Boiling Heat Transfer Of CO₂ In A Horizontal Mini-Tube

Chlorine-containing CFC and HCFC refrigerants used in civil and military air conditioners are now being phased out due to their ozone depleting effect. The natural refrigerant carbon dioxide (CO₂) has been revived due to its good thermal properties and environment friendly. In recent years, the some researchers have been studying and developing the air conditioning, refrigeration and heat pump systems used CO₂ as working fluid. The evaporator is the key equipment of the system. Since thermal properties of CO₂ are different to other conventional refrigerants, the compact mini-channel heat exchangers are usually chosen as the new developing evaporators. But up to now, the papers about CO₂ boiling heat transfer in mini-channel are just few, and the results can't conduct directly and effectively to develop or produce the equipments. Therefore the dissertation presents an experimental investigation and analysis on flow boiling heat transfer characteristics of CO₂ in a horizontal smooth mini-tube.In the dissertation, CO₂ thermal properties and the related charts are systematically described firstly. The main parameters affected boiling heat transfer and pressure drop, such as working pressure, density and viscosity, are compared with other refrigerants too. In order to understand the boiling heat transfer mechanism in the mini-channel, the familiar two phase flow regime and heat transfer feature in conventional tube are presented, and then they are compared with that in mini-channel. Although there are some similar flow regimes in both conventional and mini-tube, the heat transfer characteristic are different due to effect of the geometric parameters.In order to investigate boiling heat transfer characteristics of CO₂ in a horizontal smooth mini-channel with 1mm inner diameter deeply, A new opened test-rig is designed and established. The effects of mass flow rate, heating flux, evaporating temperature and vapor quality on heat transfer coefficient are measured and analyzed. The experimental data show that: the boiling heat transfer coefficients are increased with heatflux when the mass flow rate is among 200kg/m²s and 400 kg/m²s. When the quality is in the limit of 10% to 40%, the boiling heat transfer coefficient is nearly constant in the mini-tube when mass flux increases, while the quality is lower than 10% and higher than 40%, mass flux has slight effect on the boiling heat transfer coefficient; when the boiling heat transfer coefficient is almost constant when the evaporating temperature changes.Compared the experimental results with the five correlations used in predicting boiling heat transfer coefficient, it is found that the Kandlikar's correlation with F_fl =1.41 has the same tendency with the experimental data for low vapor quality regions. With a dimensionless confinement number N_conf, the mean deviation between experimental data and Gungor-Winterton (1987) correlation is in the range of +9% when the quality is among 10% and 60%.Ma Fuqin (Heating ventilation and air-conditioning ) Supervised by Professor Fan Xiaowei...