An experimental study of heat transfer and flow regimes during condensation of refrigerants in horizontal tubes was conducted. Measurements were made in smooth, round tubes with diameters ranging from 3.14 mm to 7.04 mm. The refrigerants tested were R-12, R-22, R-134a, and near-azeotropic blends of R-32/R-125 in 50 percent/50 percent and 60 percent/40 percent compositions. The study focused primarily on measurement and prediction of condensing heat transfer coefficients and the relationship between heat transfer coefficients and two-phase flow regimes. Flow 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. True mist flow without a stable wall film was not observed during condensation tests. The experimental results were compared with existing flow regime maps and some corrections are suggested. The heat transfer behavior was controlled by the prevailing flow regime. For the purpose of analyzing condensing heat transfer behavior, the various flow regimes were divided into two broad categories of gravity-dominated and shear-dominated flows. In the gravity dominated flow regime, the dominant heat transfer mode was laminar film condensation in the top of the tube. This regime was characterized by heat transfer coefficients that depended on the wall-to-refrigerant temperature difference but were nearly independent of mass flux. In the shear-dominated flow regime, forced-convective condensation was the dominant heat transfer mechanism. This regime was characterized by heat transfer coefficients that were independent of temperature difference but very dependent on mass flux and quality. Heat transfer correlations that were developed for each of these flow regimes successfully predicted data from the present study and from several other sources.

Condensation in Smooth Horizontal Tubes

Two-phase flow in high-heat-flux micro-channel heat sink for refrigeration cooling applications: Part II—heat transfer characteristics

Condensation in horizontal tubes, part 2: new heat transfer model based on flow regimes

Measurement and prediction of pressure drop in two-phase micro-channel heat sinks

Condensation in Horizontal Tubes, Part 1: Two-Phase Flow Pattern Map

An analysis of steam-void fraction in two-phase flow is carried out, utilizing the principle that in a steady-state thermodynamic process the rate of entropy production is minimum. The two-phase flow is idealized in the analysis to be a truly steady-state process. The effects of liquid entrainment and wall friction on the void fraction and slip ratio are evaluated. It is found that the slip-ratio in an idealized two-phase flow with zero wall friction and zero entrainment equals (ρf /ρg )1/3 . Data from a number of experiments are found to be bracketed between this result and the result obtained by assuming complete entrainment (slip ratio = 1).

Estimation of Steady-State Steam Void-Fraction by Means of the Principle of Minimum Entropy Production

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Closure to “Discussions of ‘Estimation of Steady-State Steam Void-Fraction by Means of the Principle of Minimum Entropy Production’” (1964, ASME J. Heat Transfer, 86, pp. 251–252)

S. M. Zivi

Papers

Estimation of Steady-State Steam Void-Fraction by Means of the Principle of Minimum Entropy Production

Closure to “Discussions of ‘Estimation of Steady-State Steam Void-Fraction by Means of the Principle of Minimum Entropy Production’” (1964, ASME J. Heat Transfer, 86, pp. 251–252)