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Effective Thermal Conductivity (effective + thermal_conductivity)
Selected AbstractsThe Uncertainty in SCHF-DT Thermal Conductivity Measurements of Lotus-Type Porous CopperADVANCED ENGINEERING MATERIALS, Issue 10 2009Hiroshi Chiba Abstract Lotus-type porous metals with many straight pores are attractive for use as heat-sinks because a large heat-transfer capacity can be obtained, due to the small diameter of the pores. In order to use lotus-type porous copper effectively as a heat sink, it is important to know the effective thermal conductivity considering the effect of pores on heat conduction in the material. Since these metals have anisotropic pores, a steady-state comparative longitudinal heat-flow method for measuring thermal conductivity, referring to an ASTM standard, is better than other methods. So far, the effective thermal conductivity of lotus-type porous copper has been measured by using specimens of different thickness (the SCHF-DT method). In this paper, the uncertainty in the effective thermal conductivity of a specimen measured using this method was evaluated by comparison between numerical analysis and current experimental data. The following conclusions were drawn: 1) The uncertainty showed good agreement with the uncertainty analysis; 2) The contribution of the thermal grease thickness was large, based on a combined standard uncertainty analysis; and, 3) The effective thermal conductivity perpendicular to the pores of lotus copper can be measured within 10% uncertainty by this method. [source] Effective thermal actions and thermal properties of timber members in natural firesFIRE AND MATERIALS, Issue 1 2006Jürgen KönigArticle first published online: 28 JUL 200 Abstract For the thermal analysis of structural or non-structural timber members, using conventional simplified heat transfer models, thermal conductivity values of timber are normally calibrated to test results such that they implicitly take into account influences such as mass transport that are not included in the model. Various researchers and designers have used such effective thermal conductivity values, originally determined for standard fire exposure, to evaluate other fire scenarios such as natural fires. This paper discusses in qualitative terms some parameters that govern the burning of wood and their influence on effective conductivity values. Reviewing fire tests of timber slabs under natural fire conditions, the study explains why effective conductivity values, giving correct results for the ISO 834 standard fire scenario, should not be used in other fire scenarios. For this reason, the thermal properties of timber given in EN 1995-1-2 are limited to standard fire exposure. As shown by heat transfer calculations, the effective thermal conductivity of the char layer is strongly dependent on the charring rate and therefore varies during a natural fire scenario. It has also been shown that char oxidation during the decay phase in a natural fire has a significant influence on the temperature development in the timber member, since char surface temperatures exceed the gas temperature in the compartment or furnace. Using increased effective gas temperature as thermal action during the decay phase, and varying conductivity values for the char layer, fairly good agreement could be obtained regarding the temperature development in the timber member and the char depth. Copyright © 2005 John Wiley & Sons, Ltd. [source] Thermal and Structural Characterizations of Individual Single-, Double-, and Multi-Walled Carbon NanotubesADVANCED FUNCTIONAL MATERIALS, Issue 24 2009Michael T. Pettes Abstract Thermal conductance measurements of individual single- (S), double- (D), and multi- (M) walled (W) carbon nanotubes (CNTs) grown using thermal chemical vapor deposition between two suspended microthermometers are reported. The crystal structure of the measured CNT samples is characterized in detail using transmission electron microscopy (TEM). The thermal conductance, diameter, and chirality are all determined on the same individual SWCNT. The thermal contact resistance per unit length is obtained as 78,585,m,K,W,1 for three as-grown 10,14,nm diameter MWCNTs on rough Pt electrodes, and decreases by more than 2 times after the deposition of amorphous platinum,carbon composites at the contacts. The obtained intrinsic thermal conductivity of approximately 42,48, 178,336, and 269,343,W,m,1,K,1 at room-temperature for the three MWCNT samples correlates well with TEM-observed defects spaced approximately 13, 20, and 29,nm apart, respectively; whereas the effective thermal conductivity is found to be limited by the thermal contact resistance to be about 600,W,m,1,K,1 at room temperature for the as-grown DWCNT and SWCNT samples without the contact deposition. [source] Inter-particle contact heat transfer model: an extension to soils at elevated temperaturesINTERNATIONAL