The influence of microstructure on the thermal conductivity of aluminium nitride

Starting from three different commercial powders, AIN materials were densified by pressureless sintering under various temperature and time values in order to investigate the influence of microstructure on thermal conductivity. The influence of the sintering aids (3 wt% Y2O3 and 2 wt% CaC2) and of the forming processes (cold isostatic pressing and thermocompression of tape cast pieces) were also been evaluated. Thermal conductivity increased with the purity level of the starting powder and with an increasing the sintering temperature and soaking time. The highest thermal conductivity values (196 Wm−1 K−1) were obtained with the purest powder and high temperature (1800 °C) sintering over long periods (6 h). No influence on thermal conductivity was detected from the forming technique.

Transparent chemical vapor deposited β-SiC

Transparent β-SiC has been fabricated by the pyrolysis of methyltrichlorosilane in a hot wall chemical vapor deposition reactor. Characterization of material indicates that transparent SiC is a theoretically dense, highly pure (99.9996%), highly oriented 〈111〉, β-phase (cubic) material possessing high optical transmission in the wavelength region 0.5–6 μm, a high value of hardness, electrical resistivity and thermal conductivity, and a low value of thermal expansion coefficient. These properties make it a good candidate material for use as domes and windows in severe environments such as high speed missiles, laser, combustion and space systems.

Effects of thermal shunt to substrate on normal zone propagation in high Tc superconducting thin films

Normal zone propagation is examined in high T c superconducting (HTSC) thin film grown on Al 2O 3 substrates. the transient heat equation is solved by a two-dimensional numerical model. In fact it appears that conventional analyses are inappropriate for this kind of HTSC system. The computational results indicate that the substrate, with its high value of thermal conductivity, is an efficient thermal shunt for the normal zone propagation in the thin film. This effect induces a significant increase in the quench velocity. Furthermore, the substrate is a good stabilizer, the minimum quench energy of the system is high for a current/far from the critical value/ c: in addition, the HTSC film is cooled efficiently. Thus the HTSC thin films deposited on a Al 2O 3 substrate are very stable against a pulsed disturbance or an accidental transition (/⪡/ c).

Lower Density HCFC Blown Appliance Foams

New, more stringent electrical efficiency standards for refrigerated home appliances will go into effect in 1993. In order to meet these new standards, manufacturers of appliances may have to use foams for insulation that have lower thermal conductivity values than their current production foams. In 1994, the substantial tax increase on CFC-11 used in insulating foams will likely result in the replacement of CFC-11 blown foams with HCFC blown foams. It is well known however, that HCFCs have about 25% higher thermal conductivity values than CFC-11, and drop in replacement foams with HCFCs have about 5 to 10% higher values. Miles Inc. has developed and previously published data [1] on microcell HCFC polyurethane foams that offered about 2 to 3% lower thermal conductivities than conventional CFC-11 blown production foams

Monolithic β SiC parts produced by CVD

Production of bulk, cubic (β), chemically vapour deposited (CVD) silicon carbide (SiC) has been scaled up to support commercial requirements. Free-standing CVD SiC substrates have been produced in sheets up to 1.5 m (60 inches) across and over 25 m (one inch) thick. The material can also be grown as smaller, monolithic near-net-shape parts in many different geometries. The material, commercially known as ‘CVD silicon carbide’, is single-phase, theoretically dense, and 99.999% pure, β-SiC. These characteristics lead to product attributes such as high values of hardness, flexural strength, and thermal conductivity. Additionally, the product had demonstrated good oxidation resistance and polishability. Interest has been shown in the material for use in optics, wear parts, computers and electronic packaging.

Thermal maturity patterns of Cretaceous and Tertiary rocks, San Juan Basin, Colorado and New Mexico

Horizontal and vertical thermal maturity patterns and time-temperature modeling of Cretaceous and Tertiary rocks in the San Juan Basin of southwestern Colorado and northwestern New Mexico indicate that the high levels of thermal maturity in the northern part of the basin are due to either (1) convective heat transfer associated with a deeply buried heat source located directly below the northern part of the basin or (2) the circulation of relatively hot fluids into the basin from a heat source north of the basin located near the San Juan Mountains. Vitrinite-reflectance (R m ) well profiles through Cretaceous and Tertiary rocks in the basin are commonly nonlinear, with two to four segments having different gradients. The different gradients most likely represent the combined effects of contrasting thermal conductivities associated with lithologic variations and differences in heat transfer processes (conductive versus convective). Time-temperature and kinetic modeling of nonlinear R m profiles indicates that present-day heat flow is insufficient to account for the measured levels of thermal maturity. Furthermore, in order to match the nonlinear R m profiles, it is necessary to assign artificially high thermal-conductivity values to some of the stratigraphic units. These unrealistically high thermal conductivities are interpreted as evidence of convective heat transfer.

The SiCAlN system: Influence of elaboration routes on the solid solution formation and its mechanical properties

Highly densified SiCAlN samples were prepared from commercial powder mixtures by hot-pressing in the range 2050–2150°C. Due to the respective high values of thermal conductivity of both SiC and AlN and the electrical resistivity of AlN, it should be interesting to study the mixture, or the solid solution, for potential substrate applications. Using XRD analysis and calculation of lattice parameters, the solid solution under the 2H structure was pointed out. However, high temperature polytypes were also determined in samples with high SiC contents even at a sintering temperature of 2150°C. In order to improve the mechanical properties, slurry elaboration routes and sintering cycles adapted to SiC contents were investigated—microhardness, fracture toughness and flexural strength were measured. According to the chosen sintering cycle, flexural strength values were different; and values in the range of 1000 MPa were obtained for high SiC contents. Thermal conductivity was measured, but the solid solution formation induced a decrease of it.

