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Flow regime map for condensation from superheated vapor.

Number: pap. 2287

Author(s) : XIAO J., HRNJAK P.

Summary

An update on the flow regime map for condensation inside horizontal smooth round tubes accounting for the non-equilibrium existed in a vapor compression system is introduced. It is usually by default to assume that a flow regime map should be made between bulk quality 1 and 0. However, the temperature gradient required by condensation means that the thermal equilibrium assumed in a thermodynamic point of view does not exist in a real condenser, especially at the entrance stages of a condenser in a vapor compression system. By focusing on the development of the liquid film when the superheated vapor is condensed on the tube wall whose temperature is below saturation temperature at the corresponding pressure, the real onset and end of condensation could be theoretically calculated. The flow regime map, therefore, should be constructed in between the real onset and end of condensation that are manually set as superficial quality 1 and 0. Two-phase flows of R32, R134a, R1234ze(E), R245fa and R1233zd(E) under mass fluxes from 100 to 400 kg/m2-s, heat fluxes from 5 to 10 kW/m2, and tube diameters of 4.0 and 6.1 mm at saturation temperatures of 30 and 50 oC are observed in transparent tube-in-tube heat exchangers where refrigerants are cooled by glycol. The visualizations shows three deficiencies in a conventional flow map. First, a conventional flow map does not provide any information beyond quality 1 and 0 where there is two-phase flow as mentioned above. Second, at the entrance stages of the condensation, the film-forming mechanism guarantees an annular flow. This mechanism, however, is usually not employed in a conventional flow map. Third, the conventional flow regime map does not provide information for both micro and macro tubes. The flow map for those two different sized tubes are usually separately made while putting surface tension into the picture could potentially unify them. The transition criteria with the non-equilibrium is taken into account are redefined and the new flow regime map addresses the issues above. A comparison between visualization and the prediction of the new flow map shows good agreement.

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Pages: 10

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Details

  • Original title: Flow regime map for condensation from superheated vapor.
  • Record ID : 30024482
  • Languages: English
  • Subject: HFCs alternatives
  • Source: 2018 Purdue Conferences. 17th International Refrigeration and Air-Conditioning Conference at Purdue.
  • Publication date: 2018/07/09

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