共查询到20条相似文献,搜索用时 31 毫秒
1.
Y. Shi L. Sun F. Tian J. E. S. Venart R. C. Prasad 《Journal of Thermal Analysis and Calorimetry》2007,90(3):693-698
The transient hot-wire technique is widely used for absolute measurements of the thermal conductivity of fluids. Refinement
of this method has resulted in a capability for accurate and simultaneous measurement of both thermal conductivity and thermal
diffusivity together with a determination of the specific heat. However, these measurements, especially those for the thermal
diffusivity, may be significantly influenced by fluid radiation.
The present work investigates the effect of fluid radiation on the measurements of the thermal conductivity of propane. Recently
developed corrections have been used to examine this assumption and rectify the influence of even weak fluid radiation. Measurements
at 372 K with a hot-wire instrument demonstrate the presence of radiation effects in both the liquid and vapor phase. The
influence is much more pronounced in liquid propane at 15.5 MPa than in the vapor phase at 881.5 kPa. The technique employed
to obtain radiation-free thermal conductivity measurements is described. 相似文献
2.
Summary The various techniques and methodologies of thermal conductivity measurement have been conventionally based on the determination of the rate of directional heat flow through a material having a unit temperature differential between its opposing faces. The constancy of this rate depends on the material density, its thermal resistance and the heat flow path itself. The last of these variables contributes most significantly to the true value of steady-state axial and radial heat dissipation depending on the magnitude of transient thermal diffusivity along these directions. The purpose of this paper is to exemplify the above features by defined parameters of heat flow measurement by existing methodologies. No new method is proposed here. Importantly, the relationship between the rate of heat transfer, total heat transferred and thermal conductivity at a given temperature under steady-state conditions for a fixed heat flow path will be illustrated. 相似文献
3.
Y. Shi L. Sun J. E. S. Venart R. C. Prasad 《Journal of Thermal Analysis and Calorimetry》2006,86(3):585-590
The transient hot-wire technique is widely used for absolute measurements
of the thermal conductivity and thermal diffusivity of fluids. It is well
established that fluid radiation effects significantly influence these measurements,
especially those for the thermal diffusivity. Corrections for radiation effects
are based on the models developed and deviations of the measured data from
the ideal line source model. In this paper, the effect of fluid radiation
on the measurements of the thermal conductivity of n-pentane
is presented. For comparison, the influence of thermal radiation effect on
measurement of transparent fluids, such as argon is also shown. The difference
between the influence of natural convection and thermal radiation is also
demonstrated. 相似文献
4.
F. Tian L. Sun J. E. S. Venart R. C. Prasad 《Journal of Thermal Analysis and Calorimetry》2009,96(1):67-71
The paper describes a new transient hot wire instrument which employs 25.4 μm diameter tantalum wire with an insulating tantalum
pentoxide coating. This hot-wire cell with a thin insulating layer is suitable for measurement of the thermal conductivity
and the thermal diffusivity of electrically conducting and polar liquids. This instrument has been used for experimental measurement
of the thermal conductivity and the thermal diffusivity of poly(acrylic acid) solution (50 mass%) in the temperature range
of 299 to 368 K at atmospheric pressure. The thermal conductivity data is estimated to be accurate within ±4%. Thermal diffusivity
measurements have a much higher uncertainty (±30%) and need further refinement. 相似文献
5.
6.
Hakim F. Abbasov 《Journal of Dispersion Science and Technology》2019,40(4):594-603
Nanofluids having high thermal conductivity enhancement relative to conventional pure fluids are fluids engineered by suspending solid nanoparticles into base fluids. In the present study, calculating the Van der Waals interaction energy between a nanoparticle and an ordered liquid nanolayer around it, the nanolayer thickness was determined, the average velocity of the Brownian motion of nanoparticles in a fluid was estimated, and by taking into account both the aggregation of nanoparticles and the presence of a nanolayer a new thermal conductivity model for nanofluids was proposed. It has been shown that the nanolayer thickness in nanofluids is independent on the radius of nanoparticles when the radius of the nanoparticles is much greater than the nanolayer thickness and determines by the specific interaction of the given liquid and solid nanoparticle through the Hamaker constant, the surface tension and the wetting angle. It was proved that the frequency of heat exchange by fluid molecules is two orders of magnitude higher than the frequency of heat transfer by nanoparticles, so that the contribution due to the Brownian motion of nanoparticles in the thermal conductivity of nanofluids can be neglected. The predictions of the proposed model of thermal conductivity were compared with the experimental data and a good correlation was achieved. 相似文献
7.
