A computer-controlled apparatus for thermal conductivity measurement by the transient hot wire method (original) (raw)
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Mediterranean Journal of Basic and Applied Sciences, 2020
The measurement of the physical properties (density, viscosity, surface tension, thermal conductivity, etc) is of great importance to the research industry and for the physical, chemical and biomedical applications. The thermal conductivity is a measurement of the material’s ability to conduct heat. The transient hot wire method is a suitable method to measure the thermal conductivity due to its very cheap cost of construction, accuracy and because it is a fast method of measurement. The implementation requires accurate temperature sensing, automatic control, data acquisition and data analysis. The basic procedure consists of measuring the temporal temperature rise in a thermoresistance (thin wire) immersed in the solution by applying an electrical current in the wire. Therefore, the wire works as a heat source and a temperature sensor. The time of measurement is very short and therefore the convection effect could be minimized. Then, the heat transfer to the infinite medium is due only to the conduction transfer effect. The thermal conductivity can be determined from the slope of the curve ΔT versus ln(t) due to the linear relation between ΔT and ln(t).
Applied Thermal Engineering, 2007
An apparatus has been designed and developed for the measurement of the thermal conductivity of samples of non-metallic materials whose thermal conductivity is quite low (in the range between 0.2 and 4 W/m K) by the transient hot-wire method. It is especially conceived to ensure short time consuming and economic measurement of the thermal conductivity of mortar and lateritic bricks for building application. Thermal conductivity is measured by tracking the thermal pulse propagation induced in the sample by a heating source consisting of a Nickel alloy wire. The temperature is measured on the wire by means of two T type (Copper-Constantan) thermocouples. The heat impulse transferred to the wire between two observed times gives a temperature increment of 2-10°C, depending on the thermal conductivity of the material tested and on the preselected level of the heating power supplied. The thermal conductivity of the materials can be obtained in a comparative way or in a semi-absolute way. In both cases a preliminary calibration of the instrument, obtained with a reference sample whose thermal conductivity previously certified by absolute methods, is in the range required. In the second case, the calibration is necessary to obtain the characteristic curves of the instrument. The paper shows the measurement obtained using materials with thermal conductivity between 0.2 and 1.5 W/m K. In good experimental conditions the accuracy of the measurements is within 5%. The proposed apparatus offers significant advantages, mostly in terms of economy and flexibility, over systems currently in use and over similar systems based on transient methods.
18th International Congress of Metrology
This work concerns the characterization of the thermal conductivity of liquids by hot wire method. A measuring cell has been specially developed for this study. The method of analysing the transient response makes it possible to obtain values of conductivity close to those expected (deviations <4%) for two reference fluids and measurement uncertainties in the order of 5%. The use of the so-called steady state method (ASTM D 2717) leads to relative deviations and uncertainties of the measurements about 3 times higher.
International Journal of Thermophysics, 2006
The paper deals with an assessment of the repeatability of a method for measuring thermal conductivity of high temperature melts. The main goal is to demonstrate that a novel approach to the transient hot wire technique can reproduce highly accurate results, independently of previous measurements. The paper summarises the modified transient hot wire method, presents improvements in FE analysis and briefly discusses deviations from available analytical equations. The transient hot wire instrument and experimental configuration are also described. Results from measurements of molten metals, in particular tin and indium, in the temperature range from their melting points up to 750K are presented. Finally a comparison with previously measured values is given and the accuracy and repeatability of the method are discussed.
Thermal Conductivity of Reference Solid Materials
International Journal of Thermophysics, 2000
The thermal conductivity of three thermal-conductivity reference materials, Pyrex 7740, Pyroceram 9606, and stainless steel AISI 304L, has been studied. The technique employed is the transient hot-wire technique, and measurements cover a temperature range from room temperature up to 570 K. The technique is applied here in a novel way that eliminates all remaining contact resistances. This allows the apparatus to operate in an absolute way. The method makes use of a soft silicone paste material between the hot wires of the technique and the solid of interest. Measurements of the transient temperature rise of the wires in response to an electrical heating step in the wires over a period of 20 ms up to 20 s allow an absolute determination of the thermal conductivity of the solid, as well as of 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 0.1% for Pyrex 7740, 0.4% for Pyroceram 9606, and 0.2% for stainless steel AISI 304L, while the standard uncertainty of the technique is less than 1.5%.
