An experimental study on R22 evaporating heat transfer in round and oval microfin tubes has been performed. The oval tube was an elliptic tube of axis ratio 1:1.5, which was fabricated from the round tube with an outer diameter of 9.52 mm and 18 deg helix angle counterclockwise. The test section was a straight horizontal tube of 0.6 m in length and was heated electrically by a tape heater wound on the tube surface. Heat flux of 12 kW/m2 was maintained constant and the range of refrigerant quality was 0.2–0.8. The tests were conducted for evaporation at 15 °C for 30–60 kg/h mass flow rate (mass flux based on the oval tube: 150–300 kg/m2s) and the installation angles of the oval tube were varied between 0 and 135 deg in the circumferential direction. The local and average heat transfer and pressure drop characteristics for the oval tube were compared to those for the baseline round tube. The average two-phase heat transfer coefficients for the oval tube were 2–12 percent higher than that for the round tube and pressure drops for both tubes are similar. The single heat transfer coefficient and friction factor correlations for the round and oval microfin tubes are developed within the rms errors of ±5.6 percent and ±10.0 percent, respectively.

Issue Section:
Two-Phase Flow and Heat Transfer
Keywords:
Enhancement, Evaporation, Heat Transfer, Heat Exchangers, Two-Phase, evaporation, pipe flow, heat transfer, organic compounds, refrigeration, two-phase flow
Topics:
Evaporation, Flow (Dynamics), Heat transfer, Pressure drop, Refrigerants, Heat transfer coefficients
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