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138 M.-H. Kim et al. / Progress in Energy and Combustion Science 30 (2004) 119–174 the pressures and temperatures ranging from 7.4 to 12 MPa and 20 – 110 8C, respectively. The buoyancy force affected supercritical CO2 flow significantly. The buoyancy effect became smaller as the tube diameter decreased, however. They reported that the existing correlations for larger tubes deviated notably from their test data for the micro/ minitubes. Based on the test data, they developed a correlation for the axially averaged Nusselt number with the mean relative error of 9.8%. 5.2. Flow vaporization heat transfer and pressure drop Bredesen et al. [24] measured heat transfer and pressure drop for flow vaporization of pure CO2 in a horizontal 7 mm ID (inner diameter) aluminum test tube. The heat transfer test data indicated regimes of convective boiling at high mass flux and low evaporating temperature, and nucleate boiling regimes at lower mass flux and higher temperatures. At most conditions, the local heat transfer coefficient increased up to a vapor fraction of around 0.9, but at the highest evaporating temperature (5 8C), the behavior was quite different, with a decreasing heat transfer coefficient at increasing x. In the latter case (G 1⁄4 200 kg=m2 s; T 1⁄4 5 8C; q 1⁄4 6 kW=m2), the heat transfer coefficient dropped from about 14,000 W/m2 K at x 1⁄4 0:2 to about 8000 W/m2 K at x 1⁄4 0:9: The authors explained this by the high pressure and low liquid/vapor density near the critical point. A comparison to a few common heat transfer correlations gave poor correspondence for all test data, the experimental coefficients being about twice as high as predicted. Rieberer [14] found that common heat transfer correlations gave considerably higher predicted heat transfer coefficients than his experimental data from a rig where there was some compressor lubricant in the CO2 flow. Models that gave best fit to the data of Bredesen et al. [24] overpredicted the experimental data of Rieberer [14] by a factor of 3 – 4. These large differences were probably caused by the presence of lubricant and the data gives some indication of a possible serious impact of lubricant on nucleate boiling heat transfer. Further test data by Rieberer [14] on a 10 mm tube (still including lubricant) shows that the heat transfer coefficient is almost unaffected by a doubling of the heat flux, and that the coefficient increases with mass flux. Both these observations indicate that nucleate boiling is not a dominant mechanism of heat transfer, or that this mechanism is suppressed by a lubricant concentration. Sun and Groll [68] conducted flow vaporization experiments for pure CO2 on a horizontal 4.6 mm ID (inner diameter) stainless steel tube. Test data were recorded at CO2 mass flux between 500 and 1670 kg/m2 s, heat flux 10 – 50 kW/m2 and vapor fraction 0 – 0.95. Evaporating temperatures were maintained between 2 2 and þ 10 8C. In general, the heat transfer coefficient dropped at increasing x: A more or less abrupt drop in heat transfer above a vapor fraction of 0.4 – 0.6 was observed in most tests, and was explained by dryout of the liquid film. The heat transfer was not influenced much by varying mass flux at low vapor fractions, while heat flux variation had significant influence. This was taken as evidence of nucleate boiling as the dominant heat transfer mechanism at lower x: The heat transfer after dryout was influenced by mass flux, indicating a convection-dominated heat transfer. Hihara and Tanaka [69] conducted measurements on a horizontal stainless steel microchannel test tube with 1 mm internal diameter. The authors measured very high heat transfer coefficients (around 10 – 20 kW/m2 K) in the nucleate boiling regime at low vapor fractions. At the onset of dryout the coefficients dropped abruptly to only a small fraction of the nucleate boiling level. Onset of dryout occurred at a vapor fraction of around 0.8 at a mass flux of 360 kg/m2 s, decreasing to 0.4 at a mass flux of 1440 kg/m2 s. Pettersen [20] conducted extensive studies on flow vaporization in microchannel tubes, using an aluminum test tube with 25 channels having 0.81mm diameter. Vaporization heat transfer and pressure drop data were recorded over a wide range of conditions, including temperatures (0 – 25 8C), heat flux (5 – 20 kW/m2), mass flux (190 – 570 kg/m2 s), and vapor fraction (0.2 – 0.8). Test results showed that the nucleate boiling mechanism dominated at low/moderate vapor fractions. Dryout effects became very important at higher mass flux and temperature, where heat transfer coefficient ðhÞ dropped rapidly at increasing vapor fraction ðxÞ: Heat transfer coefficients were correlated using a combination of models for nucleate boiling, convective evaporation, dryout inception, and post-dryout heat transfer. Microchannel frictional pressure drop was correlated using the ‘CESNEF-2’ correlation by Lombardi and Carsana [70], with a mean/average deviation of 16.4/21.1%. Special small-tube correlations from literature did not reproduce the test data well. 5.3. Two-phase flow patterns Pettersen [20] conducted experiments for two-phase flow patterns at a temperature of 20 8C and for mass flux ranging from 100 to 580 kg/m2 s, using a heated glass tube with 0.98 mm ID. The observations showed a dominance of intermittent (slug) flow at low x; and wavy annular flow with entrainment of droplets at higher x: At high mass flux, the annular/entrained droplet flow pattern could be described as dispersed. The aggravated dryout problem at higher mass flux could be explained by increased entrainment. Stratified flow was not observed in the tests with heat load. Bubble formation and growth could be observed in the liquid film, and the presence of bubbles gave differences in flow pattern compared to adiabatic flow. The flow pattern observations on CO2 did not fit any of the generalized maps or transition lines, including the map proposed by Kattan et al. [71]. Only the intermittent – annularPDF Image | CO2 Vapor Compression Systems
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