Elimination of the radiant component of measured liquid thermal conductivities

Experimental determination of the thermal conductivity of liquids requires to eliminate reliably thermal radiation heat transfer. Using an approximate calculation method such eliminations have been done on several liquids in a plane layer with a steady-state heat transfer by simultaneous conduction and radiation. The method, the admissibility of simplifications, and the results are discussed. It is demonstrated that the knowledge of the optical properties of the liquid is very important, especially that of the IR-absorption spectrum. As the quantitative accuracy of available spectra is not sufficient, such spectra have to be determined particularly. The calculation method has been used to eliminate thermal radiation from effective thermal conductivities measured on a liquid within several layers of different thickness, where convection heat transfer was not present. The resulting “pure” thermal conductivities are independent of layer thickness. Thus the validity of the calculation method is verified.     

Two-step homogenization for the effective thermal conductivities of twisted multi-filamentary superconducting strand

For the accurate prediction of the effective thermal conductivities of the twisted multi-filamentary superconducting strand, a two-step homogenization method is adopted. Based on the distribution of filaments, the superconducting strand can be decomposed into a set of concentric cylinder layers. Each layer is a two-phase composite composed of the twisted filaments and copper matrix. In the first step of homogenization, the representative volume element (RVE) based finite element (FE) homogenization method with the periodic boundary condition (PBC) is adopted to evaluate the effective thermal conductivities of each layer. In the second step of homogenization, the generalized self-consistent method is used to obtain the effective thermal conductivities of all the concentric cylinder layers. The accuracy of the developed model is validated by comparing with the local and full-field FE simulation. Finally, the effects of the twist pitch on the effective thermal conductivities of twisted multi-filamentary superconducting strand are studied.     

Carbon nanotube glycol nanofluids: Photo-thermal properties,thermal conductivities and rheological behavior

The efficiency and effectiveness of solar energy capture and storage are to a large extent functions of the heat transfer and storage capacity of the medium used. This paper investigates the potential of using carbon nanotube (CNT)-glycol nanosuspension as such a medium, prepared by freeze drying-ultrasonic dispersing after oxidation treatment with HNO₃. The influences of the mass fraction of CNTs glycol nanofluids and temperatures on photo-thermal properties, thermal conductivities and rheological behavior were investigated. The results show that CNTs with oxidation treatment exhibited good dispersing performance. Strong optical absorption of the CNTs glycol nanofluids was detected in the range of 200–2500 nm. At room temperature, 18% enhancement was found in the photo-thermal conversion efficiency of the 0.5% mass fraction CNTs glycol nanofluids in comparison to the basic fluids, without significant increase in viscosity. At 55 °C, CNTs glycol nanofluids with 4.0% mass fraction exhibited much lower viscosity and 25.4% higher thermal conductivity in comparison to that of pure glycol at room temperature.     

A transient method for measuring thermal properties of soils

In connection with energy conservation studies by subsurface construction, thermal properties of soils are needed for a wide range of temperature and moisture levels. Because of several disadvantages of steady state methods, including the problem of moisture migration, a transient method has been developed which requires substantially less time than steady state tests, reduces the problem of moisture migration, and provides simultaneously thermal conductivities and thermal diffusivities. The design of the apparatus and the test procedure are discussed. The agreement of sample results with steady state measurements assures reliability of the equipment which will be used for generating a wide spectrum of thermal soil data.     

Effective conductivities of thin-interphase composites

A method is presented for approximating the effective conductivity of composite media with thin interphase regions, which is exact to first order in the interphase thickness. The approximations are computationally efficient in the sense the fields need to be computed only in a reference composite in which the interphases have been replaced by perfect interfaces. The results apply whether any two phases of the composite are separated by a single interphase or multiple interphases, whether the conductivities of the composite phases are isotropic or anisotropic, and whether the thickness of an interphase is uniform or varies as a function of position. It is assumed that the conductivities of the interphase materials have intermediate values as opposed to very high or very low conductivities.     

Laboratory techniques to evaluate thermal conductivity for some soils

The thermal conductivity of two soils was investigated through laboratory studies. These laboratory experiments used the single probe and dual probe methods to measure and compare thermal conductivities. The soils used were classified as sand and loam. Thermal conductivity measured using single probe method ranged from 0.95 to 2.11 for sand and from 0.49 to 0.76 W/m K for loam. Thermal conductivity measured using dual probe method ranged from 0.98 to 2.17 for sand and from 0.51 to 0.78 W/m K for loam. Finally, it was found that sand had higher values of thermal conductivity than loam for all soil conditions studied.     

