The Measurement of Thermal Conductivity

In general, thermal conductivity measurements comprise the measurement of the heat flow passing through the sample of certain dimensions and the temperatures on isothermal surfaces, while the distance between these surfaces is known. 

There are many methods of taking thermal conductivity measurements, and aU of them have certain inaccuracies and discrepancies (the main uncertainty is the impossibility to eliminate the heat drain). 

The methods of obtaining thermal conductivity measurements are divided into two main groups  

Based on the principle of nonstationary heat flow (dynamic methods). 

Very generally speaking, it is possible to say that the methods based on the principle of stationary heat flow are better for measurements of materials with medium and low thermal conductivity, while the methods based on the principle of 

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Many carrier gas methods were reviewed in detail by Lomax2, 3. Apparatus using the measurement of thermal conductivity to estimate permeability has been described, for example, by Yasuda and Rosengren10... 

Figure 9.7 Guarded hot-plate method for the measurement of thermal conductivity [15]. Typical specimen dimensions are disks of 25 cm diameter and 5 cm thick [14],...

Although this value is 1.9% too large it is still a useful approximation, as its deviation from the exact value is within the bounds of uncertainty associated with the measurement of thermal conductivity. 

Abstract. In this study we report a literature review on the research and development work concerning thermal conductivity of nanofluids as well as their viscosity. Different techniques used for the measurement of thermal conductivity of nanofluids are explained, especially the 3co method which was used in our measurements. The models used to predict the thermal conductivity of nanofluids are presented. Our experimental results on the effective thermal conductivity by using 3co method and effective viscosity by vibro-viscometer for Si02-water, Ti02-water and A Os-water nanofluids at different particle concentrations and temperatures are presented. Measured results showed that the effective thermal conductivity of nanofluids increase as the concentration of the particles increase but not anomalously as indicated in the some publications and this enhancement is very close to Hamilton-Crosser model, also this increase is independent of the temperature. The effective viscosities of these nanofluids increased by the increasing particle concentration and decrease by the increase in temperature, and cannot be predicted by Einstein model. 

J.S. Powell, An instrument for the measurement of thermal conductivity of liquids at high temperatures, Meas. Sci. Technol, 2, 111-117 (1991). 

Th is chapter deals with the measurement of thermal conductivity, thermal diffusivity. and specific heat. Other properties that are sometimes included under the umbrella term "thermal properties" are dealt with in other parts of this volume. In most cases it does not matter whether the sample is a rubber or a plastic, the experimental techniques arc the... 

The longitudinal heat flow (guarded hot plate) method is regarded as the most accurate and most widely used apparatus for the measurement of thermal conductivity of poor conductors of heat. This method is most suitable for dry homogeneous specimens in slab forms. The details of the technique are given by the American Society for Testing and Materials (ASTM) Standard C-177 [82]. 

Nesvadba, R, Methods for the measurement of thermal conductivity and diffusivity of foodstuffs, J. FoodEng., 1 93-113 (1982). 

McGinnis, D.S., Automated line-heat source system for the measurement of thermal conductivity and diffusivity. Can. Agric. Eng., 29 201-207 (1987). 

The temperature pulse decay technique has been used to measure both the in vivo and in vitro thermal conductivity and blood flow rate in various tissues (Xu et al., 1991 1998). The specimen does not need to be cut from the body, and this method minimizes the trauma by sensing the temperature with a very small thermistor bead. For the in vitro experimental measurement, the measurement of thermal conductivity is simple and relatively accurate. The infinitively large tissue area surrounding the probe implies that the area affected by the pulse heating is very small in comparison with the tissue region. This technique also requires that the temperature distribution before the pulse heating should reach steady state in the surrounding area of the probe. 

Kazeminejad [20] has described the construction of an apparatus for the measurement of thermal conductivity accordingly to ASTM C177 [21] and DIN 52612-2 [22]. He describes a method of determining thermal conductivity of insulating materials, based on a copper-coated printed circuit board. Thermal conductivity values are reported for pure PE, pure PC, and PE and PC mixed with conductive fillers such as aluminium powder and carbon black. 

The mechanical properties of other semicrystalline polymers similarly depend on the conditions of deformation process (4,59,60). The thermal properties of semicrystalline polymers subjected to SSE have a highly pronounced anisotropy. Thus, the measurements of thermal conductivity of linear polyethylene done in a wide temperature range in directions parallel (If ) and normal (K ) to the direction of extrusion, have shown that If" > (1,61). With EDR increase, thelf"/lf ... 

Thermal conductivities can be measured using DSC. Chiu and Fair (32) irreversibly modified a DSC cell to determine thermal conductivity. Sircar and Wells (33) developed a modification of this approach, which allowed the use of the same cell for conventional DSC work as well as for the measurement of thermal conductivity. Marcus and Blaine (34) discuss the possibilities for using MTDSC for measuring thermal conductivity. 

The measurement of thermal conductivity of plastics is a difficult process, and until recently, the availability of such data was quite limited. Conventional guarded hot-plate techniques have traditionally been used to make measurements of thermal conductivity. These remain the reference technique for thermal conductivity measurements, since they do not require calibration against a material of known thermal conductivity. 

Hot-topping Tile. Refractory insulating tiles performing the function of a MOULD BRICK (q.v.), i.e. delaying solidification of the melt in a hot-top. Hot-wire method. A dynamic method for the measurement of thermal CONDUCTIVITY (q.v.) applied to refractories up to 1500°C (PRE/R32 1978). A linear heat source embedded in the test piece gives a power output constant along its length and in time. 

Answer by Author By definition, thermal conductivity is a characteristic property of the material, and therefore independent of the geometry. This definition will apply, provided that the thermal conductivity is measured over a small temperature difference. When large temperature differences are used in the measurement of thermal conductivity, the thickness of the material will appear to influence thermal conductivity because of the effect of radiant heat transfer through the material. 

F. H. Norton and W. D. KIingerly, The Measurement of Thermal Conductivity of Refractory Materials Technical Progress Report, USAEC Report NYO-601, Massachusetts Institute of Technology, 1952,... 

Jaeger, J.C., 1958. The measurement of thermal conductivity with cylindrical probes. EOS Trans. Am. Geophys. Union 39 (4), 708-710. 

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