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1.
The transient hot-wire method, incorporating a static magnetic field, has been developed to measure thermal conductivities of liquid mercury and gallium. Prior to the measurements, the effect of an alumina-coated hot wire on the measurements has been evaluated. Natural convection in the liquid metals has been effectively suppressed by the Lorentz force acting on the liquid metals in a static magnetic field. The thermal conductivities of liquid mercury and gallium have been determined to be 7.9 W.m −1.K −1 at 291 K and 24 W.m −1.K −1 at 302.9 K, respectively.Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.  相似文献   

2.
This paper presents absolute measurements for the thermal conductivity and thermal diffusivity of toluene obtained with a transient hot-wire instrument employing coated wires over the density interval of 735 to 870 kgm–3. A new expression for the influence of the wire coating is presented, and an examination of the importance of a nonuniform wire radius is verified with measurements on argon from 296 to 323 K at pressures to 61 MPa. Four isotherms were measured in toluene between 296 and 423 K at pressures to 35 MPa. The measurements have an uncertainty of less than ±0.5% for thermal conductivity and ±2% for thermal diffusivity. Isobaric heat capacity results, derived from the measured values of thermal conductivity and thermal diffusivity, using a density determined from an equation of state, have an uncertainty of ±3% after taking into account the uncertainty of the applied equation of state. The measurements demonstrate that isobaric specific heat determinations can be obtained successfully with the transient hot wire technique over a wide range of fluid states provided density values are available.  相似文献   

3.
The paper reports new measurements of the thermal conductivity of molten lead at temperatures from 600 to 750 K. The measurements have been carried out with an updated version of a modified transient hot-wire (THW) method, where the hot-wire sensor is embedded within an insulating substrate with a planar geometry. However, unlike previous sensors of the same type, the updated sensor works with the hot-wire divided into three thermally isolated parts. The operation of this sensor has been modeled theoretically using a finite-element (FE) analysis and has subsequently been confirmed by direct observation. The new sensor is demonstrated to have a higher sensitivity and a better signal-to-noise ratio than earlier sensors. Molten lead is used as the test fluid. It has the lowest thermal conductivity of any material we have yet studied. This allows us to probe the limits of our sensor system for the thermal conductivity of high-temperature melts. It is estimated that the uncertainty of the measurements is 3% over the temperature range studied. The results are used to examine the application of the Wiedemann–Franz (W-F) relationship.  相似文献   

4.
5.
A steady-state thermal model of the nanofabricated thermal probe was proposed. The resistive type probe working in the active mode was considered. The model is based on finite element analysis of the temperature field in the probe-sample system. Determination of the temperature distribution in this system allows calculations of relative changes in the probe electrical resistance. It is shown that the modeled probe can be used for measurements of the local thermal conductivity with the spatial resolution determined by the probe apex dimensions. The probe exhibits the maximum sensitivity to the changes in the thermal conductivity of the sample between 2 W·m−1 ·K−1 and 200 W·m−1 ·K−1. The influence of the thermal conductivity of the probe substrate on metrological characteristics of the probe as well as the thermal resistance of the probe-sample contact on the determination of the sample thermal conductivity were also analyzed. The selected results of numerical analysis were compared with data of preliminary experiments.  相似文献   

6.
The thermal diffusivity of a simulated fuel with fission products forming a solid solution was measured using the laser-flash method in the temperature range from room temperature to 1673 K. The density and the grain size of the simulated fuel with the solid solutions used in the measurement were 10.49 g · cm−3 (96.9% of theoretical density) at room temperature and 9.5 μm, respectively. The diameter and thickness of the specimens were 10 and 1 mm, respectively. The thermal diffusivity decreased from 2.108 m2 · s−1 at room temperature to 0.626 m2 · s−1 at 1673 K. The thermal conductivity was calculated by combining the thermal diffusivity with the specific heat and density. The thermal conductivity of the simulated fuel with the dissolved fission products decreased from 4.973 W · m−1 · K−1 at 300 K to 2.02 W · m−1 · K−1 at 1673 K. The thermal conductivity of the simulated fuel was lower than that of UO2 by 34.36% at 300 K and by 15.05% at 1673 K. The difference in the thermal conductivity between the simulated fuel and UO2 was large at room temperature, and decreased with an increase in temperature. Paper presented at the Seventeenth European Conference on Thermophysical Properties, September 5–8, 2005, Bratislava, Slovak Republic.  相似文献   

