Simulation Study On The Refrigerant Phase-change Heat Transfer In A Vertical Direct Heat Exchange U-tube

Direct Expansion Ground-coupled Heat Pump(DX-GCHP)system is both energy-efficient and environment-friendly.Compared with Second Loop Ground-coupled Heat Pump(SL-GCHP),DX-GCHP has less heat lose,so its energy efficiency is higher.In DX-GCHP system,the ground-coupled heat exchanger is an important component to exchange heat between refrigerant and soil.The heat transfer performance of ground-coupled heat exchanger plays an important role to determine whether the ground source heat pump system can work for a long period of time efficiently and economically.Therefore,this thesis mainly studied the heat transfer process inside and outside the borehole for a vertical U-tube.In this paper,the heat transfer models of the borehole inside and outside were introduced and the models used in this paper were determined after being compared.The flow and phase change heat transfer process in the tube were elaborated clearly.In this paper,it is assumed that the refrigerant transfers heat only by changing phase in the tube.Firstly,the temperature and pressure of refrigerant in the tube under different working conditions can be calculated.The results show that: the refrigerant evaporated faster in the upward tube and condensed faster in the downward tube,the system performance decreases with the evaporation temperature of the refrigerant dropping and increases with the condensation temperature of the refrigerant dropping.Then the changes of the refrigerant temperature and pressure in the tube under different conditions are analyzed.The conditions included: increasing the tube length in evaporation,decreasing the tube length in condensation,changing the original soil temperature.It is concluded that: increasing the tube length made the changes of the pressure and temperature of the tube reducing in the evaporation condition,decreasing the tube length made the changes of the pressure and temperature of the tube reducing in the condensation condition.Decreasing the soil ordinal temperature makes the tube length increasing in the evaporation condition and reducing in the condensation condition.And then,the fluent software is used to analyze the change of the soil temperature under different operating time and conditions.Finally,the soil temperature changes in the stratified model and in the homogeneous model were compared.The simulation shows that: the diffusion radius increased and the rate of increase declined with running time and the diffusion radius in the in the evaporation condition is bigger than that in the condensation condition.Finally,the soil temperature change calculated by the stratified model is compared with that calculated by the homogeneous model.The contrast result shows that: there is deviation between the homogeneous model and the real condition.The research of this paper had some reference significance for the study and the application of the DX-GCHP system.