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Table A4 Authors Hihara/Tanaka (2000) Bredesen et al. (1997) Yun et al. (2002) Sun/Groll (2002) Bredesen et al. (1997) Pettersen (2002) ID Tube [mm] 1 (RT) 2 (RT) 1-2 (RT) 4.5 (RT) 7 (RT) 0.8 (MPE) T0 [oC] 15 -10 to 5 0 to 10 -2 to 10 -25 to 5 0 to 25 M [kg/(m2s)] 360-1440 200-400 500-3570 500-1670 200-400 190-570 q [kW/m2] 9-15 6 – 9 7-48 10-50 3-9 5-20 hCO2 1) [kW/(m2K)] 8-23 6-16 10-20 6-11 4-14 8-25 Appendix A – CO2 as a Working Fluid in Heat Pumps Measured heat transfer coefficients (hCO2) for flow boiling CO2 in horizontal round tubes (RT) and microchannel tubes (MPE) at various operating conditions. 1) Pre dry-out heat transfer coefficients. Post dry-out heat transfer coefficients typically ranged from 1500 to 3000 W/(m2K). The following observations and conclusions can be drawn with reference to Tables A3 and A4: ♦ The flow boiling heat transfer coefficient for pure CO2 typically ranged from 6 to 20 kW/(m2K), and the smaller the tube diameter the higher the value. However, the effects of lubricant on heat transfer needs to be further investigated since lubricant may have conside- rable influence on the convective evaporation and nucleate boiling processes (Pettersen, 2002). ♦ Due to the small surface tension of pure CO2, nucleate boiling was the dominating heat transfer mechanism at low and moderate vapour fractions (x<0.5). Hence, the heat transfer coefficient was mainly a function of the heat flux and the evaporation temperature rather than variations in the mass flux and the vapour fraction. ♦ For the small diameter tubes (ID<4.5 mm), the average heat transfer coefficients were significantly affected by the existence of a liquid film dry-out. The probable reason for the dry-out phenomena is that the liquid film breaks down due to the low surface tension and the increased vapour velocity, and the liquid becomes entrained as drop- lets in the gas core of the flow (Pettersen, 2002). A7PDF Image | Residential CO2 Heat Pump System for Combined
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CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
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