EES software was used to simulate the flow of refrigerant and oil in underground U-tube of the Direct Expansion Ground Source Heat Pump. The results show that: the pressure of refrigerant/oil mixture first increases and then decreases along the U-tube;the temperature of refrigerant/oil mixture increases firstly and then decreases

in superheat region

the temperature rises sharply; the refrigerant/oil mixture exits in U-tube mainly as annular flow; the minimum refrigerant/oil return velocity is increasing when the U-tube diameter increases; comparing the simulation results with the experimental results find that the oil in the U-tube of the experimental system can return normally.