On thermal diffusivity

The large fluctuations in temperature and composition likely to be encountered in turbulence (B6) opens the possibility that the influence of these coupling effects may be even more pronounced than under the steady conditions rather close to equilibrium where Eq. (56) is strictly applicable. For this reason there exists the possibility that outside the laminar boundary layer the mutual interaction of material and thermal transfer upon the over-all transport behavior may be somewhat different from that indicated in Eq. (56). The foregoing thoughts are primarily suppositions but appear to be supported by some as yet unpublished experimental work on thermal diffusion in turbulent flow. Jeener and Thomaes (J3) have recently made some measurements on thermal diffusion in liquids. Drickamer and co-workers (G2, R4, R5, T2) studied such behavior in gases and in the critical region. 

Thermal diffusivity of oxidation-resistant SiC/C compositionally graded graphite materials has been measured by using the laser flash method. In order to study the effect of the SiC/C graded layer on the diffusivity, the thickness of the graded layer and the SiC content were changed. In addition, the specific surface areas of the SiC/C materials have been measured. It is shown that the effect of the SiC/C graded layer on thermal diffusivity was small within SiC contents (0.27-8.52 mass%) used in this study. 

The effect of the SiC/C graded layer on thermal diffusivity was slight within... 

The linear relations depicted in Figs. 16-3a and 16-3b may be forced. The data from which these plots were constructed have some deficiencies in precision and self-consistency. Data of good precision are available from the work of Melpolder ei a .[10] on thermally diffused fractions of a petroleum oil, as listed in Tables 16-6 and 16-7 ... 

Polymers have unique properties important for processing, viz., low thermal diffusivity, high viscosity and viscoelasticity. Because of the low thermal conductivity, efficient plastication cannot be based on thermal diffusivity alone. Polymer melting requires generating heat dissipation through intensive deformation of the highly viscous melts, leading to thin films. 

Smaller components move into the faster flows farther away from the accumulation wall and therefore elute before large molecules. This normal elution mode is reverse to that observed in SEC. The separation can be performed with different force fields. In thermal FEE, the separation is based on thermal diffusion between temperature difference of top and bottom wall. In case of electric FEE, the separation is driven by charge differences and electrophoretic mobility of the molecules applying an electrical field. Another way to separate components depending on their density properties is sedimentation FEE, where a circular channel rotates and generates a gravitational force field. 

In continuous systems, the situation is not so clear. On account of experimental difficulties, exhaustive tests as in the above case have not been performed. Nevertheless, experimental results on thermal diffusion and Dofour effect/Soret do support the theoretical predictions although in a much limited range, again on account of limited range of validity of Gibbs equation. 

Oppenheimer informed Groves that Philip Abelson s experiments on thermal diffusion at the Philadelphia Naval Yard deserved a closer look. Abelson was building a plant to produce enriched uranium to be completed in early July. It might be possible, Oppenheimer thought, to help Abelson complete and expand his plant and use its sBghtly enriched product as feed for Y-12 until problems with K-25 could be resolved. 

Scanning thermal microscopy based on thermal diffusion represents an exciting new SPM tool to probe the thickness of thin polymer layers on conductive substrates. 

Figure 7.5 Influence of boron content in Mg/PTFE on thermal diffusivity of pyrolant [9].

We have observed that countercurrent separation devices achieve considerable separation under driving forces such as chemical potential gradient and the external force of a centrifugal force field. As illustrated in equations (3.1.44) and (3.1.50), there is another type of force, the thermal diffusion force. The separation achieved thereby in a closed two-bulb cell has been illustrated in Section 4.2.5.I. We have already illustrated conceptually how thermal diffusion can achieve separation in a countercurrent column via Figures 8.1.1(a)-(h). For UF isotope separation, however, the radial separation factor in a two-bulb cell is much smaller than for other isotope separation processes. In a countercurrent column, this value is reduced by about 50%. As a result, thermal diffusion columns are not used at all for any practical/large-scale separation. More details on thermal diffusion columns are available in Pratt (1967, chap, viii) and Benedict et al. (1981, pp. 906-915), where one can find information on the primary references. 

Most materials, but dependent on thermal diffusivity and to a lesser extent the optical characteristics of material, rather than chemical composition, electrical conductivity or hardness. 

When the temperature changes with time at any point in a material, the rate of change of temperafure wifh time is dependent on thermal diffusivity. Thermal diffusivity is the ratio of thermal conductivity to heat capacity per unit volume. The heat flux for a transient heat flow can then be written as Equation 16.10. 

The time parameter a depends on thermal diffusivity and on the length of the ring. The larger a is, the better the ability of the materials to propagate heat flux. 

Molecular orientation can have a significant effect on thermal conductivity, and presumably also on thermal diffusivity. Generally, thermal conductivity values can be expected to increase along the direction of orientation or machine direction and decrease in the cross directions. For example, rubber vulcanizates show increases of up to 50% in the direction of stretch (64), and a glassy pol3rmer such as poly(methyl methacrylate) shows increases up to 20% (37). 

Using carbon ions, this technique has been used to carburize a substrate surface prior to deposition of a hard coating. The process is similar to ionitriding, where the reaction in-depth depends on thermal diffusion. In plasma source ion implantation (PSII), the plasma is formed in a separate plasma source and a pulsed negative bias attracts the ions from the plasma to bombard and heat the surface. 

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