Study On The Enhancement Of Boiling Heat Transfer Of Natural Refrigerant CO₂ On Horizontal Tube

Natural refrigerant CO2 has a broad perspective of application. This thesis aims to study flooded evaporator used in CO2 refrigeration system by digging into the relevant questions of adopting flooded evaporator in water-to-water heat pump system and the enhancement of boiling heat transfer of CO2. Also this thesis emphasizes the experimental study on boiling heat transfer of CO2 on smooth and fabricated horizontal copper tubes.Based upon the thorough investigation and research of boiling heat transfer of refrigeration preparation, a deep analysis and study on boiling heat transfer of CO2 was performed. The research result shows that the boiling of CO2 is according with the rules of that of ordinary substances. And comparing to other general refrigerants, CO2 has the characteristics of high heat transfer coefficient, easy dry-out phenomenon, less influenced by the liquid height.The thesis studies the boiling heat transfer of CO2 on smooth and fabricated horizontal copper tubes on a water-to-water heat pump system test platform. The experimental device solved the technical difficulties of sealability, the measurement of temperature and visualization of heat transfer under relatively high pressure. This thesis provides the necessary experimental data for the calculation of heat transfer in flooded evaporator and has important project value. Correlations for the average boiling heat transfer coefficients of CO2 on smooth and fabricated horizontal tubes in water-to-water heat pump with certain evaporation pressure and heat flux are obtained from the experimental data.The enhancement ratios of fabricated tube are 2.02-2.24 within the range of experimental heat flux (28.69-54.27kW/m2) when the saturation pressure is 3.2MPa. The experimental results show that the existing commercial enhanced tubes have good strengthening effect for the pool boiling heat transfer of CO2 and thus can be directly applied to flooded evaporator in CO2 system.The thesis performs a finite element analysis of the intensity of smooth tube and internal-thread tube under both internal and external pressure. It points out that for the internal-thread tube, the distance between threads has no significant impact on the strength of the tube. When under the same pressure, the maximum equivalent stress occurs at the inner wall of the root of the internal-thread as the tube is under high internal pressure. It is easier to be damaged than under the impact of high external pressure. This result provides the gist of choosing enhanced heat transfer tubes for flooded evaporator.The thesis performs an analysis of adopting flooded evaporator in CO2 water-to-water heat pump system and brings forward the structural suggestions for flooded evaporator used in CO2 system.The thesis also analyzes the phenomenon of Nano bubbles and indicates that the tinny heterogeneous bubbles might exist in liquid state of CO2.