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Appendix A – CO2 as a Working Fluid in Heat Pumps ♦ The critical vapour quality where dry-out occurred, typically ranged from 0.3 to 0.6 for the small diameter tubes, and the critical quality was strongly dependent on the mass flux. For the test with the ID 7 mm tubes, the critical vapour quality was about 0.9. ♦ The post dry-out heat transfer coefficients were typically 5 to 10 times lower than the pre dry-out values, i.e. 1000 to 3000 W/(m2K). ♦ Pettersen (2002) recommended that compact evaporators (MPE) for unitary applications should be designed for a low mass flux, since increased mass flux leads to an early dry-out and does not improve heat transfer in the pre dry-out region. Supercritical Heat Rejection At supercritical pressures, the thermophysical properties of CO2 have a strong temperature and pressure dependency, which in turn affects the Reynolds and Prandtl numbers and because of that the local single-phase heat transfer coefficient in the gas cooler. Figure A4 shows the specific heat capacity and the density of CO2 at supercritical pressures ranging from 8 to 12 MPa, whereas Figure A5 shows the dynamic viscosity and the thermal conductivity at the same pressure levels (RnLib, 2003). 40 30 20 10 8 MPa 9 MPa 10 MPa 11 MPa 12 MPa 0 0 20 40 60 80 100 120 Temperature [°C] 120 100 80 60 40 20 0 8 to 12 MPa → 0 20 40 60 80 100 120 Temperature [°C] Figure A4 The specific heat capacity and density of CO2 at super- critical pressures ranging from 8 to 12 MPa (RnLib, 2003). Figure A6 shows, as an example, the calculated convective heat transfer coefficient for in-tube cooling of supercritical CO2 in an ID 6 mm round tube. The calculations are based on the commonly used Gnielinski corre- lation for single-phase flow (VDI, 1993), and the mass fluxes are 900 and 1200 kg/(m2s). The effects of lubricant are not included. A8 Cp [kJ/kgK] Dyn. Viscosity [mPas]PDF 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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