7.
蔡杰  沈斐 《计量学报》2016,37(2):143-147
基于传统热桥法基本原理,提出了新的非对称结构模型--瞬态热带恒功率法用于测量材料的导热率。非对称结构模型将热源热带与温度传感热带电路彼此独立,消除了热带电阻自热以及环境温度变化带来的影响。通过短时间内快速测量恒流电路中两热带的电压差随时间的变化关系准确测量材料的导热率。依据此模型设计了热带加热片,建立了基于LabVIEW测试软件的实验装置自动测量平台,实验测量结果与参考材料导热率具有良好的一致性,相对偏差在3%左右,验证了该理论模型的有效性。  相似文献   

8.
This article deals with the theory and performance of a sensor for measuring thermal conductivity. The sensor, in the form of a small ball, generates heat and simultaneously measures its temperature response. An ideal model of the hollow sphere in an infinite medium furnishes a working equation of the hot-ball method. A constant heat flux through the surface of the ball generates the temperature field. The thermal conductivity of the surrounding medium is to be determined by the stabilized value of the temperature response, i.e., when the steady-state regime is attained. Error components of the sensor are discussed due to analysis of the deviations of the real hot-ball construction from the ideal model. The functionality of a set of hot balls has been tested, and the calibration for a limited range of thermal conductivities was performed. A working range of thermal conductivities of tested materials has been estimated to be from 0.06 W· m−1 · K−1 up to 1 W· m−1 · K−1.  相似文献   

9.
Vacuum insulation panels (VIPs) have a thermal resistance that is about a factor of 10 higher than that of equally thick conventional polystyrene boards. VIPs nowadays mostly consist of a load-bearing kernel of fumed silica. The kernel is evacuated to below 1 mbar and sealed in a high- barrier laminate, which consists of several layers of Al-coated polyethylene (PE) or polyethylene terephthalate (PET). The laminate is optimized for extremely low leakage rates for air and moisture and thus for a long service life, which is required especially for building applications. The evacuated kernel has a thermal conductivity of about 4 × 10−3 W · m−1 · K−1 at room temperature, which results mainly from solid thermal conduction along the tenuous silica backbone. A U-value of 0.2 W · m−2 · K−1 results from a thickness of 2 cm. Thus slim, yet highly insulating fa?ade constructions can be realized. As the kernel has nano-size pores, the gaseous thermal conductivity becomes noticeable only for pressures above 10 mbar. Only above 100 mbar the thermal conductivity doubles to about 8 × 10−3 W · m−1 · K−1, such a pressure could occur after several decades of usage in a middle European climate. These investigations revealed that the pressure increase is due to water vapor permeating the laminate itself, and to N2 and O2, which tend to penetrate the VIP via the sealed edges. An extremely important innovation is the integration of a thermo-sensor into the VIP to nondestructively measure the thermal performance in situ. A successful “self-trial” was the integration of about 100 hand-made VIPs into the new ZAE-building in Würzburg. Afterwards, several other buildings were super-insulated using VIPs within a large joint R&D project initiated and coordinated by ZAE Bayern and funded by the Bavarian Ministry of Economics in Munich. These VIPs were manufactured commercially and integrated into floorings, the gable fa?ade of an old building under protection, the roof and the facades of a terraced house as well as into an ultra-low-energy “passive house” and the slim balustrade of a hospital. The thermal reliability of these constructions was monitored using an infrared camera.Invited paper presented at the Seventh European Conference on Thermophysical Properties, September 5-8, 2005, Bratislava, Slovak Republic.  相似文献   

10.
This paper presents new absolute measurements for the thermal conductivity and thermal diffusivity of gaseous argon obtained with a transient hot-wire instrument. Six isotherms were measured in the supercritical dense gas at temperatures between 296 and 423 K and pressures up to 61 MPa. A new analysis for the influence of temperature-dependent properties and residual bridge unbalance is used to obtain the thermal conductivity with an uncertainty of less than 1% and the thermal diffusivity with an uncertainty of less than 4%. Isobaric heat capacity results were derived from measured values of thermal conductivity and thermal diffusivity using a density calculated from an equation of state. The heat capacities presented here have a nominal uncertainty of 4% and demonstrate that this property can be obtained successfully with the transient hot wire technique over a wide range of fluid states. The technique will be useful when applied to fluids which lack specific heat data.  相似文献   

11.
Carbon aerogels, monolithic porous carbons derived via pyrolysis of porous organic precursors synthesized via the sol–gel route, are excellent materials for high-temperature thermal insulation applications both in vacuum and inert gas atmospheres. Measurements at 1773K reveal for the aerogels investigated thermal conductivities of 0.09W · m−1 · K−1 in vacuum and 0.12W · m−1 · K−1 in 0.1MPa argon atmosphere. Analysis of the different contributions to the overall thermal transport in the carbon aerogels shows that the heat transfer via the solid phase dominates the thermal conductivity even at high temperatures. This is due to the fact that the radiative heat transfer is strongly suppressed as a consequence of a high infrared extinction coefficient and the gaseous contribution is reduced since the average pore diameter of about 600nm is limiting the mean free path of the gas molecules in the pores at high temperatures. Based on the thermal conductivity data detected up to 1773K as well as specific extinction coefficients determined via infrared-optical measurements, the thermal conductivity can be extrapolated to 2773K yielding a value of only 0.14W· m−1 · K−1 in vacuum.  相似文献   

