Coaxial-cylinder technique

In this technique, the fluid is placed in a ring-shaped gap between two coaxial cylinders with a common vertical axis. According to the ideal model, a thin layer of a homogeneous 

In the above relation, di is the external radius of the inner cylinder and di is the internal radius of the external cylinder. The ends of the cylinders can be made in different shapes flat, conical or spherical. 

When measuring the thermal conductivity with this technique, some corrections should be made for (1) radiation-induced heat transfer (radiative correction) (2) parasitic heat transfer from the inner to the outer cylinder through a central solid pintle, electric wires, and thermocouples (3) convective heat transfer and (4) effects of possible temperature jumps at the interface between the liquid layer and the cylinder surface. 

The correction for radiation is usually calculated by the Stefan-Boltzmann law - see Equation (5.7) - assuming the radiation absorption in the liquid is negligibly small. Le Neindre et al. (1976) employed silver cylinders with perfectly polished surfaces to reduce the heat transfer by radiation. The emissivity of these cylinders was small and Qj estimated from Equation (5.7) is negligible by comparison with the heat transfer by conduction. 

The convective heat transfer, can be calculated fi om the relation (Johannin, 1958) 

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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). 

In the radial heat-flow method the specimen is in the shape of a hollow cylinder, which is positioned in the annulus between two coaxial cylinders with the internal cylinder acting as a radial heat source. The temperature profile across the specimen is determined by thermocouples placed on the inside walls of the two cylinders. This method requires a large isothermal zone in the furnace, which is difficult to achieve at high temperatures. When this technique is used for measurements on liquids, errors can occur from convective heat transfer. 

Thermal diffusion of petroleum samples is carried out in the annular space defined by two coaxial cylinders whose surfaces are separated by distances of approximately 0.2 mm. These surfaces are maintained at different temperatures. Figure 3.15 demonstrates a laboratory setup34 used to practice this technique. Separation is performed by filling the annular space with the sample, then allowing the system to equilibrate for a period up to several days. In one configuration, the cylinder diameters are about 0.5 in., the annular spacing is about 0.0115 in., and the vertical length is 6 ft. The sample is injected at the center position and the product samples are taken off at a number of sample points, frequently ten of these, on the vertical axis after an appropriate time (3 to 10 days in practice) for the diffusional separation to take place. The samples, a few milliliters or less in size, are then analyzed by modem instrumental methods. 

The majority of detectors with radioisotope sources in current use are based on either the coaxial cylinder or asymmetric configurations, as shown in Figure 3. The low specific activity of the Ni source requires a relatively larger source area to provide a suitable background current that is easier to accommodate in these designs. Virtually all contemporary detectors employ pulse-sampling techniques to collect the thermal electrons based on the variable frequency constant... 

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