Comparative Study on Boiling Heat Transfer Characteristics and Performance of Low-Temperature Heating System of R744 and Its Azeotropic Refrigerant

R744 is the most competitive and ideal natural refrigerant when flammability and toxicity are strictly limited. However, there are still some problems when it is applied to a heating system. For example, the discharge pressure of the system exceeds 10 MPa, it increases the cost of the system, and the cycle efficiency is also low. To solve these problems, this paper proposes to replace R744 by mixing R744 and ethane at a ratio of (77.6/22.4) to form an azeotropic refrigerant. At present, there is little research on R744 azeotropic refrigerant. Therefore, this paper first establishes the CFD model and compiles the UDF program to focus on flow boiling heat transfer characteristics, and then, it analyzes the performance of R744 and its azeotropic refrigerant in a low-temperature heating system. The results show that the heat transfer coefficient of R744 and its azeotropic refrigerant decreases with an increase in mass flux and increases with an increase in heat flux and saturation temperature; the heat transfer coefficient of azeotropic refrigerant is greater than R744; and there is no dryness under the same conditions. Under a given operating condition, there is a critical point that makes the performance of azeotropic refrigerant better than R744, and this critical point is related to the outlet temperature of a gas cooler, and the system discharge temperature of azeotropic refrigerant is significantly lower than that of R744. In conclusion, azeotropic refrigerant has certain advantages in heat transfer and system performance compared with R744, which will also play an important role in promoting the replacement of refrigerant in the future.