Condensation Heat Transfer Inside Smooth Horizontal Tubes for R-22 and R-32/125 Mixture
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Regionally averaged heat transfer coefficients (h) for forced convective condensation inside smooth horizontal tubes were experimentally determined for an R-32/125 azeotropic mixture (50% by mass). A twophase test loop was built and calibrated using R-22. The refrigerant was condensed in a 6.1 m (20 ft.) long, 8 mm (0.315 inch) ID copper tube by water in the annulus of a 38 mm (1.5 inch) ID copper tube. Calculations for h were made at the three innermost locations of the five local measurement points along the test condenser, for different mass flux rates and condensing temperatures in the range of 160 to 480 kg/(m2s) (1.2 to 3.6 105lbm/hft2) and 24 to 38C (75 to 100F), respectively. An uncertainty analysis showed that the uncertainty in the h-values is 17%. The h-values compare with Shah (1981) and Traviss et al. (1973) correlations to within 20 and 15%, respectively. The effect of condensing temperature on regionally averaged h-values was about 10% while mass flux rate was found to be the dominant parameter affecting h-values. The effect of oil (EAL series polyolester) on the h-values for R-32/125 mixture was investigated. Although the hvalues seemed to have decreased by 10 and 15%, respectively, for 2.6 and 5.37% oil concentrations (by mass), they were within the experimental uncertainties and further studies are needed to confirm them. R-32/125 was found to have about 15% higher h-values than R-22 on an average (for the total range). A correlation was developed from the experimental data for predicting the local Nusselt number (Nu) with an average deviation of 9% from the experimental values and a coefficient of determination (COD) of 0.95. Flow regime studies using a Taitel and Dukler map (1976) indicate annular flow existed for most of the condensation except for mass flux rates lower than 208 kg/(m2s) (1.5 105lbm/hft2) and/or refrigerant qualities of less than 25%. 1996 Taylor & Francis Group, LLC.
