Research On The Heat Transfer Properties Of CO₂ Vapor-liquid Two Phase In Micro-channel（s） Evaporator

With the environment problems becoming increasingly serious, there is a growing recognition that the economic development should not be promoted at the cost of environment but by the means of sustainable development. Due to its remarkable heat transfer performances and incomparable environment-friendly advantages, the natural carbon dioxide(CO2) refrigerant is considered to have an irresistible trend to replace other synthetic refrigerants. In this thesis, Vapor-liquid two phase flow and heat transfer properties of CO2 in single channel and micro-channel evaporator are investigated through the method of numerical simulations combined with theoretical analysis, whose main contents are summarized as follows:Firstly, a brief introduction to CO2 refrigerant is given from the perspectives of environment protection and energy efficiency, with emphasis on the principles and properties of the transcritical refrigeration cycle.Secondly, to date researches available on the vapor-liquid two phase flow problems of micro-channel(s) are still not mature and have not yet reached a consensus. Aiming at this issue, a simplified method named homogeneous model is presented and some fundamental principles and thermodynamic equilibrium equations are also illustrated in this thesis.Thirdly, on the basis of homogeneous model and apparent heat capacity method, some numerical simulations about the CO2 vapor-liquid two phase flow inside the micro-channel are performed utilizing the finite element analysis software and some analysis on performances of heat transfer and flow field, including temperature, speed, pressure and the fraction of gas phase distribution, is given. By comparison of the simulation results and theoretical analysis, it is verified that the homogeneous model method is both rational and precise.Finally, based on the same method related above, several simulations about CO2 vapor-liquid two phase flow within the micro-channels evaporator are also performed. Besides, the effects of the refrigerant dryness, mass flow rate and heat source power on the heat transfer performance are investigated. In accordance with the simulation results and further analysis, some improvement measures are proposed for the purpose of enhance the capability of the micro-channel evaporator, the improved capability of which is also quantized.In this thesis, the homogeneous model is proved to be suitable for those micro-channels with vapor-liquid two phase flow and the researches on the heat transfer properties of CO2 vapor-liquid two phase in micro-channel(s) evaporator are investigated, along with some improvement measures. These results and analysis provide a valuable reference to other academic researches and engineering applications in the field of CO2 micro-channel evaporator.