Temperature dependence of thermal conductivity and thermal diffusivity of some composites using the transient plane source technique

The recently developed transient plane source (TPS) technique has been applied for the simultaneous measurement of thermal conductivity and thermal diffusivity of two composite materials namely, marble and magnesium oxychloride cement in the range of temperatures from 30 to 150°C. The experimental results of these samples show that there is very slight variation in thermal conductivity and thermal diffusivity of these materials in this range of temperature. An effort has been made to express this variation of thermal conductivity and diffusivity with temperature by a linear relation, in these materials.     

Thermal conductivity and its temperature dependence in selected steel samples

The transient plane source (TPS) technique was employed to measure experimentally the thermal conductivity of three selected steel samples. Measured values of thermal conductivities are critically analysed, taking into account factors such as the composition and the nature of the alloying elements. An effort is made to determine the temperature dependence of thermal conductivity of these samples from room temperature to 300°C. Results of the present measurements are also discussed in the light of the values of thermal conductivity on equivalent steels measured by other workers.     

Simultaneous measurements of thermal conductivity and thermal diffusivity of insulators,fluids and conductors using the transient plane source (TPS) technique

Simultaneous measurements of thermal conductivity and thermal diffusivity of composite red-sand bricks, glycerine and mercury have been made at room temperature by the recently developed transient plane source (TPS) technique. This paper describes, in brief, the theory and the experimental conditions for the simultaneous measurements of thermal conductivity and thermal diffusivity of insulators, fluids and metals. The source of heat is a hot disc made out of bifilar spirals. The disc also serves as a sensor of temperature increase in the sample. The measured values of the thermal conductivity and thermal diffusivity of these samples are in agreement with the values reported earlier using other methods. The advantage of the TPS technique is the simplicity of the equipment, simultaneous information on thermal conductivity and thermal diffusivity, and also the applicability of the technique to insulators, fluids and metals.     

Measurements of the thermal diffusivity of linear-medium-density-polyethylene/aluminium composites using a transient comparative method

Metal-powder/polymer composites are an interesting class of material because their physical properties may, within limits, be selected to match a particular application. It is therefore important to be able to measure and model the physical properties of these composites. The effective diffusivity of linear-medium-density-polyethylene/aluminium composites was measured for a range of volume fractions using a simple, transient comparative method. Effective thermal conductivity data were calculated from the effective thermal diffusivity data. The effective thermal conductivity data were modelled well by the EMT equation.     

Determination of the thermal transport properties of ammonia borane and its thermolysis product (polyiminoborane) using the transient plane source technique

Reliable thermal property data are necessary to improve the fidelity of chemical hydride thermal decomposition models. The thermal diffusivity and conductivity of ammonia borane (NH₃BH₃) and its partial thermolysis product (polyiminoborane) were measured at various packing densities using a transient plane source technique under ambient conditions. The particle size of the ammonia borane powder was between 200 and 600 μm, while the particle size of the polyiminoborane powder was between 10 and 30 μm. The thermal diffusivity and conductivity of the ammonia borane increased from 0.17 to 0.24 mm²/s and 0.19 to 0.44 W/m K (±10%), respectively, when its packing density was increased from 0.37 to 0.58 g/cm³. The increase in thermal conductivity is due to the increase in contact area between particles and the increase in the thermal diffusivity is related to an increase in density and volumetric heat capacity caused by compaction. The thermal conductivity of the polyiminoborane powder was approximately three times lower, likely due to its higher porosity. The thermal diffusivity and conductivity of this product changed from 0.21 to 0.12 mm²/s and 0.068 to 0.23 W/m K (±10%), respectively, when its packing density was increased from 0.13 to 0.96 g/cm³.     

Determination of the thermal conductivities of building and insulating materials by the probe method

A simple and inexpensive electrical circuit based on the transient probe method has been developed for the determination of thermal conductivities of building and insulating materials namely limebrick, gypsum, rockwool and polystrene foam. The solution of the unsteady state heat conduction equation has been approximated by considering certain simplifying assumptions which have been justified by experimental observations. The overall accuracy of the thermal conductivity measurements is estimated to be within ± 3%.     

