Hot-Wire Technique

This technique has been used to measure the specific heat of polyamide 6, polypropylene, polymethylene methacrylate, rigid polyvinyl chloride and cellular polyurethane foam, high-density polyethylene, low-density polyethylene and polystyrene [48], epoxy resins [73], polypropylene [74], polymethyl methacrylate [75], high-density polyethylene, low-density polyethylene, polypropylene and polystyrene [77], and polytetrafluoroethylene [76], 

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Bel 2 using a hot wire technique, but none of these processes has industrial application. 

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 hot wire technique appears to be the most useful one among the various electrical methods. However, the hot wire anemometer can operate only at relatively low droplet velocities. 

J. de Boer, J. Butter, B. Grosskopf, and P. Jeschke, Hot Wire Technique for Determining High Thermal Conductivities , Refract., J. 22-28 (1980). 

Two other ways are available for obtaining metallic uranium (i) by the electrolysis of KUFg in fused CaClg (80%) and NaCl (20%) at 900° with the graphite container as anode and a molybdenum cathode. The current density employed is 1.5 amp/cm and the product better than 99.9%. (ii) by the hot-wire technique (Van Arkel and De Boer, 1925), also used for Ti (p. 449), Zr and W, applied to UI4. 

Deposition on 0.15-mil Tungsten Wire. The 0.15-mil tungsten substrate wire was cleaned by the hot-wire technique at hydrogen flow rate of 140 cc./min. and at a d.c. power input of 208 volts and 90 milliamperes. While the input vapors were split between the three plating cells, only one cell was utilized for plating studies. 

Figure 11-3. Reaction kinetics of sulfur compounds by the hot-wire technique. 1 0.75% Elemental sulfur in white oil, 763 K. 2 Didodecyl disulfide, 1% S diphenyl disulfide, 1% S 763 K. 3 Uncompounded white oil, 763 K. 4 0.75% Elemental sulfur in white oil, 703 K. 5 0.75%...

Figure 11-5. Reaction kinetics of chlorine compounds by the hot-wire technique. 1,3 IK Benzyl chloride. 2 1% Hexachloroethane. 4 1% Chlorinated paraffin. 5 1% Pentachlorodiphenyl. a Carrier oil without additive. From data by Sakurai, Sato and Yamamoto [37].

The accuracy of this technique is to within 0.3%. Both tungsten and platinum wires are used with diameters of below 7 i im, more typically 4 ixm (for low-pressure gas measurements). Small diameters reduce errors introduced by assuming the wire is a line source (infinitely thin). The effect becomes significant for gases and increases with decreasing pressure. The thermal diffusivity of gases is inversely proportional to the pressure and thus thermal waves may extend to the cell wall. For these cases, a steady state hot-wire technique is used for which the design equation is ... 

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

Figure 26.11. Influence of the system pressure (vacuum quality) on the effective thermal conductivity of granular and monolithic silica aerogel (as measured by the hot-wire technique under ambient conditions) [28].

A laser flash technique has been used to determine the diffusivity of pyroelectric polymers such as polyvinylidene fluoride [83], whereas hot-wire techniques have been used to determine the thermal diffusivity of high-density polyethylene, low-density polyethylene propylene, and polystyrene [83], Dos Santos and coworkers [84] utilized the laser flash technique to study the effect of recycling on the thermal properties of selected polymers. Thermal diffusivity expresses how fast heat propagates across a bulk material, and thermal conductivity determines the woiking temperature levels of a material. Hence, it is possible to assert that those properties are important if a polymer is used as an insulator, and also if it is used in applications in which heat transfer is desirable. Five sets of virgin and recycled commercial polymers widely used in many applications (including food wrapping) were selected for this study. 

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). 

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. 

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. 

The first fiow characteristics of single phase stirred tank reactors were performed using the HFA technique. For example, Gtinkel and Weber [77] used a hot-wire technique to measure instantaneous fluid velocities in the bulk flow and between the impeller blades in baffled stirred vessels. The HFA technique was later used to measure the first instantaneous fluid velocities in dispersed two-phase (gas-liquid) dispersions. Attempts were also performed to measure the local gas void fraction. Many examples of such investigations of two-phase bubble column flows can be found in the literature [67, 72, 76, 135, 199]. 

Murshed, S. M. S., Leong, K C. Yang, C. (2006). Determination of the effective thermal diffusivity of nanofluids by the double hot-wire technique. Journal of Physics D Applied Physics, Vol. 39, pp.5316-5322. 

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