Flow Pattern, Heat Transfer and Pressure Drop in Flow Condensation Part II: Zeotropic Refrigerant Mixtures (NARMs)

Abstract

This paper, which is part II of a study on flow condensation is focused on zeotropic (or non-azeotropic) refrigerant mixtures (NARMs). In the experiments, condensation in a horizontal tube of inner diameter 6 mm and 10 m length were studied with fluids R-404A, R-407C, and three mixtures of R-32 and R-134a. A flow pattern map by Tandon et al. (1982) roughly predicts flow patterns associated with NARMs studied in this work. Most tests recorded are in the annular or semi annular flow region. The Froude number is, however, found to be an additional indicator to identify transition between annular and wavy flows. The transition in the experiments occurs mostly at Fr = 15 to 20 for the fluids tested. For NARMs with a small temperature glide (e.g. R-404A), as observed in the case of pure and azeotropic fluids, the heat transfer coefficient is independent of the mass flux in wavy flow regions, and increases with an increasing mass flux in annular flow regions. For other NARMs tested, the heat transfer coefficient (starting from a lower level) always increases with an increasing mass flux within the tested ranges. The heat transfer data from the tests with R-404A, R-407C, and R-32/R-134a mixtures can be predicted reasonably well by a modified Tandon et al. (1985b, 1995) equation with a correction proposed by Granryd (1989) for NARMs. The classical correlations for the pressure drop do not work well. Instead, the data for local pressure drop are correlated within ±15% by means of the same correlation as for the pure and azeotropic fluids. A simple correlation for the overall pressure drop based on the experimental data for pure and azeotropic fluids is good also for R-404A (with a small glide), but overpre-dicts the pressure drop (by up to 50%) for NARMs with glide, such as R-407C.