Numerical simulation on boiling heat transfer in a spiral heat exchanger for an open-loop geothermal system

The "taking heat without water" geothermal technology based on a single well heat exchanger is environmentally friendly and economical, which can effectively protect groundwater resources and ecological environment. However, due to the low outlet temperature, it needs to be heated by a heat pump to meet heating needs. In open-loop geothermal system, the compact heat exchanger installed in the upper part can improve the heat transfer coefficient through the flushing of groundwater outside the pipe and the phase change of working fluid inside the pipe, while reducing pump power consumption. A numeriacal model was established to study the flow and heat transfer characteristics of heat exchangers. Injected along the spiral tube, the working fluid forms countercurrent heat exchange with the hot groundwater outside the tube, accelerating the generation and detachment of bubbles, causing a 19% increase in heat extraction power compared to that of the vertical pipe. Affected by gravity and centrifugal force, the vapor phase accumulates on the upper and inner sides of the pipeline, and the interface between the two phases presents a tilted crescent shaped distribution. The heat transfer coefficient of R134a is 16% and 18% higher than R1234yf and R1234ze(Z), respectively. When the flow rate is increased from 0.08 kg/s to 0.24 kg/s, the starting point of vaporization of R134a in the pipe is delayed. However, the increased turbulence inside the tube leads to an increase in the speed of bubble detachment, ultimately resulting in a 136% increase in heat transfer coefficient.