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Study on two-phase flow pattern in the gas cooler of supercritical carbon dioxide entrained with PAG-type lubricating oil.

Author(s) : DANG C., KOJI I., HIHARA E.

Summary

In order to clarify the heat-transfer mechanism of supercritical carbon dioxide flowing with a small amount of lubricating oil, visualization experiments have been conducted using two sight-glass tubes with inner diameters of 2 and 6 mm. The visualization images were recorded using a high-speed CCD camera. PAG-type oil, which is partially miscible with supercritical carbon dioxide, was used. The experiments were conducted with lubricating oil concentrations of 1 and 5%, pressures between 8 and 10 MPa, and mass fluxes between 200 to 1200 kg/m2.s. The visualization images revealed that two-phase flow pattern inside the gas cooler was determined by many factors, including tube diameter, oil concentration, temperature, pressure and mass flux. For the small size tube of 2 mm I.D., the formation of both oil droplets in the bulk region and the oil film along the inner wall of the tube are confirmed. At low temperatures, a large number of oil droplets are observed to flow with CO2 with a slip ratio of approximately 0.7. With an increase in the bulk temperature, both size and quantity of the oil droplets entrained with CO2 decrease, and the flow of the oil film becomes clearly visible. For a large tube with 6 mm I.D., the flow pattern at low mass flow rate is separated wavy flow, with the increase in mass flow, the flow pattern changes to annular flow, which corresponds to a distinct decrease in heat transfer coefficient due to the heat resistance of oil layer. The transition of flow pattern is considered due to the shear stress between the oil-layer and bulk CO2. In addition, the dissolution of CO2 into PAG oil, as well as the change of the solubility and thermodynamic of CO2 with temperature and pressure makes the prediction of flow pattern a challenging task. The relationship between the flow pattern and heat transfer characteristics is also clarified.

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Pages: ICR07-B1-1094

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Details

  • Original title: Study on two-phase flow pattern in the gas cooler of supercritical carbon dioxide entrained with PAG-type lubricating oil.
  • Record ID : 2007-2535
  • Languages: English
  • Source: ICR 2007. Refrigeration Creates the Future. Proceedings of the 22nd IIR International Congress of Refrigeration.
  • Publication date: 2007/08/21

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