Pseudocritical region

Pseudocritical region is a narrow region around a pseudocritical point where aU thermophysical properties of a pure fluid exhibit rapid variations. For H2O, it is about 25°C from pseudocritical temperature. 

The specific heat of He, Na, Pb, and Pb—Bi (Fig. A2.6) is nearly constant over the whole range of operational parameters. In the case of CO2, the specific heat increases linearly and reaches the same value as Na at around 750°C. The specific heat of water goes through a peak (where its value increases almost 8 times) within the pseudocritical region. The specific heats of Pb and LBE are nearly identical and 10 times less than those of Na and CO2, and almost 40 times less than that of He. At temperatures higher than 450°C, the specific heat of He is higher than that of SCW. 

Figure A3.13 Variations of selected thermophysical properties of water near pseudocritical point (384.9°C at 25 MPa) pseudocritical region is about 25°C around pseudocritical point.

Analyses of Figs. A3.25—A3.27 for helium show that helium as a reactor coolant will perform as a compressed gas because of operating range of pressures and, especially because temperatures are way above those of critical/pseudocritical regions. 

Supercritical fluids are used intensively in various industries. Therefore, understanding specifics of thermophysical properties and their behavior at critical and supercritical pressures is an important task. Supercritical fluids are considered as single-phase substances in spite of significant variations of all thermophysical properties within critical or pseudocritical regions. Some of these variations in thermophysical properties are similar to those at subcritical pressures during crossing of the saturation line. 

NHT can be characterized, in general, with HTCs similar to those of subcritical convective heat transfer far from the critical or pseudocritical regions, when they are calculated according to the conventional single-phase Dittus—Boelter-type correlations Nu = 0.023 Re ... 

In general, experimental heat transfer coefficient values show just a moderate increase within the pseudocritical region. This increase depends on flow conditions and heat flux higher heat flux—less increase. Thus, the bulk fluid temperature might not be the best characteristic temperamre at which all thermophysical properties should be evaluated. Therefore, the cross-sectional averaged Prandfl number (see Fig. A4.8), which accounts for thermophysical properties variations within a cross section due to... 

Chueh s method gives consistently good results for mixtures except in the immediate vicinity of the critical region (T/TCml > 0.93). For the critical region, his procedure was modified by using true critical constants, rather than pseudocritical constants in Eq. (56). For this purpose, he has established a separate correlation of true critical volumes and temperatures (C3). 

Figures 2, 3, and 4 present isothermal pressure-composition diagrams of the liquid-vapor regions close to the pseudocritical locus, at - 90 , -95°, and -100°F. Since no pseudocritical points were determined in this particular study, the pseudocritical points shown are taken from the work published by Donnelly and Katz [1].

Figures 2, 3, 4, 5, and 6 are diagrams representing the best lines through a considerable amount of data. Some of the data were found to scatter considerably, particularly in the regions near the pseudocritical point and near the triple point. Several times, apparently valid liquid samples were taken in a region which should have contained only vapor and solid, indicating a strong tendency toward a metastable liquid state. Consequently, the brute force method of obtaining a large number of points was used to define the boundaries of the liquid-vapor region.

The foregoing expressions for both viscosity and thermal conductivity are for pures. For mixtures it is recommended that the pseudocritical method be used where possible with liquid regions of Figures 1-5 through 1-8. 

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