L. Vozár 《Journal of Thermal Analysis and Calorimetry》1996,46(2):495-505
The aim of this paper is to review the transient hot wire method for measurement of thermal conductivity, which is based on the measurement of temporal history of the temperature rise caused by linear heat source (hot wire) embedded in a test material. If a current is passed through the wire, the rise in temperature will be dependent, among other factors, on the thermal conductivity of the medium, surrounding the wire. Here the mathematical basis, as well as main modifications of the hot wire method — cross technique, resistance modifications with potential and compensated lead methods; hot wire probe method and parallel wire technique, are described and discussed. A fully automated computer-controlled transient hot wire apparatus is presented and tested, which allows measurement of thermal conductivity of solid, powder and granular materials at high temperatures. 相似文献
8.
Naoto Tsutsumi Satoshi Ishida Tsuyoshi Kiyotsukuri 《Journal of Polymer Science.Polymer Physics》1994,32(11):1899-1906
This article presents measurement of thermal diffusivity and electrical conductivity of polypyrrole films prepared by electropolymerization. Thermal diffusivity was measured by laser radiometry (former flash radiometry). Electrical conductivity was determined by a conventional four-probe method. Increase of thermal diffusivity is observed when increasing the supporting electrolyte concentration, which is also shared with the increase of electrical conductivity. Both thermal diffusivity and electrical conductivity significantly depended on the types of counter anion incorporating into polymer bulk. Thermal diffusivity of polypyrrole film is larger than that for common nonelectrical conductive polymers. Temperature profile of thermal diffusivity for as-grown polypyrrole films shows that thermal diffusivity increases with increasing temperature (first running profile), whereas remeasured temperature profile of thermal diffusivity (second or third running profiles) shows the decrease of thermal diffusivity with increasing temperature. Electrical conductivity monotonically increases until the significant decrease of it occurs at the temperature above 130°C. Investigation of these temperature profiles of thermal diffusivity and electrical conductivity has been made by corresponding to thermal analysis data. © 1994 John Wiley & Sons, Inc. 相似文献
9.
Martin Rides Junko Morikawa Lars Halldahl Bruno Hay Hubert Lobo Angela Dawson Crispin Allen 《Polymer Testing》2009,28(5):480-489
An intercomparison of measurements of the thermal conductivity and thermal diffusivity of two poly(methyl methacrylates) is reported. A wide variety of methods were used: temperature wave analysis, laser flash, transient plane source (Hot Disk®), transient line-source probe, and heat flux meter methods. Very good agreement of thermal conductivity results and, separately, of thermal diffusivity results was obtained. Similarly, good agreement between thermal conductivity and thermal diffusivity results, when converted using specific heat capacity and density values, was also obtained. Typically, the values were within a range of approximately ±10%. Considering the significant differences between the methods and the requirements on specimen dimensions, the level of agreement between results was considered to be good. 相似文献
10.
We have developed a laser flash apparatus for simultaneous measurements of thermal diffusivity and specific heat capacity
of solid materials by introducing recent technical progress: uniform heating by a homogenized laser beam using an optical
fiber with a mode mixer, measuring transient temperature of a specimen with a calibrated radiation thermometer, analyzing
a transient temperature curve with a curve fitting method, to achieve differential laser flash calorimetry. Thermal diffusivity,
specific heat capacity and thermal conductivity of glassy carbon and molybdenum were measured in the temperature range from
300 to 1100 K.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
Effect of thermal decomposition processes on the thermal properties of carbon fiber reinforced cement composites in high-temperature range 总被引:1,自引:0,他引:1
R. Černý Jitka Němečková Pavla Rovnaníková P. Bayer 《Journal of Thermal Analysis and Calorimetry》2007,90(2):475-488
Thermal conductivity, specific heat capacity, thermal diffusivity and linear thermal expansion coefficient of two types of
carbon fiber reinforced cement composites are measured in the temperature range up to 800°C. Thermal conductivity and thermal
diffusivity are also determined for the specimens exposed to thermal load up to 800°C before the measurement. Differential
thermal analysis (DTA), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and X-ray diffraction analysis
(XRD) are utilized for the assessment of thermal decomposition processes taking place in the high temperature range under
consideration.
