Thermal radiative resistance

Here the thermal radiative resistance must assume a more complex form... 

Figure 4 Spectral specific extinction (absorption) of pure silica aerogel (dashed), silica aerogel doped with 5% carbon black (dotted), and RF aerogel (solid). Note that the transmission window in pure silica aerogel between 3 and 5 j4m leads to a dramatic increase in thermal radiative transport and renders pure Si02 aerogels ineffective as thermal insulators above 100°C. The integration of an opacifier markedly improves the thermal resistance of Si02 aerogels.

Figure 3 shows the temperature profile of 10 mm size particle, similar to that of the 2 mm size particle. The shapes of these curves are very similar to those of small particle. However, the variation of temperature increasing rate for every layer is much more distinct than the case of 2 mm particle. This is because the high char yield of the larger particle reduces the radiative fraction of thermal conductivity, and increases the resistance of heat transfer. 

One of the most common manifestations of a deposition problem is reduced heat transfer in the radiant zone of a furnace. Decreased heat transfer due to a reduction in surface absorptivity is a result of the combination of radiative properties of the deposit (emiss-ivity/absorptivity) and thermal resistance (conductivity) of a deposit. Thermal resistance (thermal conductivity and deposit overall thickness) is usually more significant because of its effect on absorbing surface temperature. 

From a radiative thermal network, normalizing the heat transfer to surface i of area A, and where all other surface resistances Rj Ri, then... 

M. M. Yovanovich, General Expressions for Constriction Resistances of Arbitrary Flux Distributions, in Radiative Transfer and Thermal Control, AlAA Progress in Astronautics and Aeronautics, Vol. 49, pp. 381-396, New York, 1976. 

Thermal conductivity Radiative Solid Gaseous Specific heat Young s modulus Pyrolyzed electrical resistivity Density ... 

FIGURE 17.13 Thermal conduction representation of an electrical component by two thermal resistances, ic and 0ji,. The resistance to ambient (R ) is the result of convective and radiative heat loss. 

Farnworth also presented a theoretical model ofthe combined conductive and radiative heat flows through fibrous insulating materials and compared them with experimental values of the thermal resistances of several synthetic fibre battings and of a down and feather mixture (Table 4.9). No evidence of convective heat transfer is found, even in very low-density battings. The differences in resistance per unit thickness among the various materials may be attributed to their different absorption constants. 

It is worth emphasizing, however, that more work is needed in this very critical area. For instance. Ref. 127 shows the existence of a sigitificant dispersion of the estimates in the literature for foam thermal conductivities the role of the additional contributions due to convection and radiative heat transfer still need to be assessed and tbe role of the thermal resistance at the interface between the foam and the inner reactor tube wall has not been addressed so far. 

V. Z. Geller and V. G. Peredri [2.5] used a modified version of the stationary heated-filament method. A thin-wall platinum capillary was adopted as an external resistance thermometer this allowed an increase in the accuracy of determination of thermal conductivity. The absence of convection was ensured by measurements at two to four different temperature gradients in the layer (Rayleigh number did not exceed 1500). When computing, corrections were introduced to compensate for the eccentricity of the filament, heat outflow from the ends, and the change in geometric sizes of the measuring cell. In all, the corrections did not exceed 0.3-0.5%. Correction for radiative heat losses were not introduced due to the absence of infrared absorption in the spectrum of liquid Freon-21. This, however, as shown by the calculations did not noticeably distort the results of the measurements. The tests were carried out at a gap of 0.5 mm. 

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