液体导热系数的瞬态双热线测量

研究设计瞬态双热线液体导热系数测量仪,分析了该测试仪器的基本原理及其模型修正,并通过测量去离子水对该装置进行检定。通过与标准值比较,测量的平均误差为0.61%,最大误差为-0.9%,该装置有较高的测试精度。     

Revisiting the block method for evaluating thermal conductivities of clay and granite

Determination of thermal conductivities of porous media using the contact method was revisited and revalidated. Thermal conductivities of granite and clay were determined in the laboratory with and without the use of thermal interface material (TIM) (Arctic Silver®) to reduce contact resistance. KD2 probe was also used with and without TIM to compare results. Thermal conductivity of dry clay sample increased from 0.68 W/mK to 0.85 W/mK while that of granite sample increased from 2.95 W/mK to 3.95 W/mK with TIM. The difference in thermal conductivities with and without TIM was significant at (P > 0.05).     

Simultaneous measurement of thermal properties by thermal probe using stochastic approximation method

A novel thermal probe method is proposed for the simultaneous measurement of the thermal properties by the Monte Carlo stochastic approximation method. In this method, thermal capacity of probe and thermal contact resistance between probe and sample are considered. An experimental system is set up with the method to validate the measurement accuracy of the method. The thermal properties of several liquid samples as well as solid samples are measured. The results show that: (1) the thermal conductivity and the volumetric heat capacity can be measured with an error of less than 1.2% and 3% respectively, therefore, the measurement accuracy by the method is much higher than the conventional method and (2) the thermal contact resistance has a great effect on thermal conductivity for solid sample, while little influence on thermal conductivity for liquid sample and volumetric heat capacity.     

An Absolute Method for Determination ofThermal Conductivities of Thermal Insulators by Microcalorimetry

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Measurements of thermal conductivity and thermal diffusivity of alternative refrigerants in liquid phase with a transient short‐hot‐wire method

A transient short‐hot‐wire method has been proposed to simultaneously measure the thermal conductivity and thermal diffusivity of liquids. The method has been applied to the refrigerant HCFC‐22, alternative refrigerants HFC‐32, HFC‐125, HFC‐134a and refrigerant mixtures HFC‐32/125 [35/65, 50/50 (HFC‐410A), 68/32 wt%], HFC‐32/125/134a [23/25/52 (HFC‐407C) wt%]. From the present study, it is shown that the measured results agree well with the literature values on thermal conductivity and those on thermal diffusivity obtained from NIST's thermophysical property database, REFPROP Ver. 6.0. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(8): 540–552, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20032     

Measurement and modelling of the thermal conductivity of dispersed aluminium composites

Measurements of the thermal conductivity of a model system (squat aluminium cylinders suspended in an aqueous carbohydrate polymer gel) have been performed for a range of compositions up to the packing factor limit of the aluminium (approximately 0.6). Of a variety of models considered, the Cheng–Vachon model provided the most accurate predictions of thermal conductivity, and it was argued that this would also be the case for suspensions of most metals in polymer matrices. A modified form of the Cheng–Vachon model was used to obtain an even closer fit to the experimental data.     

On-site experimental estimation of thermal conductivities and heat capacities in multilayer walls under arbitrary transient conditions using explicit and implicit finite difference schemes

It is often required to estimate the thermal properties of the layers of a multilayer wall, which is already part of an existing building. Such cases are encountered when an ex post check is required in order to find out if the design specifications have been followed, or if air conditioning loads have to be calculated in old buildings, the walls of which are composed of layers of unknown materials and thermal properties. In the present study, a method is proposed for estimating the thermal conductivities and heat capacities of the layers a multilayer wall is composed of. The method is based on explicit and implicit finite difference schemes and uses on-site temperature measurements at various locations within the wall. It is applicable to multilayer walls which are already parts of buildings. The outdoor and indoor conditions may be arbitrary, i.e. transient, nonperiodic, with solar radiation. The accuracy of the method, which has been verified by numerical and experimental applications, depends on the available number of temperature values in space. For example, in a 10-cm thick wall layer, measurement at five locations gives satisfactory accuracy, which is considerably improved by increasing the number of values in space using fourth-order polynomial interpolation.     

Determination of thermal parameters of poor conductors by transient techniques

The authors compare the transient hot wire method and the parallel wire method of determining thermal conductivity, using the second to find the thermal diffusivity of two materials. The wires are sandwiched between two samples of the material to be investigated. The influence of pressure is also studied in order to identify the measurement conditions that can be easily achieved. A method is chosen to evaluate thermal parameters and to determine the field of data to be used in relation to sample dimensions.     

