Global warming has emerged as a world-wide concern; therefore, research on the fourth generation of new environmentally friendly refrigerants is an urgent requirement. The boiling heat transfer characteristics of a refrigerant in an evaporator are directly related to its application. Thus, the R513A refrigerant is a suitable alternative to R134a. The boiling heat transfer characteristics of R513A and R134A in a 12.7 mm horizontal smooth tube and microfin tube were experimentally studied. The effects of mass flow rate, evaporation temperature, heat flux, and internal thread structure on heat transfer coefficient and pressure drop were also analyzed. The experimental mass flow rate of refrigerant was 100-200 kg/（m²·s）and the evaporating temperature was 5 ℃-10 ℃. The results showed that the boiling heat transfer coefficient changed significantly with an increase in the heat flux. With an increase in mass flow rate, the boiling heat transfer coefficient increased by 15.06% to 42.33%, and the pressure drop increased by 26.16% to 61.83%. As the evaporating temperature increased, the boiling heat transfer coefficient increased by 13.27% to 38.25%, and the pressure drop decreased by 19.53% to 33.27%. The heat transfer coefficient of the microfin tube was significantly higher than that of the smooth tube, with a reinforcement multiplier of up to 2. Under the same conditions, the boiling heat transfer coefficient of R513A was 25.61%-30.74% higher than that of R134a. The pressure drop inside the microfin tube of R134a was approximately 12.33% higher than that of R513A. With an increase in the heat flux, the flow pattern transition of R513A occurred earlier than that of R134a, and the boiling heat transfer coefficient decreased sharply.

This study investigates the condensation heat transfer and pressure drop characteristics in horizontal tubes with different fin profiles and reveals their respective heat transfer enhancement mechanisms. Experiments are conducted to assess the heat-transfer coefficient and pressure-drop of condensation in heat-exchanger tubes with an outer diameter of 8 mm. The results of the study demonstrate that both the condensation heat-transfer coefficient and pressure drop exhibited within the examined tubes increase with an increase in the mass flux, whereas both decrease with an increase in the condensation temperature. The heat-transfer coefficient of the enhanced tubes increases by 38.5%-115.6% compared to that of the smooth tubes, whereas the pressure drop increases by 49%-173%. The secondary circulation formed by the spiral structure within the tube enhances heat transfer. Larger fin heights and smaller apex angles enhance the turbulence in the refrigerant fluid as it flows over the fin tips. An increased fin density increases heat transfer by expanding the heat-exchange area. A comparison of the heat-transfer coefficients per unit pressure drop shows that the enhanced tube with 18° spiral angle exhibits the best overall performance. The spiral angle enhances the heat transfer and significantly increases the pressure drop, indicating that improving heat transfer through more aggressive enhancement structures is not advisable. Finally, a comparison of the experimental values with various heat-transfer and pressure-drop correlations shows that the correlation by Olivier et al. alters the effect of turbulence on the results by reducing the weight of the equivalent Reynolds number in the correlation, resulting in a better prediction accuracy for the heat-transfer coefficient. Hirose et al. considered factors such as the Lockhart-Martinelli parameter and two-phase pressure-drop multiplier, making the predicted pressure drop more accurate. The average relative deviations of these factors are 11.4% and 18.2%, respectively.