The high temperature thermal properties of the studied materials are found to be positively affected by the application of
the high alumina cement and in the case of the Portland cement based composite also by using the autoclaving procedure in
the production process. Also, the randomly distributed carbon fibers that can reduce the damage of the pore structure by the
thermal decomposition processes are identified as a positive factor in this respect. A comparison of thermal conductivity
vs. temperature curves obtained for the specimens pre-heated to different temperatures is found to be a useful tool in the identification
of major dynamic effects in the specimens due to the thermal decomposition reactions. The results are in a good agreement
with the DTA, MIP, SEM and XRD analyses. The character of the thermal conductivity measurements that in fact includes the
effects of convection and radiation into the thermal conductivity coefficient can be beneficial for a simple assessment of
the influence of the fire on a dividing structure. 相似文献
12.
Thermal conductivity measurements using the flash method 总被引:1,自引:0,他引:1
Gaal P. S. Thermitus M.-A. Stroe Daniela E. 《Journal of Thermal Analysis and Calorimetry》2004,78(1):185-189
Thermal diffusivity is the speed with which heat propagates through a material. It has a multitude of direct applications,
such as determining heat transfer through brake pads at the moment of contact, etc., but more often it is used to derive thermal
conductivity from the fundamental relationship tying it with specific heat capacity and density. Using a new multi-sample
configuration system, and testing a reference sample adjacent to the unknown, specific heat capacity can be obtained parallel
with thermal diffusivity. Thus, a single test yields thermal diffusivity and thermal conductivity with prior knowledge of
density. The method is fast and produces results with high accuracy and very good repeatability. The sample size, 12 to 30
mm diameter and 2 to 5 mm thickness, is easy to handle and is well suited for a broad range of materials, even for composites,
often a problem for other methods. Typical data on two polymers, Pyrex glass and Pyroceram 9606 are presented.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
O. A. Almanza M. A. Rodríguez‐Prez J. A. De Saja 《Journal of Polymer Science.Polymer Physics》2000,38(7):993-1004
The thermal conductivity and the cellular structure as well as the matrix polymer morphology of a collection of chemically crosslinked low‐density closed cell polyolefin foams, manufactured by a high‐pressure nitrogen gas solution process, have been studied. With the aid of a useful theoretical model, the relative contribution of each heat‐transfer mechanism (conduction through the gas and solid phases and thermal radiation) has been evaluated. The thermal radiation can be calculated by using a theoretical model, which takes into account the dependence of this heat‐transfer mechanism with cell size, foam thickness, chemical composition, and matrix polymer morphology. A simple equation, which can be used to predict the thermal conductivity of a given material, is presented. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 993–1004, 2000 相似文献
14.
Ana Paula F. Albers Thomaz A. G. Restivo Luciano Pagano Joo B. Baldo 《Thermochimica Acta》2001,370(1-2):111-118
Several experimental techniques either under steady state or transient heat transfer conditions, have been developed to evaluate thermal conductivity and thermal diffusivity of materials. However, testing difficulties resulting from specimen size, extended testing time and heat losses, have somewhat impaired the applicability of many of them. In this respect, the use of the laser flash technique for thermal diffusivity measurements, is a very convenient alternative, considering its basic modeling equation is independent of the temperature gradient as well as the heat flow, and in addition the heat losses can be analytically treated. Another important advantage of the technique is its rapid experimental execution. In this work, it is presented as an investigation concerning how the testing conditions such as specimen coating, laser power and pulse duration, base line adoption, heat losses correction methods, and specimen thickness, may affect the thermal diffusivity measurements of some ceramic materials using the laser flash technique. 相似文献
15.
Advances in production are leading to increasing use of polymeric thin films in applications such as automotive bearings. Two approaches have been developed to study the thermophysical properties of these thin films: The first technique based on Flash theory uses a scanning thermal microscopy (SThM) tip in temperature contrast mode to measure thermal diffusivity over a nano-scale area. The SThM tip is in contact with the upper surface of the film to detect a heat pulse delivered by a microelectromechanical heater platform from the lower surface. The second technique is a conductivity contrast mode SThM based approach for measuring the size and distribution of thermally conducting particles in thin film polymeric coatings. Topographical and thermal conductivity data are combined to produce a “correlation analysis value” 3D particle map of the coating. Good practice and a case study are highlighted. 相似文献
16.