12.
Thermal Transport Properties of Water and Ice from One Single Experiment   总被引:2,自引:0,他引:2  
For the first time, the transient hot wire (THW) and the transient hot strip (THS) techniques were used to measure the thermal conductivity and thermal diffusivity of ice and the thermal conductivity of liquid water simultaneously in one run. With the additional knowledge of the thermal diffusivity of water from a subsequent single-phase run, the latent heat of melting can be determined as well as the time dependent position of the interface between both phases during an experiment. The results of the dual-phase measurements are compared with those obtained in the single-phase experiments using the same simple setup. The composite THS and THW signals are interpreted based on the underlying phase-change-theory of Stefan and Neumann, as outlined briefly in the text.  相似文献   

13.
The newly developed quasi-steady state (QSS) method to measure the thermal conductivity combines characteristic advantages of transient and steady-state techniques but avoids their major drawbacks. Based upon a transient hot strip setup, the QSS technique can be realized by adding only two temperature sensors at different radial distances from the strip. After a short settling time, the QSS output signal which is the measure for the thermal conductivity is constant in time as it is for steady-state instruments. Moreover, in contrast to transient techniques, the QSS signal is not altered by homogeneous boundary conditions. Thus, there is no need to locate a time window as has to be done with the transient hot wire or transient hot strip techniques. This paper describes the assessment of the QSS standard uncertainty of thermal conductivity according to the corresponding ISO Guide. As has already been done in previous papers on the uncertainty of the transient hot wire and transient hot strip techniques, first, the most significant sources of error are analyzed and numerically evaluated. Then the results are combined to yield an estimated overall uncertainty of 3.8%. Simultaneously, the present assessment is used as an aid in planning an experiment and in designing a QSS sensor to achieve minimal uncertainty. Such a sensor is used to verify the above mentioned standard uncertainty from a run on the candidate reference material polymethyl methacrylate.  相似文献   

14.
Low-pressure thermal conductivity and thermal diffusivity measurements are reported for argon and nitrogen in the temperature range from 295 to 350 K at pressures from 0.34 to 6.9 MPa using an absolute transient hot-wire instrument. Thermal conductivity measurements were also made with the same instrument in its steady-state mode of operation. The measurements are estimated to have an uncertainty of 1% for the transient thermal conductivity, 3% for the steady-state thermal conductivity, and 4% for thermal diffusivity. The values of isobaric specific heat, derived from the measured thermal conductivity and thermal diffusivity, are considered accurate to 5% although this is dependent upon the uncertainty of the equation of state utilized.Paper presented at the Sixteenth European Conference on Thermophysical Properties, September 1–4, 2002, London, United Kingdom  相似文献   

15.
New measurements of the thermal conductivity of stainless steel AISI 304L over the temperature range 300 to 550 K are reported. To perform the measurements, the transient hot-wire technique was employed, with a new wire sensor. The sensor makes use of a soft silicone paste material and of two thin polyimide films, between the hot wires of the apparatus and the stainless steel specimen. The transient temperature rise of the wire sensor is measured in response to an electrical heating step over a period of 40 s to 2 s, allowing an absolute determination of the thermal conductivity of the solid, as well as of the polyimide film and the silicone paste. The method is based on a full theoretical model with equations solved by a two-dimensional finite-element method applied to the exact geometry. At the 95% confidence level, the standard deviation of the thermal conductivity measurements is 0.6%, while the standard uncertainty of the technique is less than 1.5%.  相似文献   

16.
The transient fin model introduced recently for determination of the in-plane thermal diffusivity of planar samples with the help of infrared thermography was modified so as to be applicable to poor heat conductors. The new model now includes a temperature-dependent heat loss by convective heat transfer, suitable for an experimental setup in which the sample is aligned parallel to a weak, forced air flow stabilizing otherwise the convective heat transfer. The temperature field in the sample was measured with an infrared camera while the sample was heated at one edge. The symmetric temperature field created was averaged over the central fifth of the sample to obtain one-dimensional temperature profiles, both transient and stationary, which were fitted by a numerical solution of the fin model. One of the fitting parameters was the thermal diffusivity, and with a known density and specific heat capacity, the thermal conductivity was thus determined. The test measurements with tantalum samples gave the result (57.5 ± 0.2) W · m−1 · K−1 in excellent agreement with the known value. The other fitting parameter was a temperature-dependent heat loss coefficient from which the lower limit for the temperature-dependent convection coefficient was determined. For the stationary state the result was (1.0 ± 0.2) W · m−2 · K−1 at the temperature of the flowing air, and its temperature dependence was found to be (0.22 ± 0.01) W ·m−2 · K−2.  相似文献   

