Abstract
Supercritical Fluids (SCFs) have unique thermophyscial properties and heat-transfer characteristics, which make them very attractive for use in power industry. In this chapter, specifics of thermophysical properties and heat transfer of SCFs such as water, carbon dioxide, and helium are considered and discussed. Also, particularities of heat transfer at Supercritical Pressures (SCPs) are presented, and the most accurate heat-transfer correlations are listed. Supercritical Water (SCW) is widely used as the working fluid in the SCP Rankine “steam”-turbine cycle in fossil-fuel thermal power plants. This increase in thermal efficiency is possible by application of high-temperature reactors and power cycles. Currently, six concepts of Generation-IV reactors are being developed, with coolant outlet temperatures of 500°C~1000°C. SCFs will be used as coolants (helium in GFRs and VHTRs, and SCW in SCWRs) and/or working fluids in power cycles (helium, mixture of nitrogen (80%) and helium (20%), nitrogen and carbon dioxide in Brayton gas-turbine cycles, and SCW/“steam” in Rankine cycle).
TopIntroduction
SuperCritical Fluids (SCFs) are not a human invention. Actually, compressed fluid (i.e., fluid at pressures above the critical one, but at temperatures below the critical one) - water exists in oceans below the depth of ~2.2 km, which corresponds to the critical pressure of water - 22.064 MPa. If there is an underwater volcano with molten lava at this depth then water can reach temperatures above the critical one (373.95°C) and becomes SCF.
Also, nature has been processing minerals in aqueous solutions at near or above the critical point of water for billions of years (Levelt Sengers, 2000). In the late 1800s, scientists started to use this natural process in their labs for creating various crystals. During the last 50 - 60 years, this process, called hydrothermal processing (operating parameters: water pressure from 20 to 200 MPa and temperatures from 300 to 500ºC), has been widely used in the industrial production of high-quality single crystals (mainly gem stones) such as sapphire, tourmaline, zircon and others.
First works dedicated to specifics of thermophysical properties of fluids and heat transfer at SuperCritical Pressures (SCPs) started as early as the 1930s. E. Schmidt and his associates (Schmidt, 1960; Schmidt et al., 1946) investigated free-convection heat transfer of fluids at the near-critical point with the application to a new effective cooling system for turbine blades in jet engines. They found that free-convection Heat Transfer Coefficient (HTC) at the near-critical state was quite high and decided to use this advantage in single-phase thermosyphons with an intermediate working fluid at the near-critical point.
In the 1950s, the idea of using SuperCritical Water (SCW) appeared to be rather attractive for SC “steam” generators. At SCPs, there is no liquid-vapour-phase transition; due to that there is no such phenomenon as critical heat flux or dryout. Only within a certain range of parameters a deterioration of heat transfer may occur. The main objective of operating “steam” generators at SCPs was to increase the total thermal efficiency of a power plant. Work in this area was mainly done in the USA, former USSR, Germany and some other countries in the 1950s - 1980s. Currently, use of SCW in coal-fired power plants is the largest application of fluid at SCPs in industry.
At the end of the 1950s and the beginning of the 1960s, some studies were conducted to investigate a possibility of using SCFs in nuclear reactors (Pioro and Duffey, 2007) with the same objective as in the thermal-power engineering, i.e., to increase thermal efficiency of water-cooled-reactors Nuclear Power Plants (NPPs). Several concepts of nuclear reactors using SCW as a coolant were developed in the USA, former USSR and some other countries. However, this idea was abandoned for almost 30 years and regained momentum in the 1990s.
At the end of 20th century, leading nuclear-power countries started to look into a possibility to develop next generation nuclear-power reactors. And in 2001, under the leadership of the U.S. Department Of Energy’s (DOE) Office of Nuclear Energy, Science and Technology a framework for international co-operation in research and development for the next generation of nuclear-energy systems has been launched as the Generation-IV International Forum (GIF) Initiative, which was originally signed by Argentina, Brazil, Canada, France, Japan, Republic of Korea, South Africa, the United Kingdom (UK) and the United States (US). Later on, Switzerland (2002), Euratom (2003), the People’s Republic of China (2006), the Russian Federation (2006), and Australia (2016) have joined the GIF (https://www.gen-4.org/gif/jcms/c_9260/public).