Correct Use of the Transient Hot-Wire Technique for Thermal Conductivity Measurements on Fluids
International Journal of Thermophysics
The paper summarizes the conditions that are necessary to secure accurate measurements of the thermal conductivity of fluids using the transient hot-wire technique. The paper draws upon the development of the method over five decades to produce a prescription for its use. The purpose is to provide guidance on the implementation of the method to those who wish to make use of it for the first time. It is shown that instruments of the transient hot-wire type can produce measurements of the thermal conductivity with the smallest uncertainty yet achieved (± 0.2%). This can be achieved either when a finite element method (FEM) is employed to solve the relevant heat transfer equations for the instrument or when an approximate analytic solution is used to describe it over a limited range of experimental times from 0.1 s to 1 s. As well as establishing the constraints for the proper operation of the instrument we consider the means that should be employed to demonstrate that the experiment o...
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 1999
The nonstationary hot wire method with a silica-coated probe has been developed to measure thermal conductivities of molten metals at high temperatures. Measurements were carried out on mercury and lead as test liquids. The thermal conductivities of liquid mercury ranged between 7.6 and 8.1 W/m K at temperatures between 273 and 293 K, and those of molten lead displayed constant values of about 15 W/m K at temperatures between 673 and 973 K. Factors affecting the thermal conductivity measurement using this method have been discussed. It has been concluded that the nonstationary hot wire method with an insulator-coated probe can be applied to various molten metals, as long as materials for probe coating are pertinent.
Thermal Conductivity of Solid Materials
INTERNATIONAL JOURNAL OF RESEARCH IN ENGINEERING & SCIENCE
Thermo physical properties such as specific heat, thermal conductivity and density are unique for any material. Within a conductive material, thermal conductivity is the main property in the thermal energy transfer. Specific heat and density are the important components involved in the analysis of energy and mass balances. When combining these three properties, we get the thermal diffusivity, which is used in the analysis of unsteady-state heat transfer. Two major methods are used to measure the thermal conductivity of any material. The steady state method named guarded plate method which depends on a constant temperature difference achieved in the specimen of the material. This method needs complicated measurement system, it is unsuitable for the field application [1]. The transient (unsteady-state) techniques generally use a heater of a particular geometry inserted in the sample, and heated within a period of time. These measurement systems are less complex with respect to steady state methods and are better suited for field measurements. The experimental studies for determination of the thermal properties in a transient state has been conducted with numerous methods and techniques. The linear heat source method relies on a steady heat source ,for example, a hot wire, that generate s a temperature field inside an infinite volume of a material. Based on the thermal conductivity of the desired material, the temperature rise in its sample will vary from one material to other. The thermal conductivity, then, can be calculated from the temperature rise at two unique times and the power of the heater. This method was used to measure the thermal conductivity of many solids , fluids and soil [2-4].
Measurement of thermal conductivity of fluid using single and dual wire transient techniques
Measurement, 2013
A modified measurement device to measure thermal conductivity of fluids using transient hot-wire technique has been designed, developed, tested and presented in this paper. The equipment is designed such that the thermal conductivity could be measured using both single wire sensor of different length and dual wire sensor. The sensor, which is also a heater, is a platinum micro-wire of 50 lm diameter. The influence of wire length on the measurement of thermal conductivity of fluids is tested using two single wires of length 50 mm and 100 mm. The thermal conductivity is also measured using a dual hot wire arrangement; which is achieved by placing the 100 mm and 50 mm wires in a Wheatstone bridge with the 100 mm wire as the sensor and 50 mm wire as a compensation wire. The apparatus requires a 100 ml of test fluid to perform the experiment. The testing temperature of the test fluid during the experimentation can be suitably varied by the choice of heat exchange fluid used in the apparatus. Water is chosen as testing fluids for primary standards. When compared to single wires, the thermal conductivity of the fluids measured is consistent with dual-wire method with an uncertainty of ±0.25%.
Absolute Steady-State Thermal Conductivity Measurements by Use of a Transient Hot-Wire System
2000
A transient hot-wire apparatus was used to measure the thermal conductivity of ar- gon with both steady-state and transient methods. The effects of wire diameter, eccentricity of the wire in the cavity, axial conduction, and natural convection were accounted for in the analysis of the steady- state measurements. Based on measure- ments on argon, the relative uncertainty at the 95