| Nowadays, ozone depletion has become an environmental problem attracting more attention around the world. Pure refrigerants with zero ODP and low GWP can prevent the environmental problems from taking place. They have higher latent heat and better performance is showed during the heat transfer process. Researchers also focus on blends mixed by environmentally friendly refrigerants and cheap ones with similar thermo-physical properties which can improve the performance of systems. Mini-channel heat exchangers are able to decrease the charging amount for their small volume and ensure the safety and economic efficiency. It is essential to conduct the research on refrigerants and mixtures with low GWP and ODP condensing in mini-channel tubes.Research on condensation characters of R290, R161, R41 and R1234 ze was conducted and was compared with those of R134 a and R32. Compared with other refrigerants, they have better environmentally friendly properties. Changes and characters of flow patterns during condensation were analyzed. Considering of the difference between pure refrigerants and mixtures during condensation, models for calculating condensation heat transfer coefficients were summarized with their conditions. Analysis on temperature glide in condensation was also conducted in order to prepare for the discussion of experiments and data analysis.Experiments were conducted based on the experimental system with environmental friendly refrigerants. Pure refrigerants such as R134 a, R290, R161 and R41 and mixtures such as R32/R134a(55.5%/44.5%), R32/R1234ze(60.5%/39.5%), R32/R134a(23%/77%) and R32/R1234ze(26.5%/73.5%) were tested while condensing in the horizontal round tube. The saturation temperature was set as 35 °C, 40 °C and 45 °C. Mass flow is set as 100—500 kg/(m2·s) inside the channel with the vapor quality changing from 0 to 1.The experimental results show that the heat transfer coefficient increases with the increase of mass flow and vapor quality and decreases with the increase of saturation temperature. It is latent heat, dynamic viscosity and liquid conductivity that have impact on coefficients. R161 has the highest coefficient and R1234 ze has the lowest. Because of the mass transfer resistance between the interfaces of mixtures, degradation appears during the condensation especially when the temperature glide is large enough. The degradation is more obvious under small mass flow. Heat transfer coefficients affected by the latent heat are different under the same condition. Calculated value using the heat transfer models were compared with the experimental value. It is found that Barid et al. model, Koyama et al. model and Garimella et al. model predict the coefficients of pure refrigerants better. Modification was done for Koyama et al. model. A factor was added to the gravity controlled Nusselt number. As a result, the deviations of R134 a, R290, R161, R41, R32 and R1234 ze were almost within 20%.There exist more deviations in the prediction of mixtures. Only the deviation of prediction by Fuji-Nataka model is within the acceptable scale. Therefore, more modification is essential for the heat transfer models used for mixtures. |
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