Forced convective boiling of nonazeotropic mixtures inside horizontal tubes was investigated experimentally. The heat transfer coefficient and pressure drop of pure refrigerant R123 and a mixture of R123 and R134a were measured in both a smooth tube and a spirally grooved tube. The heat transfer coefficient for the mixture was found to be lower than that for an equivalent pure refrigerant with the same phsycial properties, not only in the boiling-dominant region but also in the convection-dominant region. On the basis of this experiment, correlations were proposed for heat transfer coefficients in smooth and grooved tubes; the reduction in heat transfer coefficient for the mixture is attributed to the mixture effects on nucleate boiling and to the heat transfer resistance in the vapor phase. This heat transfer resistance is caused by the sensible heating of the vapor phase accompanying the rise in saturation temperature. These correlations are able to predict the heat transfer data within ± 20 percent

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