CONDENSATION IN SMOOTH HORIZONTAL TUBES

An experimental study of heat transfer and flow regimes during condens ation of refrigerants in horizontal tubes was conducted. Measurements were made in smooth, round tubes with diameters ranging from 3.14 mm t o 7.04 mm. The refrigerants tested were R-12, R-22, R-134a, and near-a zeotropic blends of R-32/R-125 in 50 percent/50 percent and 60 percent /40 percent compositions. The study focused primarily on measurement a nd prediction of condensing heat transfer coefficients and the relatio nship between heat transfer coefficients and two-phase flow regimes. F low regimes were observed visually at the inlet and outlet of the test condenser as the heat transfer data were collected. Stratified, wavy, wavy annular, annular, annular mist, and slug flows were observed. Tr ue mist flow without a stable wall film was not observed during conden sation tests. The experimental results were compared with existing flo w regime maps and some corrections are suggested. The heat transfer be havior was controlled by the prevailing flow regime. For the purpose o f analyzing condensing heat transfer behavior, the various flow regime s were divided into two broad categories of gravity-dominated and shea r-dominated flows. In the gravity dominated flow regime, the dominant heat transfer mode was laminar film condensation in the top of the the tube. This regime was characterized by heat transfer coefficients tha t depended on the wall-to-refrigerant temperature difference but were nearly independent of mass flux. In the shear-dominated flow regime, f orced-convective condensation was the dominant heat transfer mechanism . This regime was characterized by heat transfer coefficients that wer e independent of temperature difference but very dependent on mass flu x and quality. Heat transfer correlations that were developed for each of these flow regimes successfully predicted data from the present st udy and from several other sources.