Critical phenomena pure fluid

Similar problems will arise in the analysis of other properties. For example, the observed heat capacity Cj, will not display an actual divergence. Only one layer in the cell is at the true critical point and the total enthalpy is averaged over all heights and will not show singular behaviour. The phenomenon is entirely analogous to the gravitational perturbation of the measurement of Cv for a pure fluid. ... 

For large particles, is given very accurately by > e with = 6 even for small particles, however, ) e given a good estimate of D. In fact, Ie with = 4, is amazingly accurate for of a dense pure fluid. It follows that the mode-mode contribution to must always be important in dense fluids, even for small particles. This state of affairs holds for other types of diffusion, but not for nondiffusive processes. Diffusion is the slowest of transport processes, since it may only proceed by actual physical transport of particles, while other transport utilizes this mechanism plus collisional transport of the quantity of interest from particle to particle. However, the mode-mode part of all transport coefficients is comparable to D e, i e., to D, Thus, mode-mode coupling becomes important in nondiffusive transport only when some phenomenon, such as a critical point, slows down the processes that usually dominate the transport. 

Compare the behavior of the ethylene-n-propanol system to a type I system, CO2-hexane, shown in Figure 13. Note that in both cases, solubility of the supercritical component in the liquid phase increases rapidly as the pressure increases. This phenomenon can lead to substantial swelling of the liquid phase. The solubility of the heavy component in the lighter phase does not increase rapidly with an increase in pressure until the mixture critical point is approached or an additional phase is formed. The conditions of these occurrences may be substantially removed from the pure supercritical fluid critical conditions. 

In thermally non-homogeneous supercritical fluids, very intense convective motion can occur [Ij. Moreovei thermal transport measurements report a very fast heat transport although the heat diffusivity is extremely small. In 1985, experiments were performed in a sounding rocket in which the bulk temperature followed the wall temperature with a very short time delay [11]. This implies that instead of a critical slowing down of heat transport, an adiabatic critical speeding up was observed, although this was not interpreted as such at that time. In 1990 the thermo-compressive nature of this phenomenon was explained in a pure thermodynamic approach in which the phenomenon has been called adiabatic effect [12]. Based on a semi-hydrodynamic method [13] and numerically solved Navier-Stokes equations for a Van der Waals fluid [14], the speeding effect is called the piston effecf. The piston effect can be observed in the very close vicinity of the critical point and has some remarkable properties [1, 15] ... 

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