The thermal properties of a silicon oxide‐based low‐k film and a thermally oxidized silicon film were investigated using the 3‐omega and laser thermo‐reflectance (LTR) methods. Thermal conductivity and effusivity were successfully estimated by the 3‐omega and LTR methods, respectively. It was confirmed that the combination of thermal effusivity and conductivity can successfully provide the heat capacity and thermal diffusivity of the films. The thermal parameters thus obtained suggested that the lower thermal conductivity of the examined low‐k film comes mainly from the rather low level of thermal diffusivity. Based on an analysis of the X‐ray diffraction profiles of the films, it was found that the low thermal diffusivity of the low‐k film can be attributed to the discontinuity of the network structure of their clusters. The heat resistance at the interface between the film and Si substrate was also evaluated. We found that the low‐k film exhibited, interestingly, negative interfacial heat resistance, although interfacial heat resistance should have a positive value in general. In order to determine the origin of the negative interfacial heat resistance, the interface state of the films was analyzed in detail on the basis of X‐ray reflectivity (XRR) measurements. The XRR results showed clearly that a thin, high‐density layer was present at the interface of the low‐k films. This high‐density layer presumably promoted heat flow to the substrate, resulting in the apparent negative interfacial heat resistance. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
17.
DongLiang Li JianWei Du Song He DeQing Liang XiangYong Zhao XiangYang Yang 《中国科学:化学(英文版)》2012,55(3):373-379
The effective thermal conductivities of gas-saturated porous methane hydrates were measured by a single-sided transient plane source (TPS) technique and simulated by a generalized fractal model of porous media that based on self-similarity.The density of porous hydrate,measured by the volume of the sample in the experimental system,was used to evaluate the porosity of methane hydrate samples.The fractal model was based on Sierpinski carpet,a thermal-electrical analogy technique and one-dimensional heat flow assumption.Both the experimental and computational results show the effective thermal conductivity of methane hydrate decreases with the porosity increase.The porosity of 0.3 can reduce the thermal conductivity of the methane hydrate by 25%.By analysis of the experimental data and the simulative result,the optimized thermal conductivity of the zero-porosity methane hydrate is about 0.7 W m-1K-1. 相似文献
18.
The validity of two approaches widely used to determine the radiant thermal conductivity in plastic foams is discussed. While one approach is based on the solution of a geometric model, the other is derived from the experimental determination of the extinction coefficient. A comparison to recently reported experimental data shows that the geometric approach predicts values that are in good agreement. In contrast, values deduced from measurements of the mean extinction coefficient significantly underestimate the radiant thermal conductivity, an effect that can be traced to the way that the extinction coefficient is measured. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 190–192, 2005 相似文献
19.
The advanced construction of a two-channel ac calorimeter for simultaneous measurements of frequency-dependent complex heat capacity C(ω) and complex thermal conductivity λ(ω) is presented. In the new calorimeter, the number of interfaces with thermal-wave reflections was reduced. Thus, the new construction can be easily calibrated with higher precision and is simpler in handling than the previous one. The new construction allows to measure thermal conductivity in steady-state mode, as well as frequency-dependent complex thermal properties in ac mode, in the same measuring cell. The capabilities of this technique were demonstrated, being applied for simultaneous measurements of complex effusivity, diffusivity, heat capacity, and thermal conductivity of glycerol in the glass transition region. The so-called ac and dc thermal conductivities of glycerol were measured as a function of temperature. It was shown that the double-channel ac calorimetry is a technique, which can be used for reliable distinguishing of relaxation processes related to relaxing thermal conductivity or relaxing heat capacity.In the region apart from phase transitions, the calorimeter provides the unique possibility of simultaneous measurements of the thermal contact properties together with the sample’s thermal parameters. The improvement of the accuracy gave us the possibility to observe the thermal contact resistance, leading to a step of 1 and 5% in the temperature-modulation amplitude at the cell/sample interface in the case of liquid samples such as Apiezon™-H grease and glycerol, respectively. A step of 25% was observed in the case of a dry thermal contact between the cell and an ethylene-1-octene copolymer sample. Thus, the thermal contact resistance must be taken into account in the temperature-modulated calorimetry, especially in the case of a dry cell/sample contact. 相似文献
20.
A. V. Kondyurin A. P. Sviridov M. V. Obrezkova V. V. Lunin 《Russian Journal of Physical Chemistry A, Focus on Chemistry》2009,83(8):1405-1413
A technique for simultaneous measurements of the thermal diffusivity, specific heat, and effective absorbtion coefficient
was developed. The technique is based on local heating of a sample by laser radiation and thermal imager measurement of the
temperature field dynamics in the surface layer in both the heating and cooling stages. The technique includes a program for
calculating the laser-induced temperature field in the sample volume and the determination of three parameters by the Levenberg-Marquardt
algorithm to provide the best fit of calculations to experimental results. The statistical error of thermal diffusivity, specific
heat, and effective absorbtion coefficient measurements was 5–6%. The technique efficiency was demonstrated by the example
of the development of a thermal and optical equivalent of cartilage tissue, based on polyacrylamide hydrogel. 相似文献