Hot-Wire Method

The theoretical model assumes a line heat source dissipating heat radially into an infinite solid, initially at uniform temperature. The fundamental heat conduction equation in cylindrical coordinates, assuming uniform radial heat transfer, is [3]  

The solution [4] [5] [6] is obtained using Laplace transforms and the convolution theorm [7]  

Thermal conductivity measurements are taken at a series of isothermal temperatures, created by an external furnace. The furnace must provide enough stability so that there are negligible temperature gradients within the refractory blocks. The furnace is maintained at a particular temperature for a matter of hours before power is applied to the wire in order to assure temperature uniformity. 

The power through the wire can be ac or dc. In order to accurately determine voltage, SCR regulation of ac power is not recommended since the ac waveform is disturbed, mak- 

Various configurations [8, 10] of the test have been demon- 

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The hot-wire method is a transient method and can yield an absolute value. A long, thin wire, through which electric current is passed stepwise, is placed in a melt, when... 

A number of variations of the transient hot-wire method have been devised, and an optical method to detect the temperature rise has been used. A modified transient hot-wire technique using a mercury-incapillary probe was introduced by Nagashima et al., in which a thin mercury thread was used as a heater-thermometer and the capillary wall as an insulator. Using this method, they measured the thermal conductivity in mixture systems such as (Na, K)N03, (Li, Na)N03, and HTS(KN03-NaN03-NaN02, 44-7-49 mol.%). ... 

The data on molten NaNO, are summarized in Table 16. Nagasaka and Nagashima reeommended the following empirieal relation obtained by the transient hot-wire method with eeramic-coated probes ... 

A square end is more easily obtained by the hot wire method. The tubing is first scratched all the way round. This may be done by... 

Hori, T. 1985. Objective measurement of the process of curd formation during rennet treatment of milks by the hot wire method. J. Food Sci. 50, 911-917. 

A common transient method is the line source technique, and such an apparatus was developed by Lobo and Cohen41 which could be used with melts. Oehmke and Wiegmann42 used the line source technique for measurements as a function of temperature and pressure. A hot wire parallel technique43 yielded conductivity and specific heat from the same transient, and then diffusivity was calculated. Zhang and Fujii44 obtained conductivity, diffusivity and the product of density and specific heat from a short hot wire method. 

A comparison of guarded hot plate, transient plane source and modified hot wire methods has been made54 using polyurethane foam, and the strengths and weaknesses of the techniques discussed,... 

W. R. Davis, Hot-Wire Method for the Measurement of the Thermal Conductivity of Refractory Materials , in Compendium of Thermophysical Property Measurement Methods, (K. D Maglic, A. Cezairliyan, and V. E. Pelet-sky, eds.), Vol. 1, Plenum Press, NY (1984). 

A careful test of Nernst s theory was reported by Isnardi243 in 1915. His work consisted of the measurement of the thermal conductivity of iodine (whose dissociation as a function of temperature and pressure was well known), comparison of the experimental results with those of Nernst s theory, and the use of the method to determine JD(H2) which was not then known. Isnardi measured thermal conductivities by the hot wire method, in which the rate of heat removal by conduction from the wire arranged concentrically in a cylinder containing the gas is given by... 

Z)(N2) has also been determined by the flame and by the detonation method, and the experiments have been described and discussed in Section 3.5. Both measurements favour the higher value, as does the hot wire method Although these results are disturbing, it is possible to advance more or less plausible reasons for doubting them see Section 3.5), whereas it does not appear possible on the present evidence to deny the validity of the electron impact results. The author has therefore chosen Z>(N2) =7 38 eV, or A///(N)25og =85 57 kcal 529. 

A very doubtful direct v alue for Z)(GH3 Gl) has been obtained by Doty using the hot wire method It is 74 kcal, which is to be compared with the thermochemical]y derived value of 80 kcal. 

One source of error in methods in which the liquid is in contact with metal plates at fixed temperatures is the discontinuity of temperature at the solid-liquid interface. The Schleiermacher hot-wire method ( 4.VIIG) has been used by several experimenters but its reliability is not beyond question, and a continuous flow method devised by Graetz, in which the liquid flows through a metal tube in a mantle at constant temperature, and the inlet and outlet temperatures in the tube are measured, has also been modified.5... 

A newer method is the transient hot-wire method, where an electric current is passed through a metal wire immersed in the fluid. The resistance of the wire is affected by its temperature, which in turn is affected by the dissipation of heat from the wire s surface, which depends on the thermal conductivity of the fluid. These instruments require sophisticated data analysis, but that is no longer an obstacle with the ready availability of personal computers. The absence of convection is relatively easy to verify. The best research instruments can achieve an accuracy of better than 1%. Measurements on conducting fluids (such as polar liquids) are more difficult because of the need to electrically insulate the wire. Other geometries, such as needle-shaped cylinders and thin strips, are also sometimes used for transient measurements. 