Measurement of thermal expansion coefficients of composites using strain gages

The measurement of the coefficients of thermal expansion (CTEs) of composite materials using electrical resistance strain gages is addressed. Analytical expressions for the CTEs of an orthotropic lamina are derived, accounting for the effects of transverse sensitivity and possible misalignment of the gages. Experiments are performed for the characterization of the thermal expansion behavior of a fiber-glass-reinforced epoxy unidirectional lamina using an invar specimen as reference material. Preliminary training cycles are performed for the determination of an optimal heating rate for the measurements, which ensures thermal equilibrium conditions. Three measurement cycles yield the principal CTEs of the lamina α₁, α₂ and α₁₂ with repeatability within ±0.34×10⁻⁶, ±0.85×10⁻⁶ and ±2.8×10⁻⁶/°C, respectively. It is noted that inhomogeneity of the specimen and variation in thermomechanical properties of the gages can cause a noticeable spead in the measurements.     

Measurement of thermal stresses within an experimental nuclear-reactor vessel head

This paper describes certain tests and techniques employed in measuring stresses within an experimental nuclear-reactor head of unusual design. The incorporation of certain desired design features necessitated that the head be extremely thick. Due to the thickness and its complex geometry, it was considered desirable to determine stress distribution within the head under conditions of steady-state pressure combined with rapid heating and cooling transients within the reactor, in order to determine safe limits for the operation of the head. A photoelastic study of a three-dimensional model of the reactor head was completed in 1956; this study permitted prediction of the stress distribution throughout the head as a function of internal pressure, but it was not possible to assimilate the head thermal stresses by photoelastic means. It is the purpose of this paper to describe the technique employed in measuring thermal stresses in the interior of the head, under simulated operating conditions of steady-state pressure and temperature transients.     

Measurement of temperature-dependent thermal conductivity and viscosity of TiO2-water nanofluids

Nanofluid is an innovative heat transfer fluid with superior potential for enhancing the heat transfer performance of conventional fluids. Many attempts have been made to investigate its thermal conductivity and viscosity, which are important thermophysical properties. No definitive agreements have emerged, however, about these properties. This article reports the thermal conductivity and dynamic viscosity of nanofluids experimentally. TiO₂ nanoparticles dispersed in water with volume concentration of 0.2–2 vol.% are used in the present study. A transient hot-wire apparatus is used for measuring the thermal conductivity of nanofluids whereas the Bohlin rotational rheometer (Malvern Instrument) is used to measure the viscosity of nanofluids. The data are collected for temperatures ranging from 15 °C to 35 °C. The results show that the measured viscosity and thermal conductivity of nanofluids increased as the particle concentrations increased and are higher than the values of the base liquids. Furthermore, thermal conductivity of nanofluids increased with increasing nanofluid temperatures and, conversely, the viscosity of nanofluids decreased with increasing temperature of nanofluids. Moreover, the measured thermal conductivity and viscosity of nanofluids are quite different from the predicted values from the existing correlations and the data reported by other researchers. Finally, new thermophysical correlations are proposed for predicting the thermal conductivity and viscosity of nanofluids.     

Steady state Joule heating with temperature dependent conductivities

A general solution is given to the nonlinear steady state heat conduction equation for the case in which a metal is heated by electrical conduction currents. The solution is valid for any temperature variation in the thermal and electrical conductivities and is illustrated by application to a typical Joule heating situation in one dimension. Comparison of the case of temperature dependent conductivities for a metal with the solution which assumes both of these conductivities to remain constant with varying temperature reveals some interesting differences between the two cases.     

Effective hydraulic conductivities in unsaturated heterogeneous media by Monte Carlo simulation

When modeling flow and transport through unsaturated heterogeneous geological deposits, it may be neither computationally nor technically feasible to account for the actual heterogeneity in the simulations. One would fall short in terms of technical feasibility because there is simply no way that the entire spatial domain could be characterized (e.g., you cannot measure hydraulic conductivity at every location at a site). With respect to computational feasibility, the non-linear nature of the Richards equation (which is used to model the flow process) makes simulation of most sites extremely computationally intensive. The computational roadblock is being dismantled as computer hardware advances, but our inability to precisely characterize geological heterogeneity is expected to remain with us for a very long time. To address this problem, the analyst typically uses average or effective properties to model flow and transport behavior through heterogeneous media. In this paper, a variety of approaches for developing effective unsaturated flow properties are assessed. Computational results have been obtained which give the hydraulic conductivity ratios (K _parallel/K _nomal) for highly nonisotropic layered materials. These results are compared with analytical models. Good agreement was obtained for all soil saturation levels except for extremely dry conditions.This work was performed at Sandia National Laboratories, which is operated for the U.S. Department of Energy under contract number DE-AC04-76DP00789.     