17.
The thermal conductivities, thermal diffusivity, thermal anisotropy ratio, and thermal boundary resistance for the multilayered microstructure of a carbon nanotube (CNT) array are reconstructed experimentally using the 3ω method with two different width metal heaters. The thermal impedance in the frequency domain and sensitivity coefficients are introduced to simultaneously determine the multiple thermal parameters. The thermal conductivity at 295 K is 38 W · m−1 · K−1 along the nanotube growth direction, and two orders of magnitude lower in the direction perpendicular to the tubes with the anisotropy ratio as large as 86. Separation of the contact and CNT array resistances is realized through circuit modeling. The measured thermal boundary resistances of the CNT array/Si substrate and insulating diamond film interfaces are 3.1 m2 · K · MW−1 and 18.4 m2 · K · MW−1, respectively. The measured thermal boundary resistance between the heater and diamond film is 0.085 m2 · K · MW−1 using a reference sample without a CNT array. The thermal conductivity for a CNT array already exceeds those of phase-changing thermal interface materials used in microelectronics.  相似文献   

18.
A new apparatus for measuring both the thermal conductivity and thermal diffusivity of fluids at temperatures from 220 to 775 K at pressures to 70 MPa is described. The instrument is based on the step-power-forced transient hot-wire technique. Two hot wires are arranged in different arms of a Wheatstone bridge such that the response of the shorter compensating wire is subtracted from the response of the primary wire. Both hot wires are 12.7 µm diameter platinum wire and are simultaneously used as electrical heat sources and as resistance thermometers. A microcomputer controls bridge nulling, applies the power pulse, monitors the bridge response, and stores the results. Performance of the instrument was verified with measurements on liquid toluene as well as argon and nitrogen gas. In particular, new data for the thermal conductivity of liquid toluene near the saturation line, between 298 and 550 K, are presented. These new data can be used to illustrate the importance of radiative heat transfer in transient hot-wire measurements. Thermal conductivity data for liquid toluene, which are corrected for radiation, are reported. The precision of the thermal conductivity data is ± 0.3% and the accuracy is about ±1%. The accuracy of the thermal diffusivity data is about ± 5%. From the measured thermal conductivity and thermal diffusivity, we can calculate the specific heat, Cp, of the fluid, provided that the density is measured, or available through an equation of state.  相似文献   

19.
This article describes the development of a method to measure the normal-to-plane thermal conductivity of a very thin electrically insulating film on a substrate. In this method, a metal film, which is deposited on the thin insulating films, is Joule heated periodically, and the ac-temperature response at the center of the metal film surface is measured by a thermo-reflectance technique. The one-dimensional thermal conduction equation of the metal/film/substrate system was solved analytically, and a simple approximate equation was derived. The thermal conductivities of the thermally oxidized SiO2 films obtained in this study agreed with those of VAMAS TWA23 within ± 4%. In this study, an attempt was made to estimate the interfacial thermal resistance between the thermally oxidized SiO2 film and the silicon wafer. The difference between the apparent thermal resistances of the thermally oxidized SiO2 film with the gold film deposited by two different methods was examined. It was concluded that rf-sputtering produces a significant thermal resistance ((20 ± 4.5) × 10−9 m2·K·W−1) between the gold film and the thermally oxidized SiO2 film, but evaporation provides no significant interfacial thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1). The apparent interfacial thermal resistances between the thermally oxidized SiO2 film and the silicon wafer were found to scatter significantly (± 9 × 10−9 m2·K·W−1) around a very small thermal resistance (less than ± 4.5 × 10−9 m2·K·W−1).  相似文献   

20.
A Quasi-Steady State Technique to Measure the Thermal Conductivity   总被引:3,自引:0,他引:3  
A new method is developed for the measurement of thermal conductivity. It combines characteristic advantages of steady-state and transient techniques but avoids major drawbacks of both these classes of methods. On the basis of a simple transient hot wire (THW) or transient hot-strip (THS) arrangement, a direct indicating thermal-conductivity meter is realized by adding only one temperature sensor. After a short settling time during which all transients die out, the instrument operates under quasi-steady state conditions. No guard heaters are required because outer boundaries are free to change with time. The instrument's uncertainty is provisionally estimated to be 3%.  相似文献   

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