Average Heat Transfer Coefficient for Pool Boiling of R-134a and R-123 on Smooth and Enhanced Tubes (RP-1316)

Abstract

This paper presents information about nucleate pool boiling of R-134a and R-123 on TBIIHP and TBIILP tubes, respectively, as well as on smooth tubes. This is the first phase of ASHRAE project RP-1316, Experimental Evaluation of Heat Transfer Impacts of Tube Pitch in a Highly Enhanced Surface Tube Bundle. Data were taken at a saturation temperature of 4.44°C (40°F). The test tubes' outer diameter was 19.05 mm (0.75 in.) and had lengths of 1 m (39.37 in.). Tubes were water heated with an insert in the water passage. A new analytical method known as enthalpy-based heat transfer analysis (EBHT) was introduced and used to determine the refrigerant heat transfer coefficient. The heat flux range was 4–135 kW/m2 (1269–42,823 Btu/h·ft2) for R-134a on TBIIHP and 5–60 kW/m2 (1586–19,033 Btu/h·ft2) for R-123 on TBIILP. The resulting heat transfer coefficient range for R-134a on TBIIHP was 6270–23,268 W/m2·°C (1105–4100 Btu/h·ft2·°F) and 6748–23,338 W/m2·°C (1189–4113 Btu/h·ft2·°F) for R-123 on TBIILP. The present study provided one of the widest heat flux ranges studied with this type of tubes and showed significant structure to the pool boiling curve, which had not been traditionally observed. The experimental data of the smooth tubes were compared against the Cooper and Stephan-Abdelsalam correlations. The Cooper correlation provided a better fit with the experimental data, with an average uncertainty of 27% for R-123 and 16% for R-134a.