The transient hot wire (THW) method has been well developed and widely used for measurements of the thermal conductivities and, in some cases, the thermal diffusivities of fluids with a high degree of accuracy [6, 42]. More than 80% of the thermal conductivity measurements on nanofluids were performed by transient hot wire method [6, 8, 18, 19, 45-47]. 

Y. Nagasaka and A. Nagashima, Absolute measmement of the thermal conductivity of electrically conducting liquids by the transient hot wire method, J Phys E Sci Instrum., 14, 1435—1440 (1981). 

M. Chirtoc. and J.F. Hemy, 3co hot wire method for micro-heat transfer measurements From anemometry to scaiming thermal microscopy (SThM), Europ. Phys. J., Special Topics, 153, 343-348 (2008). 

Thermal conductivity was measured by the hot wire method. The principle involved in the measurement has been well explained by Carislaw and Jaeger (1959). Specific heat was measured by DSC (Differential Scanning Calorimeter) System TA 2910 (DuPont, U.S.A.) with heat flux type. The temperature differences between the reference material and the target specimen were measured during heating. Measurements were conducted twice at a specified temperature and temperature is varied from room temperature to 100 °C. Relationship between specific heat and temperature was linear and the result is summarized in table 1 and figure 2. 

Figure 3 Total thermal conductivity X of various RF aerogels versus gas pressure Pg p = 82 kg/m3 (circles) p = 157 kg/m (triangles) p = 303 kg/m3 (squares). The data were measured with the hot-wire method at room temperature.

To reveal the thermal properties of aerogels, stationary hot-plate measurements are usually employed [45]. In such a measurement two equal aerogel specimens are sandwiched between a hot plate and two cold plates. If the electrical power fed into the hot plate and the temperature difference between the hot and the cold plates, as well as the thickness of the specimens, are known, the thermal conductivity can be derived. For the thermal characterization of opacified aerogels, the faster nonstationary hot-wire method can also be used. In this case a thin platinum wire is embedded into the aerogel specimen and a constant power is delivered into the wire, which also serves as a temperature sensor. From the temperature increase in the wire as a function of time, the thermal conductivity of the aerogel specimen can be determined [49]. 

A. C.-HEATED HOT WIRE METHOD /DOWTHERM A AND E, DOW CORNING 710, ISOPAR M, LIQUID G AND N204/. 

MEASUREMENT OF THE THERMAL CONDUCTIVITY OF LIQUIDS BY A STATIONARY HOT WIRE METHOD. PH.D. THESIS. 

THERMAL CONDUCTIVITY OF LIQUID REFRIGERANTS MEASURED BY AN UNSTEADY-STATE HOT-WIRE METHOD. 

The transient hot-wire method can be used to measure the thermal conductivity of nanofluids in this study. As schematically shown in Fig. 4, the transient hot-wire measurement system contains a DC power supply, the Wheatstone bridge, a data acquisition system, a nanofluid container, and a computer system to analyze the... 

Deviations of approx. 20% should be expected when Eq. (3.145) is used, which shows that it is worth to perform a data inquiry or to carry out a comparably inexpensive measurement, for instance with the hot-wire method [83], at least in critical cases. 

Transient hot wire methods are most extensively used to measure the thermal conductivity of polymers including PA, PMMA, PP, PVC, LDPE, and PS. This technique has been used by several researchers [26, 32-36]. 

Ebert, H.-P., Bock, V., Nilsson, O. and Fricke, J. Errors From Rarliative Heat Transfer In The Determinatirm Of The Thermal Conductivity In Semitransparent Media Using The Hot-Wire Method, in 21th Eurotherm Seminar Heat Transfer in Semi-Transparent Media. 1992. Lyon. 

Figure 26.7. Monolithic silicaaerogelmonolith synthesized by a two-step catalytic process from TEOS [30], presenting a density of 0.18 g cm and an effective thermal conductivity of 0.015 W m (as respectively measured by mercury pycnometry and with the hot-wire method at room temperature) with couite of Rigacci A. and Achard P.

Dynamic hot-wire method It allows to determine the thermal conductivity by embedding a wire into the material to be investigated. The wire deals as heating element and temperature sensor at the same time and is heated with a constant heat power. From the time-dependent temperature increase of the wire the thermal cmiductivity is derived... 

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