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5 – Experimental Results A1.6, Heat Exchanger Performance). At -10oC, 400 kg/(m2s) and 9000 W/m2, which are virtually the same operating conditions as for the prototype CO2 heat pump unit, the heat transfer coefficients ranged from 10 000 to 12 000 at increasing vapour fraction. Due to the relatively high mass flux, convective evaporation was the predominant boiling regime at vapour qualities above approximately 0.2 to 0.3 The estimated mean CO2-side heat transfer coefficient was about 3 times lower than that of the heat transfer coefficients measured by Bredesen et al. The dramatic difference was most likely a result of the 6 to 9 weight% lubricant in the CO2 flow. Both nucleate boiling and convective evapo- ration is considerably hampered by large oil concentrations in the flow, since it increases the thermal resistance of the liquid boundary layer and more or less offsets the advantage of the very low viscosity and surface tension of the CO2. The latter property is of particular importance when it comes to nucleate boiling, and the formation and growth of vapour bubbles at low vapour qualities and low mass fluxes. If the local heat transfer coefficient for the flow boiling CO2 in the proto- type evaporator had been in the same order of magnitude as measured by Bredesen et al., the mean overall heat transfer coefficient would have risen in average by roughly 80%. Pressure Drop The total pressure drop at the CO2-side of the evaporator was measured at varying operating conditions. The pressure drop ranged from typically 25 to 45 kPa at varying inlet vapour qualities, which corresponds to a mean pressure drop gradient of about 2100 to 3800 Pa/m. Bredesen et al. mea- sured a mean pressure drop gradient of about 2700 Pa/m at -10°C evapo- ration temperature and a mass flux of 400 kg/(m2s). 5.1.3.8 Performance and Main Operating Characteristics of the Tripartite Gas Cooler Heat Transfer Efficiency The tube-in-tube tripartite gas cooler was not instrumented for measuring the local convective heat transfer coefficient for the supercritical CO2. In contrary to the evaporator, it was not possible to estimate the mean heat transfer coefficient hi for the CO2 by means of the temperature measure- ments at the inlet and outlet of the gas cooler units and the mass flow rates 152PDF 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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