ChemInform Abstract: Wide‐Range Determination of Thermal Conductivity by Differential Scanning Calorimetry.

AbstractThe applicability of the DSC technique over a wide range of conductivity values is tested using the materials Teflon, duraluminum, and copper having low, intermediate and high values of thermal conductivity, respectively.

Thermal Maturity Patterns and Time-Temperature Modeling of Cretaceous and Tertiary Rocks, San Juan Basin, Colorado and New Mexico: ABSTRACT

Mean random vitrinite reflectance (R{sub m}) values of coal beds in the Upper Cretaceous Fruitland Formation in the San Juan basin of Colorado and New Mexico range from 0.42% in the southern part of the basin to 1.54% in the northern part. R{sub m} profiles of wells drilled through approximately 8,000 ft of cretaceous and Tertiary rocks in the area of high thermal maturity are commonly nonlinear, with three or more segments. R{sub m} gradients through the coal-bearing part of the Fruitland are high, ranging from 1.0 to 1.5%/1,000 ft and 0.30%/1,000 ft, respectively. These gradients most likely represent the combined effects of contrasting thermal conductivities associated with lithologic variations and differences in heat transfer processes (conductive vs. convective). Time-temperature modeling of a nonlinear vitrinite reflectance well profile in the area of high thermal maturity indicates that the present heat flow of 96 m W/m{sup 2} is insufficient to account for the measured levels of thermal maturity. Furthermore, in order to match the nonlinear R{sub m} well profile, it was necessary to assign artificially high thermal conductivity values to nearly all of the stratigraphic units penetrated in the well. The authors interpret the unreasonably high thermal conductivity values as evidencemore » of convective heat transfer and conclude that the high level of thermal maturity in the northern part of the San Juan basin is due to a deep, localized source of high heat flow in conjunction with convective heat transfer associated with a strong component of vertically directed fluid flow.« less

The temperature variation of the thermal conductivity of benzene-derived carbon fibers

The temperature variation of the in-plane thermal conductivity of benzene-derived carbon fibers (BDF) measured from 5 to 300 K is reported and discussed. Very high thermal conductivity values — comparable to that of the best HOPG heat treated at the same temperature — are found. The data confirms the high structural order previously reported for BDF.

Dependence of ground heat loss upon solar pond size and perimeter insulation calculated and experimental results

Ground heat losses from solar ponds are modelled numerically for various perimeter insulation strategies and several solar pond sizes. The numerical simulations are steady state calculations of heat loss from a circular or square pond to a heat sink at the outer boundaries of an earth volume that surrounds the pond on the bottom and sides. Simulation results indicate that insulation on top of the ground around the pond perimeter is rather ineffective in reducing heat loss, and that uninsulated sloping side walls are slightly more effective than insulated vertical side walls, except for very small ponds. The numerical results are used to derive coefficients for a semi-empirical equation describing ground heat loss as a function of pond area, pond perimeter and insulation strategy. Experimental results for ground heat loss and energy balance in the 400 m 2 solar pond at the Ohio State University are reported. Analysis of this data, along with data on solar energy input, heat gain by the pond, heat loss through the gradient zone, and heat extraction from the pond yields a good energy balance. Numerical simulation of ground heat loss from this pond shows good agreement with the results obtained from pond measurements. Loss turns out to be large because of unexpectedly high values of earth thermal conductivity in the region.

Response of fusion reactor first walls to major plasma disruptions

The melting and vaporization responses of a reactor's stainless steel first wall to plasma disruption heat fluxes are predicted using several analytical techniques. A comparison of the results shows that the simplest techniques can produce results which differ by orders of magnitude from the more sophisticated models. These more complex models generally exhibit good agreement over the range of heat fluxes selected, and also indicate the importance of the high temperature thermal conductivity value, especially on vaporization predictions.

Energy Usage for Production of Omelets in Restaurants ,

Energy usage was determined for the production of three kinds of omelets under actual operating conditions in a fast food and a family dining facility. Information was obtained by on-site data collection during normal breakfast service times within each facility during a three week period. An energy accounting model previously developed for various menu items was used. Omelets prepared in the fast food facility had a significantly longer heat processing time and used greater amounts of food ingredients per omelet and heavier pans. The family dining situation had a significantly greater energy input due to the use of a larger gas burner. Energy losses in excess of 80% were revealed for all products. Energy/nutrient ratios were reported for megajoules expended per kilocalorie and gram of protein at point of service. Recommended changes given for improving energy efficiency included controlling the time the gas burners were ignited, altering the configuration of the pan and the burner, using pans with higher thermal conductivity values, and altering food service procedures to maximize energy efficiencies.

Thermal conductivity of rare earth fluoride crystals

Thermal resistance of LaF3 and HoF3 monocrystals is linearly dependent on temperature. In LaF3 crystals a deviation from the linear law was observed at T>170°K in the direction of higher thermal-conductivity values. In doped LaF3:Eu2+ in the region T>170°K thermal resistance is lower than in pure specimens.

放電加工の電極消耗比について

In the electro-discharge machining, the discharge current is usually oscillatory. But the majority of energy which is discharged from charged condenser is dissipated in the first half cycle of the discharge current, so even if the second half cycle (which is reverse polarity) of the discharge current were stopped by germanium rectifiers, the ratio of electrode-wear (=wear of cathode/that of anode) does not decrease so much.According to the results of the experiments made on various metals of anode and cathode respectively, an alloyed cathode metal having higher value of thermal conductivity, vaporizing point and latent heat of vaporization, has smaller ratio of the electrode-wear. Furthermore, a cathode metal which contains lower value of work-function has smaller ratio of electrode-wear. In the case of ferromagnetic cathode metal, the ratio of electrode-wear is getting higher value by longitudinal magnetic field.