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126 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 Fig. 8. Density of CO2. and 14% of the CO2 vapor density, respectively. The low- density ratio of CO2 may give more homogenous two-phase flow than with other refrigerants [24]. The liquid to vapor density ratio plays an important role in an evaporator since it determines the flow pattern and thus the heat transfer coefficient [20]. The higher vapor density gives the high volumetric refrigeration capacity of CO2, which is defined as product of vapor density and latent heat of evaporation. The volumetric refrigeration capacity of CO2 increases with temperature, has a maximum at 228C, and then decreases again. By definition it is zero at the critical point as shown in Fig. 10. Surface tension of the refrigerants influences nucleate boiling and two-phase flow characteristics. A small surface tension reduces the superheat required for nucleation and growth of vapor bubbles, which may positively affect heat transfer. Wetting characteristics of the liquid is affected by surface tension, thus influencing evaporation heat transfer. Reduced liquid surface stability with small surface tension may affect heat transfer Fig. 10. Volumetric refrigeration capacity for refrigerants. negatively due to increased droplet formation and entrainment [20]. Fig. 11 presents surface tension of saturated CO2 liquid at varying temperatures, compared to other fluids. The surface tension of the refrigerants decreases with temperature and becomes zero at the critical point. As shown in Fig. 11, the surface tension of CO2 is smaller than those of other fluids. For instance at 0 8C it is 0.0044 N/m, which is 2.5 times smaller than that of R-134a at the same temperature. Surface tension data for CO2 can be estimated based on the publication of Rathjen and Straub [25], and speed-of-sound data were derived by Estrada-Alexanders and Trusler [26]. One of the most important characteristics of supercritical fluids near the critical point is that their properties change rapidly with temperature in an isobaric process, especially near the pseudocritical points (the temperature at which the specific heat becomes a maximum for a given pressure). This may be clearly seen from Figs. 12 and 13, where the isobaric specific heat and pseudocritical temperature are depicted. It should be noted that the 1-NTU or LMTD method requires that the specific heat be constant over the test section. Thus when the data are analyzed using Fig. 9. Ratio of liquid to vapor density at saturation for refrigerants. Fig. 11. Surface tension for refrigerants.PDF Image | CO2 Vapor Compression Systems
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