Thermal conductivities values

Values of thermal conductivity are temperature-dependent and vary widely for different materials. Table 1 summarizes the thermal conductivity values of a few materials relevant to heat-exchanger analysis (1,2). 

Polyurethane. Polyurethanes (pu) are predominantly thermosets. The preparation processes for polyurethane foams have several steps (see Urethane polymers) and many variations that lead to products of widely differing properties. Polyurethane foams can have quite low thermal conductivity values, among the lowest of all types of thermal insulation, and have replaced polystyrene and glass fiber as insulation in refrigeration. The sprayed-on foam can be appHed to walls, roofs, tanks, and pipes, and between walls or surfacing materials directly. The slabs can be used as insulation in the usual ways. 

TABLE 11-26 Representative Apparent Thermal Conductivity Values... 

The effective therm conductivity values generally obtained in practice are at least a factor of two greater than the one-dimensional thermal conductivity values measured in the laboratoiy with carefully controlled techniques. This degradation in insulation thermal performance is caused by the combined presence of edge exposure to isothermal boundaries, gaps, joints, or penetrations in the insulation blanket required for structure supports, fill and vent hnes, and high lateral thermal conductivity of these insulation systems. 

Figures 7 and 8 show thermal conductivity data for CBCF after exposure to temperatures of 2673, 2873, 3073, and 3273 K, for 5.7 and 15 7 seconds, respectively. The symbols in the Figs. 7 and 8 represent measured thermal conductivity values, and the solid lines are the predicted behavior from Eqs. (5) through (8) The model clearly accounts for the effects of measurement temperature, exposure tune, and exposure temperature The fit to the data is good (typically within 10%). However, the fit to the as fabricated CBCF data (Fig 6) was less good (-20%), although the scatter in the data was larger because of the much lower heat treatment temperature (1873 K) in that case.

With the exception of the last calibration run (calibrating the parameters in series) all ground thermal conductivity and borehole conductivity values are very comparable. The ground thermal conductivity values estimated are significantly lower than the estimates obtained with the line source method (on the first 40 h of data). When the parameters are estimated separately, estimated ground thermal conductivity is higher. 

Table 2 Thermal Conductivity Values for Selected Materials... 

It is important to note that Vie and Kjelstrup [250] designed a method of measuring fhe fhermal conductivities of different components of a fuel cell while fhe cell was rurming (i.e., in situ tests). They added four thermocouples inside an MEA (i.e., an invasive method) one on each side of the membrane and one on each diffusion layer (on the surface facing the FF channels). The temperature values from the thermocouples near the membrane and in the DL were used to calculate the average thermal conductivity of the DL and CL using Fourier s law. Unfortunately, the thermal conductivity values presented in their work were given for both the DL and CL combined. Therefore, these values are useful for mathematical models but not to determine the exact thermal characteristics of different DLs. 

Figure 8.17 A materials selection bar chart showing ranges of thermal conductivity values for three of the material classes. Reprinted, by permission, from M. F. Ashby, Materials Selection in Mechanical Design, p. 33, 2nd ed. Copyright 1999 by Michael F. Ashby.

Two other references (M13, SI) have discussed the problem of choice of correct thermal conductivity values. Miller (M13) worked with water-graphite and kerosene-graphite suspensions at concentrations of the solid between 4.7 and 13.5% by weight (0.02 < xv < 0.08). This investigator noted the results of attempts to correlate the heat transfer data in two manners. 

Fig. t. Thermal conductivity values for a wide range of substances and over a temperature range of t lo It)4 K... 

The thermal conductivity values of some common explosives, in cal/sec/cm/ C, are as... 

Coal is a low-to-medium conductor of heat, with thermal conductivity values ranging from about 3 x 10-4 to 9 x 10 4 cal/s cm °C (Speight, 1994, and references cited therein). The thermal conductivity X of coal is related to thermal diffusivity (a) and the heat capacity (Cp) by... 

Thermal conductivity increases with increasing apparent density, volatile matter, ash, and mineral matter content. Due to the high porosity of coal, thermal conductivity is also strongly dependent on the nature of gas, vapor, or fluid in the pores, even for monolithic samples (van Krevelen, 1961). Moisture has a similar effect and increases the thermal conductivity of coal since its thermal conductivity value is approximately three times higher than that of dry coal (Speight, 1994, and references cited therein). However, the thermal diffusivity of coal is practically unaffected by moisture since the /Cp value is not essentially changed by moisture. 

The following table gives the properties of common gas chromatographic carrier gases. These properties are those used most often in designing separation and optimizing detector performance. The density values are determined at CPC and 0.101 MPa (760 torr).1 The thermal conductivity values, X, are determined at 48.9°C (120°F).1 The viscosity values are determined at the temperatures listed and at 0.101 MPa (760 torr).1 The heat capacity (constant pressure) values are determined at 15°C and 0.101 MPa (750 torr).2... 

The thermal conductivity value K should also reference the liquid film value and the gas phase for a condensing film, averaging the two at the tube wall temperature. 

Some applications, however, must conduct heat but not electricity. In these applications the adhesive must permit high transfer of heat plus a degree of electrical insulation. Fillers used for achieving thermal conductivity alone include aluminum oxide, beryllium oxide, boron nitride, and silica. Table 9.9 lists thermal conductivity values for several metals as well as for beryllium oxide, aluminum oxide, and several filled and unfilled resins. 

Effective thermal conductivity values of porous materials Ap,efr range between 0.1 and 0.5Js IK 1 in gaseous atmospheres [6] and are only slightly larger than those for the gas phase. Straightforward combination of eqs 23 and 37 and integration leads to a simple general result that relates the temperature and concentration profile over a particle ... 

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