Supercritical Water Thermophysical Properties

In the phase diagram of a liquid, as shown in Fig. 2.1, the region above the critical point is called the supercritical region. In the case of water, the critical point is at 374.2°C and 22.1 MPa. Above this temperature and pressure, the water is called supercritical water. Pressures below the critical point are referred to as subcritical pressures. Due to the relatively low viscosity of supercritical water with respect to its density and high specific heat enthalpy, it has a good ability as a coolant. 

3 Changes of specific heat capacity of water with respect to temperatme 

A phenomenon similar to the boiling transition of a subcritical fluid is recognized to occur during heat removal by a supercritical fluid. It is known as the heat transfer deterioratirai phenomenon and occurs when the fluid flow rate is relatively low for the high heat flux [1-4]. However, unlike the boiling transition of a subcritical fluid. 

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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. ... 

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. 

Liquid metals are considered as efficient coolants in some fast neutron breeder reactor concepts due to their excellent thermophysical and neutron properties [36]. As already mentioned, the Generation IV reference liquid metal coolants are sodium, lead, and lead-bismuth eutectic. Some challenging corrosion issues are also studied in the molten salts and supercritical water environments. 

As mentioned above, numerical computations were carried out [5, 6] based on a k-e model by Jones-Launder. This model has a more general description for turbulence than the mixing length models. Effects of buoyancy force and fluid expansion on the heat transfer to normal fluids are successfully analyzed by the k-e model. Thermophysical properties are treated as variables in the governing equations and evaluated from a steam table library. Thus, extremely nonlinear thermophysical properties of supercritical water are evaluated directly and correctly. This approach is applicable to a wide range of flow conditions of supercritical water. Many cases of different inlet temperatures can be calculated and the relation between the heat transfer coefficient and the bulk enthalpy can be obtained in a wide range. 

Generally speaking, the conventional numerical analysis with a k-e turbulence model and accurate treatment of thermophysical properties can successfully explain the unusual heat transfer phenomena of supercritical water. Heat transfer deterioration occurs due to two mechanisms depending on the flow rate. When the flow rate is large, viscosity increases locally near the wall by heating. This makes the viscous sublayer thicker and the Prandtl number smaller. Both effects reduce the heat transfer. When the flow rate is small, buoyancy force accelerates the flow velocity near the wall. This makes the flow velocity distribution flat and generation of turbulence energy is reduced. This type of heat transfer deterioration appears at the boundary between forced and natural convection. As the heat flux increases above the deterioration heat flux, a violent oscillation of wall temperature is observed. It is explained by the unstable characteristics of the steep boundary layer of temperature. 

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