JOURNAL OF ENERGY RESEARCH, Issue 2 2005W. H. Leong Abstract A simple ,inter-particle contact heat transfer' model for predicting effective thermal conductivity of soils at moderate temperatures (0,30°C) has been extended up to 90°C. The extended model accounts for latent heat transport by water vapour diffusion in soil air above the permanent wilting point; below that point, the soil thermal conductivity is approximated by linear interpolation without latent heat effect. By and large the best results are obtained when the latent heat is used only in the ,self consistent approximation' model with an overall root mean square error of 35% for all soils under consideration or 26% when excluding volcanic soils. This option can also be applied to moderate temperatures at which the enhanced heat transfer is negligibly small. Copyright © 2005 John Wiley & Sons, Ltd. [source] Fire-resistant effect of nanoclay on intumescent nanocomposite coatingsJOURNAL OF APPLIED POLYMER SCIENCE, Issue 3 2007Zhen-yu Wang Abstract The aim of the study is the development of an intumescent nanocomposite coating to provide fire protection for the metallic substrate. Acrylic nanocomposites containing nanoclay and relative intumescent nanocoatings are prepared. The effect of nanoclay on the thermal degradation of an intumescent nanocomposite coating is analyzed by using differential thermal analysis, thermogravimetry, and X-ray diffraction. The influence of the added content of nanoclay on fire performance is studied by a fire protection test and measurements of the limiting oxygen index and effective thermal conductivity. The distribution of nanoparticles in the acrylic nanocomposite is characterized by transmission electron microscopy. The flame-retardant efficiency of the intumescent nanocomposite coating is improved by 1.5% well-distributed nanoclay particles. However, 3% nanoclay produces a negative effect on the fire performance of the coating. Fire protection tests and scanning electron microscopy observations reveal that the fire-retardant property of a conventional intumescent coating is destroyed by aging, whereas the nanocomposite coating modified with 1.5% nanoclay demonstrates good aging and fire resistance. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1681,1689, 2007 [source] Particle scale study of heat transfer in packed and bubbling fluidized bedsAICHE JOURNAL, Issue 4 2009Z. Y. Zhou Abstract The approach of combined discrete particle simulation (DPS) and computational fluid dynamics (CFD), which has been increasingly applied to the modeling of particle-fluid flow, is extended to study particle-particle and particle-fluid heat transfer in packed and bubbling fluidized beds at an individual particle scale. The development of this model is described first, involving three heat transfer mechanisms: fluid-particle convection, particle-particle conduction and particle radiation. The model is then validated by comparing the predicted results with those measured in the literature in terms of bed effective thermal conductivity and individual particle heat transfer characteristics. The contribution of each of the three heat transfer mechanisms is quantified and analyzed. The results confirm that under certain conditions, individual particle heat transfer coefficient (HTC) can be constant in a fluidized bed, independent of gas superficial velocities. However, the relationship between HTC and gas superficial velocity varies with flow conditions and material properties such as thermal conductivities. The effectiveness and possible limitation of the hot sphere approach recently used in the experimental studies of heat transfer in fluidized beds are discussed. The results show that the proposed model offers an effective method to elucidate the mechanisms governing the heat transfer in packed and bubbling fluidized beds at a particle scale. The need for further development in this area is also discussed. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Temperature-Gradient Effects in Thermal Barrier Coatings: An Investigation Through Modeling, High Heat Flux Test, and Embedded SensorJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 10 2010Yang Tan The harsh thermal environment in gas turbines, including elevated temperatures and high heat fluxes, induces significant thermal gradients in ceramic thermal barrier coatings (TBCs), which are used to protect metallic components. However, the thermal conductivity of plasma-sprayed TBC increases with exposure at high temperatures mainly due to sintering phenomena and possible phase transformation, resulting in coating performance degradation and potential thermal runaway issues. An analytical thermal model and experimentally obtained coating thermal conductivity data are used to determine the coating through-thickness temperature profile and effective thermal conductivity under gradient conditions at high temperatures. High heat flux tests are then