Flow Boiling Heat Transfer to a Non-Azeotropic Refrigerant Mixture of HFC32+HFC134a in Horizontal Tubes

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Experiments were carried out on the flow boiling heat transfer to a non-azeotropic refrigerant mixture of HFC32+HFC134a in horizontal tubes. This mixture is expected to be one of alternative refrigerants to HCFC22. Following results were obtained.1. For the flow boiling of the mixture of HFC32+HFC134a in a horizontal smooth tube, heat transfer coefficients in the forced convection dominated region are almost equal to those of HCFC22 and can be predicted by a correlation for a pure refrigerant with good precision. In the region where nucleate boiling contributes substantially to the heat transfer, however, the heat transfer to the mixture is significantly degrade, down to two-thirds the value of HCFC22, due to a mass transfer resistance.2. An internally spirally grooved tube is effective in enhancing the heat transfer to the mixture, especially at a low mass velocity, but the enhancement for the mixture is considerably smaller than for HCFC22, because in the grooved tube the heat transfer coefficient of the mixture itself is lower than that of HCFC22.3. The frictional pressure drop of the mixture is larger than that of HCFC22 by a few to about 20 percent in both the smooth and grooved tubes. For the mixture, the pressure drop in the grooved tube is larger by about 20 to 60 percent or more than in the smooth tube with the same length, while it is not so larger than in the smooth tube in the condition of the same heat transger rate, because of the heat transfer enhancement for the grooved tube.