Supercritical pressure thermophysical properties

Due to their peculiar solvent properties, supercritical fluids offer a range of unusual chemical possibilities such as in environmentally benign separation and destruction of hazardous waste, as well as for new materials synthesisd" These intriguing reaction media make it possible to sensitively control reaction rate and selectivity with changes in temperature and pressure. The thermophysical properties of water as well as more than 70 other fluid systems have been formulated and/or compiled by lAPWS and NIST. ... 

Pioro, I., Mokry, S., Draper, S., 2011. Specifics of thermophysical properties and forced-convective heat transfer at critical and supercritical pressures. Reviews in Chemical Engineering 27 (3—4), 191—214. 

Therefore, the most widely used supercritical fluids as of today and possibly in the future are water, carbon dioxide, helium, and refrigerants. Often, refrigerants, similar to carbon dioxide, are considered as modeling fluids instead of water due to significantly lower critical pressures and temperatures (for example, R-134a Per = 4.0593 MPa Per = 101.06°C), which decreases the complexity and costs of thermal hydraulic experiments. Based on the above mentioned, knowledge of thermophysical properties specifics at critical and supercritical pressures is very important for safe and efficient use of fluids in power and other industries. 

Figs. A3.5—A3.12 show variations in the basic thermophysical properties of water at three subcritical pressures [all (a) figures] (1) 7 MPa—usual operating pressure of boiling water reactors (BWRs) and many Ranldne steam turbine cycles in pressurized water reactor (PWR), BWR, and RBMK NPPs (2) 11 MPa—usual inlet pressure for CANDU reactors and (3) 15 MPa—usual pressure for PWRs and the critical (Per = 22.064 MPa) and four supercritical pressures (P = 25, 30, 35, and 40 MPa) [all (b) figures]. The range of critical and supercritical pressures covers current range... 

It should be noted that thermophysical properties of 121 pure fluids, including water, carbon dioxide, helium, refrigerants, etc. 5 pseudo-pure fluids (such as air) and mixtures with up to 20 components at different pressures and temperatures, including critical and supercritical regions, can be calculated using the NIST REFPROP software (2010), Version 9.1. 

Analysis of profiles shown in Figs. A3.5—A3.12 for subcritical water [figures (a)] and critical/supercritical water [figures (b)] shows similar trends. However, for subait-ical water, there are two different values of any thermophysical property on the saturation line one for liquid and one for vapor (steam). However, for example, at pressure of 7 MPa, values of specific heat of water (5.4025 kJ/kg K) and steam (5.3566 kJ/kg K) can be very close (see Fig. A3.9(a)). Also, it can be clearly seen that pressure has almost negligible effect of liquid properties. Just closer to the saturation line, some small differences can be seen in property profiles at various pressures. 

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. 

Eq. [A4.4] is applicable for subcritical and supercritical pressures. However, adjustment of this expression to conditions of supercritical pressures, with singlephase dense gas and significant variations in thermophysical properties near the critical and pseudocritical points, was the major task for the researchers and scientists. In general, two major approaches to solve this problem were taken an analytical approach (including numerical approach) and an experimental (empirical) approach. 

Pioro, I., 2014. Application of supercritical pressures in power engineering specifics of thermophysical properties and forced-convective heat transfer. In Anikeev, V., Fan, M. (Eds.), Supercritical Fluid Technology for Energy and Environmental Applications. Elsevier, pp. 201-233. 

Thermophysical properties of gases, liquids, and fluids at supercritical pressures used in publications of Dr. I. Pioro were calculated according to the National Institute of Standards and Technology software (2010) (http //www.nist.gov/srd/nist23.cfm). 

Technical Appendices, which provides readers with additional information and data on current nuclear power reactors and NPPs thermophysical properties of reactor coolants, thermophysical properties of fluids at suhcritical and critical/supercritical pressures, heat transfer and pressure drop in forced convection to fluids at supercritical pressures, world experience in nuclear steam reheat, etc. 

Therefore, for the higher-pressure case we may adopt a numerical analysis, based on iterative integration around the loop of the momentum equation (since mass is also conserved) for varying loop power inputs, using the thermophysical properties of the supercritical fluid as a function of actual thermodynamic state. Thus the general flow variation with major loop parameters (elevations, losses etc.) follows Equation (4) but with a non-linear expansion coefficient. 

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