Thermal diffusivity critical region

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 The foregoing thoughts are primarily suppositions but appear to be supported by some as yet unpublished experimental work on thermal diffusion in turbulent flow.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. 

The diffusion, thermal diffusion, and Soret coefficients of this system are shown in Fig. 8. Samples of two different off-critical compositions (c = 0.3 and c = 0.9) were prepared. The temperature was set to a value of a few degrees above the bin-odal. Hence, the sample was entirely within the homogeneous phase and one would expect that heating could only drive the blend further into the stable one-phase region. 

There is considerable evidence that the thermal diffusion ratio becomes very large in the critical region. > > > It is possible, but not certain, that the anomaly can be ascribed solely to the vanishing of Z)(r). ... 

Rutherford, W.M. Calculation of Thermal Diffusion Factors for the Methane-n-Butane System in the Critical and Liquid Regions." AIChE J vol. 9, p. 841,1963,... 

Fig. 6.4. The thermal diffusivity of CO2 in the critical region. The symbols indicate experimental data, and the curves represent calculated values of a = a + Acc (Vesovic et al. 1990).

To apply the above scheme, accurate experimental measurements for the transport properties of the monatomic fluids were collected. In Table 10.1 the experimental measurements of diffusion, viscosity and thermal conductivity used for the correlation scheme are shown. This table also includes a note of the experimental method used, the quoted accuracy, the temperature range, the maximum pressure and the number of data sets. The data cover the range of compressed gas and the liquid range but not the critical region, where there is an enhancement (Chapter 6) which cannot be accounted for in terms of this simple molecular model. 

Jany, P. Straub, J. (1987). Thermal diffusivity of fluids in a broad region around the critical point. Int. J. Thermophys., 8, 165-180. 

Many physical properties undergo dramatic changes in value as water is heated and pressurized from sub- to supercritical conditions, particularly in the region of the critical point where some properties such as heat capacity reach a singularity. This change in behavior means that more familiar correlations of properties measured at subcritical conditions are likely to be inaccurate when applied at supercritical conditions. There have been some experimental studies performed to measure, tabulate, and in some cases correlate values of key properties of supercritical water, such as the self-diffusion coefficient, viscosity,thermal conductivity," heat capacity at constant volume," dielectric constant," and selfdissociation constant." " Far more work has been devoted to calculation of property values from models fitted empirically to data or developed more rigorously through molecular simulation. For PVT data and its derivatives, several attempts... 

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