Transient hot-wire technique

Only two investigators (DiGuilio et ah, 1990 Bleazard et al, 1994 and Bleazard and Teja, 1995) employed the transient hot-wire technique for the measurement of the thermal conductivity of aqueous solutions above 200 °C. Hence this technique will only be briefly described here. 

A transient thermal conductivity measurement is one in which a time-dependent perturbation, in the form of a heat flux, is applied to a fluid initially at equilibrium. The thermal conductivity is obtained from an appropriate working 

The application of this methodology to liquids and gases at moderate pressures has provided many reliable thermal-conductivity data over the last two decades. Unfortunately, the analytical corrections proposed by Healy et al (1976) proved to be inadequate (Assael etal, 1998) for the description of experiments in the gas phase at low densities, where fluids exhibit exceedingly high thermal-diffusivity values. 

To overcome these difficulties, a numerical finite element method was proposed by Assael et al (1998), in order to solve the complete set of energy-conservation equations that describe the heat-transfer experimental processes. The ehoice of this particular numerical method was dictated by the high accuracy the method exhibits in computational heat transfer problems. Hence, two coupled partial differential energy-eonservation equations, one for the wire and one for the fluid, with appropriate boundary conditions, were solved. 

In praetiee, experimental means are employed to yield a finite segment of a wire that behaves as if it were part of an infinite wire. This allows the numerical solution of the differential equations to be used iteratively to determine the thermal conduetivity and diffusivity of the fluid that yields the best mateh between the experimental and calculated temperature rise of this finite segment of wire. 

---

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.%). ... 

MEASUREMENT OF THE THERMAL CONDUCTIVITY OF FROST BY A TRANSIENT HOT-WIRE TECHNIQUE. 

Most of the conventional techniques of thermal conductivity measurements are based on the steady-state solution of Equation (5.1), i.e. establishing a stationary temperature difference across a layer of liquid or gas confined between two cylinders or parallel plates (Kestin and Wakeham, 1987). In recent years, the transient hot-wire technique for the measurement of the thermal conductivity at high temperatures and high pressures has also widely been employed (Assael et al, 1981, 1988a,b, 1989, 1991, 1992, 1998 Nagasaka and Nagashima, 1981 Nagasaka et al., 1984, 1989 Mardolcar et al., 1985 Palavra et al, 1987 Roder and Perkins, 1989 Perkins et al, 1991, 1992 and Roder et al, 2000). 

In Table 5.1 all available experimental thermal conductivity data sources at high temperatures (above 200 °C) and high pressures are presented. As one can see from this table, all data were derived by the parallel-plate and the coaxial-cylinder techniques, except only two datasets for LiBr by Bleazard et al. (1994) and DiGuilio and Teja (1992) which were obtained by the transient hot-wire technique. We further note that almost all investigators quote an uncertainty of better than 2%. In this section a brief analyses of these methods is presented. The theoretical bases of the methods, and the working equations employed is presented, together with a brief description of the experimental apparatus and the measurements procedure of each technique. For a more thorough discussion of the various techniques employed, the reader is referred to relevant literature (Kestin and Wakeham, 1987 Wakeham et al., 1991 Assael et al, 1991, and Wakeham and Assael, in press). 

Uribe et al. (1991) surveyed all the modem measurements of A.mix. obtained by the refined transient hot-wire technique, in order to assess the accuracy of their prediction scheme. The deviations were not particularly systematic, indicating that no single large effect remains unaccounted for. Improvements in the accuracy of calculated A jx will therefore probably be difficult to achieve, requiring improved treatment of many effects at once. 

M. J. Assael, K. D. Antoniadis, and D. Tzetzis, "The use of the transient hot-wire technique for measurement of the thermal conductivity of an epoxy-resin reinforced with glass fibres and/or carbon multi-walled nanotubes," Composites Science and Technology, vol. 68, pp. 3178-3183,2008. 

Revisions to the technique continue to be reported. A parallel wire technique has been described wherein the thermocouple wire runs parallel, about 15 mm away from the hot wire [10]. A transient hot strip technique for thermal conductivity measurement has also been recently described [13]. 

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]. 

In this section the different techniques (parallel-plate, coaxial-cyUnder, and transient hot-wire) employed to measiue the thermal conductivity of aqueous solutions at high temperatures were discussed. In general, the uncertainty of all the available experimental thermal conductivity data derived with the first two methods should be within 1... 

Another transport property of molten salts for which there is a considerable amormt of data is their thermal conductivity Xth- In earher years the then available measuring techniques led to the conclusion that Xth increases mildly with increasing temperatures. More modem techniques, such as transient hot wire measurements, yield values of that diminish mildly and linearly with increasing temperatures. The scatter of values reported in the literature is large, however, and they have not been critically compiled so far. Gheribi et al. [277] provided an explicit model expression for 2, the required inputs for their model being the ionic radii, and the density, velocity of sound, heat capacity, and melting temperature of the salt. Table 3.21 shows the recommended predicted values in terms of the parameters of the linear temperature dependence ... 

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,... 

The hot-wire method is a transient technique in which the temperature rise of a hot wire immersed in the sample is monitored as function of time. It is better suited for molten materials, and variants have been designed to measure both melts and solids. 

Since the technique was first employed in 1931 (Stahlane and Pyk, 1931) to measure the thermal conductivity of powders, there have been significant improvements in the practical realization of the technique. In modem instm-ments the wire sensor aets both as the heat source and as a thermometer. Rapid development of analogue and digital equipment as well as of eomputer-driven data-acquisition systems, have meant that precise measurements of transient electrical signals can be made quickly. Thus, it has become possible to measure the resistance change taking place in the hot wire as a consequence of its temperature rise with a... 

---