Condensation of an azeotropic mixture inside 2.5 mm ID Minitubes

The ongoing miniaturization of air conditioning and refrigeration systems, in order to limit, as much as possible, the refrigerant charge, calls for smaller and smaller heat exchangers. Besides, the new environmental regulations are calling for new pure refrigerants or refrigerants mixtures with lower values of global warming potentials (GWPs). In this context, this paper analyzes the possible implementation of minitubes during condensation of the azeotropic mixture R513A. Two minitubes are tested: A smooth tube with an inner diameter of 2.5 mm, and a microfin tube with an inner diameter at the fin tip of 2.4 mm. The effects of vapor quality (varied in the range 0.10-0.99), of mass velocity (varied in the range 200-1000 kg m-2 s-1), and of saturation temperature (30°C and 40°C) on the heat transfer coefficient are investigated. The experimental results indicate that the heat transfer coefficient increases as both vapor quality and mass velocity increase, both in the case of the smooth tube and of the microfin tube, but the slope of the heat transfer coefficient trend respect to vapor quality is higher in the case of the microfin tube. The microfin tube shows, on average, heat transfer coefficients are 79% higher than those of the smooth tube under the same working conditions. Since R513A is a possible substitute of R134a, some experimental data during condensation heat transfer are also compared against those for R134a. Finally, the experimental results are compared against values estimated by empirical correlations available in the open literature.