Heat Transfer to Supercritical Fluids

General definitions of selected terms and expressions related to heat transfer at critical and supercritical pressuresDeteriorated Heat Transfer (DHT) is characterized with lower values of the wall heat transfer coefficient compared to those at the normal heat transfer; and hence has higher values of wall temperature within some part of a test section or within the entire test section.Improved Heat Transfer (IHT) is characterized with higher values of the wall heat transfer coefficient compared to those at the 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.Normal Heat Transfer (NHT) can be characterized in general with wall heat transfer coefficients similar to those of subcritical convective heat transfer far from the critical or pseudocritical regions, when are calculated according to the conventional single-phase Dittus-Boelter-type correlations: Nu = 0.0023 Re 0.8 Pr 0.4 .Pseudo-boiling is a physical phenomenon similar to subcritical pressure nucleate boiling, which may appear at supercritical pressures.Due to heating of supercritical fluid with a bulk-fluid temperature below the pseudocritical temperature (high-density fluid, i.e., "liquid"), some layers near a heating surface may attain temperatures above the pseudocritical temperature (low-density fluid, i.e., "gas") (for specifics of thermophysical properties, see Chapter "Thermophysical Properties at Critical and Supercritical Conditions").This low-density "gas" leaves the heating surface in the form of variable density (bubble) volumes.During the pseudo-boiling, the wall heat transfer coefficient usually increases (improved heat-transfer regime).Pseudo-film boiling is a physical phenomenon similar to subcritical-pressure film boiling, which may appear at supercritical pressures.At pseudo-film boiling, a low-density fluid (a fluid at temperatures above the pseudocritical temperature, i.e., "gas") prevents a highdensity fluid (a fluid at temperatures below the pseudocritical temperature, i.e., "liquid") from contacting ("rewetting") a heated surface (for specifics of thermophysical properties, see Chapter "Thermophysical Properties at Critical and Supercritical Conditions").Pseudofilm boiling leads to the deteriorated heat-transfer regime.Water is the most widely used coolant or working fluid at supercritical pressures.The largest application of supercritical water is in supercritical "steam" generators and turbines, which are widely used in the power industry worldwide (Pioro and Duffey, 2007).Currently, upper limits of pressures and temperatures used in the power industry are about 30 -35 MPa and 600 -625ºC, respectively.New direction in supercritical-water application in the power industry is a development of SuperCritical Water-cooled nuclear Reactor (SCWR) concepts, as part of the Generation-IV International Forum (GIF) initiative.However, other areas of using supercritical water exist (Pioro and Duffey, 2007).Supercritical carbon dioxide was mostly used as a modelling fluid instead of water due to significantly lower critical parameters (for details, see Chapter "Thermophysical Properties at Critical and Supercritical Conditions").However, currently new areas of using supercritical carbon dioxide as a coolant or working fluid have been emerged (Pioro and Duffey, 2007). www.intechopen.com Heat Transfer to Fluids at Supercritical Pressures 483The third supercritical fluid used in some special technical applications is helium (Pioro and Duffey, 2007).Supercritical helium is used in cooling coils of superconducting electromagnets, superconducting electronics and power-transmission equipment.Also, refrigerant R-134a is being considered as a perspective modelling fluid due to its lower critical parameters compared to those of water (Pioro and Duffey, 2007).Experiments at supercritical pressures are very expensive and require sophisticated equipment and measuring techniques.Therefore, some of these studies (for example, heat transfer in bundles) are proprietary and hence, were not published in the open literature.The majority of studies (Pioro and Duffey, 2007) deal with heat transfer and hydraulic resistance of working fluids, mainly water, carbon dioxide and helium, in circular bare tubes.In addition to these fluids, forced-and free-convection heat-transfer experiments were conducted at supercritical pressures, using liquefied gases such as air, argon, hydrogen; nitrogen, nitrogen tetra-oxide, oxygen and sulphur hexafluoride; alcohols such as ethanol and methanol; hydrocarbons such as n-heptane, n-hexane, di-iso-propyl-cyclohexane, n-octane, iso-butane, iso-pentane and n-pentane; aromatic hydrocarbons such as benzene and toluene, and poly-methyl-phenyl-siloxane; hydrocarbon coolants such as kerosene, TS-1 and RG-1, jet propulsion fuels RT and T-6; and refrigerants.A limited number of studies were devoted to heat transfer and pressure drop in annuli, rectangular-shaped channels and bundles.Accounting that supercritical water and carbon dioxide are the most widely used fluids and that the majority of experiments were performed in circular tubes, specifics of heat transfer and pressure drop, including generalized correlations, will be discussed in this chapter based on these conditions 1 .Specifics of thermophysical properties at critical and supercritical pressures for these fluids are discussed in the Chapter "Thermophysical Properties at Critical and Supercritical Conditions" and Pioro and Duffey (2007).

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