GIF experts have evaluated about 130 reactor concepts and selected six reactor technologies for further research and development. These technologies include (listed in the alphabetical order): 1) Gas-cooled Fast Reactor (GFR), 2) Lead-cooled Fast Reactor (LFR), 3) Molten Salt Reactor (MSR), 4) Supercritical Water-cooled Reactor (SCWR), 5) Sodium-cooled Fast Reactor (SFR), and 6) Very High Temperature Reactor (VHTR).
Key Terms in this Chapter
Pseudocritical Point: (Characterized with P and T pc ) is the point at a pressure above the critical pressure and at a temperature ( T pc > T cr ) corresponding to the maximum value of specific heat at this particular pressure.
Deteriorated Heat Transfer (DHT): Is characterized with lower values of the heat transfer coefficient compared to those for normal heat transfer; and hence, has higher values of wall temperature within some part of a test section or within the entire test section.
Normal Heat Transfer (NHT): Can be characterized in general with heat transfer coefficients 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 0.8 Pr 0.4 .
Supercritical “Steam”: Is actually supercritical water, because at supercritical pressures fluid is considered as a single-phase substance. However, this term is widely (and incorrectly) used in the literature in relation to supercritical-“steam” generators and turbines.
Superheated Steam: Is steam at pressures below the critical pressure, but at temperatures above the critical temperature.
Critical Point: (also called a critical state) is the point in which the distinction between the liquid and gas (or vapour) phases disappears, i.e., both phases have the same temperature, pressure and specific volume or density. The critical point is characterized with the phase-state parameters: T cr , P cr and v cr (or ? cr ), which have unique values for each pure substance.
Brayton Cycle: Is thermodynamic power cycle based on gas turbine. Widely used in gas-fired thermal power plants (consists of compressor, combustion chamber (source of heat) and gas turbine connected to electrical generator; open cycle; working fluid: combustion products). Might be used in Generation-IV (next generation) nuclear power plants (consists of reactor (source of heat), (heat exchanger for indirect cycle), gas turbine connected to electrical generator, recuperator, heat exchanger (cooler) to reject heat, compressors, and intercooler; closed cycle; working fluids: supercritical-pressure helium (direct and indirect cycles), nitrogen (indirect cycle), nitrogen-helium mixture (indirect cycle), and carbon dioxide (indirect cycle).
Improved Heat Transfer (IHT): Is characterized with higher values of the heat transfer coefficient compared to those for normal heat transfer; and hence, lower values of wall temperature within some part of a test section or within the entire test section. In our opinion, the improved heat-transfer regime or mode includes peaks or “humps” in the heat transfer coefficient near the critical or pseudocritical points.
Supercritical Fluid: Is the fluid at pressures and temperatures that are higher than the critical pressure and critical temperature. However, in the present Chapter, the term supercritical fluid usually includes both terms - a supercritical fluid and compressed fluid.
Rankine Cycle: Is thermodynamic power cycle based on steam turbine. Widely used in fossil-fuel thermal power plants (consists of steam generator installed inside combustion chamber or flue-gases duct (source of heat), steam turbine connected to electrical generator, condenser, circulation pump, feedwater heaters; working fluid: subcritical- or supercritical-pressure water/steam). Widely used in all current nuclear power plants (subcritical-pressure cycle). Might be used in Generation-IV nuclear power plants (consists of reactor (source of heat), (heat exchanger for indirect cycle), steam turbine connected to electrical generator, condenser, circulation pump, feedwater heaters; working fluid: subcritical- or supercritical-pressure water/steam (direct and indirect cycles) or supercritical-pressure carbon dioxide (indirect cycle).