Measurement of thermal fluxes and estimation of electrode temperature in a pulsed electromagnetic plasma accelerator

Some results of measurements of the total and radiative heat fluxes to the accelerating electrodes during a single capacitor discharge are presented for one model of an erosion-type coaxial plasma accelerator. On the basis of experimental data on the total heat transfer, the temperature at the surface of the accelerating electrodes is estimated.The author thanks G. M. Bam-Zelikovich and A. B. Vatazhin for their interest in his work.     

蜂窝夹心结构等效导热系数反演的边界单元法

对蜂窝夹心结构的等效导热系数进行反演,对于热防护系统的优化设计具有重要意义.本文应用高斯定理、径向基函数和径向积分法,推导了各向异性材料稳态热传导方程的边界单元表达式;应用Newton-Raphson迭代法和复变量求导法,对蜂窝夹心结构等效导热系数进行反演.分别应用形函数、高斯积分和径向基函数插值,对蜂窝夹心结构的边界...     

Measurement of the thermal diffusivity of metals on the temperature interval 1100–2500°K

A method of measuring the thermal diffusivity of metals on the temperature interval 1100–2500°K is described. Values of the thermal diffusivity of tantalum, molybdenum, niobium, vanadium, and cobalt are presented.Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, Vol. 10, No. 3, pp. 154–155, May–June, 1969.     

Investigation on the effective conductivities of carbon/carbon fiber composites with inhomogeneous interphase

I.Intr0ducti0nThestudyofeffectivethermalconductivitiesofcompositesisaclassicaltopic.Theearlyresearcheswerec0ncernedmainIycompositesconsistingofisotropicphases,wheretheinterfacebetweentu'oconstituentswasassumedtobeperfect,i.e.,thetemperatureandheatfluxfiel…     

Measurement of coatings' elastic properties by mechanical methods: Part 2. Application to thermal barrier coatings

Elastic properties of a thermal barrier ceramic coating composed of an NiCoCrAIY bond coat and a ZrO₂(Y₂O₃) top coat were measured by a four-point bending rig in the temperature range 20°C–900°C. Different types of specimens (i.e., with bond coat only or with bond coat and top coat, on one side or on both sides) were employed. Test procedures were based on the theory discussed in Part 1 to enhance accuracy and to estimate confidence intervals. In particular, the method employed at high temperature was calibrated at room temperature by comparing the results with those obtained by methods with low sensitivity to layer thicknesses. For the bond coat, Young's modulus was found to be temperature independent up to about 500°C; a decreasing trend was observed above this temperature. For the top coat, a slightly temperature range examined. A possible explanation is given on the basis of phase transformation and the microstructure of the two layers. At room temperature, Poisson's ratio for the bond coat was found to be near 0.3, whereas a near zero value was measured for the top coat.     

Measurement of the vibrational temperature of oxygen behind a shock wave front under thermal and chemical nonequilibrium conditions

In shock tube experiments the profiles of light absorption in oxygen are obtained for the wavelength interval 200–260 nm over the temperature range 4000–10800 K. Using these data, the vibrational temperature profiles are measured for oxygen molecules behind the shock front. The method of determination of the vibrational temperature of oxygen is based on comparing absorption measurements and detailed absorption spectrum calculations for oxygen in the Schumann-Runge system.     

Measurement of O(^1D) formation during thermal decomposition of CO_2 behind shock waves

Atomic Resonance Absorption Spectroscopy (ARAS) was applied to measure the time dependent concentration of electronically excited O(D)-atoms during the thermal decomposition of CO behind reflected shock waves. The experiments were performed in the temperature range 4102 K T 6375 K at pressures 0.2 to 1.9 bar with initial gas mixtures of 100 to 1000 ppm CO diluted in Ar. The measured O(D)-formation rate at early reaction times divided by the initial reactant concentrations was found to obey the Arrhenius law: The assumption of a fast thermalisation between the O(P) and O(D) states is in agreement with previous measurements of the O(P) formation during the thermal decomposition of CO, see Burmeister and Roth (1990). Received 24 January 1995 / Accepted 13 March 1996