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M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 127 Fig. 12. Isobaric specific heat of CO2. the 1-NTU or LMTD method, it should be carefully investigated whether the specific heat is constant. The pseudocritical temperature of CO2 was calculated using the following algebraic equation [27] Tpseudo 1⁄4 2 122:6 þ 6:124P 2 0:1657P2 þ 01773P2:5 2 0:0005608P3; 75 # P # 140 ð1Þ where the temperature ðTpseudoÞ and pressure ðPÞ are in 8C and bar, respectively. 2.2. Transport properties The transport properties of refrigerants play an important role in heat transfer and pressure drop characteristics. Fig. 14 shows the transport properties, which are thermal conduc- tivity and viscosity at subcritical and supercritical pressures at varying temperatures. A high thermal conductivity is essential for heat transfer coefficients both in single-phase and two-phase flow. Viscosity, particularly of the liquid Fig. 13. Pseudocritical temperature and maximum isobaric specific heat of CO2. Fig. 14. Transport properties of CO2. (a) Thermal conductivity, (b) viscosity. phase, and the ratio of liquid to vapor viscosity, are important parameters for the fluid flow behaviors, convec- tion characteristics and two-phase heat transfer and pressure drop. The thermal conductivities of saturated CO2 liquid and vapor at 0 8C are 20 and 60% higher than of R-134a liquid and vapor, respectively, while the viscosity of CO2 liquid is only 40% of R-134a liquid viscosity, and the vapor viscosities of the two fluids are comparable [20]. The Prandtl number is an important parameter for the heat transfer coefficient. Fig. 15 depicts the Prandtl number of supercritical and liquid/vapor CO2 at varying tempera- tures. It has a maximum at the pseudocritical temperature associated with the corresponding specific heat, and the maximum value decreases with pressure. The effect of the temperature on the Prandtl number depends on pressure. The Prandtl number becomes higher with pressure for T . about 60 8C in the supercritical region, whereas it decreases with pressure when temperature is smaller than about 20 8C. This results in a strongly varying local heat transfer coefficient depending on temperature and pressure [14]. In summary, the thermodynamic and transport properties ofPDF Image | CO2 Vapor Compression Systems
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