performed on TBCs to evaluate coating thermal behavior under temperature gradients close to service conditions. Coating internal temperature during the tests was also measured by thermally sprayed embedded thermocouples within the top coat. This combined approach provides a sintering map with a new model and allows for the assessment of temperature-gradient effects on the thermal performance of plasma-sprayed TBCs. [source] Inverse Problem for Composites with Imperfect Interface: Determination of Interfacial Thermal Resistance, Thermal Conductivity of Constituents, and Microstructural ParametersJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 4 2000Ce-Wen Nan An explicit method is introduced to solve inverse problems for composites with imperfect interfaces. We apply the method to determine the thermal conductivity of constituents and the interfacial thermal resistance in SiC-particulate-reinforced aluminum-matrix composites and to estimate the whisker thermal conductivity, the interfacial thermal resistance, and the whisker alignment distribution in two types of SiC-whisker-reinforced lithium aluminosilicate glass-ceramic composites from their measured effective thermal conductivity reported in the literature. Certain bounds for these three properties of both SiC-whisker-reinforced glass-ceramic composites are obtained, and reasonable estimates for their exact values from room temperature to 500°C are made. The inverse problem is quite sensitive to noise in the measurements. We also comment on existing estimates. [source] Physical properties of rocks from the upper part of the Yaxcopoil-1 drill hole, Chicxulub craterMETEORITICS & PLANETARY SCIENCE, Issue 6 2004Y. Popov Thermal conductivity, thermal diffusivity, density, and porosity were measured on 120 dry and water-saturated rocks with a core sampling interval of 2,2.5 m. Nondestructive, non-contact optical scanning technology was used for thermal property measurements including thermal anisotropy and inhomogeneity. Supplementary petrophysical properties (acoustic velocities, formation resisitivity factor, internal surface, and hydraulic permeability) were determined on a selected subgroup of representative samples to derive correlations with the densely measured parameters, establishing estimated depth logs to provide calibration values for the interpretation of geophysical data. Significant short- and long-scale variations of porosity (1,37%) turned out to be the dominant factor influencing thermal, acoustic, and hydraulic properties of this post impact limestone formation. Correspondingly, large variations of thermal conductivity, thermal diffusivity, acoustic velocities, and hydraulic permeability were found. These variations of physical properties allow us to subdivide the formation into several zones. A combination of experimental data on thermal conductivity for dry and water-saturated rocks and a theoretical model of effective thermal conductivity for heterogeneous media have been used to calculate thermal conductivity of mineral skeleton and pore aspect ratio for every core under study. The results on thermal parameters are the necessary basis for the determination of heat flow density, demonstrating the necessity of dense sampling in the case of inhomogeneous rock formations. [source] Influence of blood flow and millimeter wave exposure on skin temperature in different thermal modelsBIOELECTROMAGNETICS, Issue 1 2009S.I. Alekseev Abstract Recently we showed that the Pennes bioheat transfer equation was not adequate to quantify mm wave heating of the skin at high blood flow rates. To do so, it is necessary to incorporate an "effective" thermal conductivity to obtain a hybrid bioheat equation (HBHE). The main aim of this study was to determine the relationship between non-specific tissue blood flow in a homogeneous unilayer model and dermal blood flow in multilayer models providing that the skin surface temperatures before and following mm wave exposure were the same. This knowledge could be used to develop multilayer models based on the fitting parameters obtained with the homogeneous tissue models. We tested four tissue models consisting of 1,4 layers and applied the one-dimensional steady-state HBHE. To understand the role of the epidermis in skin models we added to the one- and three-layer models an external thin epidermal layer with no blood flow. Only the combination of models containing the epidermal layer was appropriate for determination of the relationship between non-specific tissue and dermal blood flows giving the same skin surface temperatures. In this case we obtained a linear relationship between non-specific tissue and dermal blood flows. The presence of the fat layer resulted in the appearance of a significant temperature gradient between the dermis and muscle layer which increased with the fat layer thickness. Bioelectromagnetics 30:52,58, 2009. © 2008 Wiley-Liss, Inc. [source] | |||||||||||||