Ex situ measurements of through-plane thermal conductivities in a polymer electrolyte fuel cell

In this paper thermal properties for materials typically used in the proton exchange membrane fuel cell (PEMFC) are reported. Thermal conductivities of Nafion membranes were measured ex situ at 20 °C to be 0.177 ± 0.008 and 0.254 ± 0.016 W K⁻¹ m⁻¹ for dry and maximally wetted membranes respectively. This paper also presents a methodology to determine the thermal conductivity of compressible materials as a function of applied load. This technique was used to measure the thermal conductivity of an uncoated SolviCore porous transport layer (PTL) at various compaction pressures. For the dry PTL at 4.6, 9.3 and 13.9 bar compaction pressures, the thermal conductivity was found to be 0.27, 0.36 and 0.40 W K⁻¹ m⁻¹ respectively and the thermal contact resistivity to the apparatus was determined to be 2.1, 1.8 and 1.1 × 10⁻⁴ m² K W⁻¹, respectively. It was shown that the thermal contact resistance between two PTLs is negligible compared to the apparatus’ thermal contact resistivity. For a humidified PTL, the thermal conductivity increases by up to 70% due to a residual liquid saturation of 25%.     

The Measurement of Thermal Conductivities of Silica and Carbon Black Powders at Different Pressures by Thermal Conductivity Probe

This investigation was done to study the gas filled powder insulation and thermal conductivity probe for themeasurement of thermal conductivity of powders.The mathematical analysis showed that the heat capacity ofthe probe itself and the thermal resistance between the probe and powder must be considered.The authorsdeveloped a slender probe and measured the effective thermal conductivity of silica and carbon black powdersunder a variety of conditions.     

Reconstruction of thermal conductivity and heat capacity using a tomographic approach

We consider the estimation of the volumetric heat capacity and the thermal conductivity as distributed parameters. The measurement scheme consists of sequentially heating the boundary of the object in different source locations and measuring the induced temperature evolutions in different measurement locations on the boundary. The estimation of the distributions of volumetric heat capacity and thermal conductivity based on these boundary data is an ill-posed inverse boundary value problem. We propose an approach which is based on transient data on the boundary and the modelling of the unknown coefficients as Markov random fields. The intended applications are non-destructive retrieval of defects as well as the estimation of macroscopic characteristics of novel materials. We evaluate the proposed approach by a numerical simulation.     

Improvement of thermal characteristics of latent heat thermal energy storage units using carbon-fiber brushes: experiments and modeling

Brushes made of carbon fibers with a high thermal conductivity are inserted on the shell side of a heat exchanger to enhance the conductive heat transfer rates in phase change materials. The experimental results show that the brushes essentially improve the heat exchange rate during the charge and discharge processes even when the volume fractions of the fibers are about one percent. A three-dimensional model describing the heat transfer in the heat exchanger is numerically solved. The model predicts well the experimental outlet fluid temperatures and the local temperatures in the composite.     

Inverse determination of thermal conductivity by differential quadrature method

The differential quadrature method (DQM) is employed to inverse determination of thermal conductivity in one-dimensional slab. Several examples including spatially dependent and temperature dependent thermal conductivities are solved to illustrate the accuracy and efficiency of the DQM. The advantages of this approach are that no a priori assumption is made on the functional form of the estimates, and that the estimated thermal conductivity can be obtained directly in the calculating process. Of interest are the effects of the number of the measurement points and magnitude of measurement errors on the accuracy of the estimations. Comparisons are made between the estimated thermal conductivities and exact forms, and the agreement is found out to be generally good. Numerical results demonstrate that the DQM is effective and stable in treating this type of problem.     

Study on the efficiency of effective thermal conductivities on melting characteristics of latent heat storage capsules

Improvement of the thermal conductivity of a phase change materials (PCM) is one effective technique to reduce phase change time in latent heat storage technology. Thermal conductivity is improved by saturating porous metals with phase change materials. The influence of effective thermal conductivity on melting time is studied by analyzing melting characteristics of a heat storage circular capsule in which porous metal saturated with PCM is inserted. Numerical and approximate analyses were made under conditions where there are uniform or non-uniform heat transfer coefficients around the cylindrical surface. Four PCMs (H₂O, octadecane, Li₂CO₃, NaCl) and three metals (copper, aluminum and carbon steel) were selected as specific materials. Porosities of the metals were restricted to be larger than 0.9 in order to keep high capacity of latent heat storage. Results show that considerable reduction in melting time was obtained, especially for low conductivity PCMs and for high heat transfer coefficient. Melting time obtained by approximate analysis agrees well with numerical analysis. A trial estimation of optimum porosity is made balancing the desirable conditions of high latent heat capacity and reduction of melting time. Optimum porosity decreases with increase in heat transfer coefficient.