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~ -"E 'Q} aoL 40~ ~f .x: 80 ....... ::t 2: LITERATURB STUDY 26 The conductivity of CO2 is increased as the temperature of the refrigerant decreases. Therefore heat transfer is high near the pseudo-critical temperature (Yang et a/., 2006). For both single- and two-phase flow a high thermal conductivity is essential for the heat transfer coefficients. Viscosity and in particular that of the liquid phase and the ratio of liquid to vapour viscosity, are important parameters when it comes to the fluid flow behaviours, convection characteristics, two-phase heat transfer and the pressure drop. For saturated CO2 liquid and vapour at O°C the thermal conductivities are 20% to 60% higher than that of R-134a liquid and vapour. The viscosity of CO2 liquid is 40% higher than the viscosity of R-134a liquid and the vapour viscosities of the two refrigerants are comparable. Figure 2.15: Viscosity of CO2 (Kim et ai., 2003). _ .... 14MPa --~ 12MPa --.10 MF'a --- 81\APa; - - 4 M P q, -20 0 20 40 ,60 eo 100 120 T~Cl The Prandtl number plays an important role in the heat transfer coefficients. It is associated with the specific heat and thus has a maximum at the pseudo-critical temperature and the maximum value decreases with pressure. The Prandtl number gets higher at temperatures exceeding 60°C in the super-critical region. It could be said that the thermodynamic and transport properties of CO2 seem to be favourable in terms of heat transfer and pressure drop when taking in account other refrigerants (Kim et a/., 2003). A Techno-Economical Analysis of a CO2 Heat Pump. School of Mechanical Engineering, North-West UniversityPDF Image | CO2 HEAT PUMP Analysis
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