Investigation On Underground Heat Exchanger Under Coupled Thermal Conduction And Groundwater Seepage Conditions

Ground Source Heat Pump(GSHP) system increasingly applied in architectural air-condition as these systems can make significant contributions to reduction in electrical energy usage,using renewable resource efficiency,and have been recognized as an environmental-friendly alternative to conventional unitary system.Vertical U-tube heat exchanger have been widely used because of many notable advantages it has,such as fewer area requirement,extensive application,running steadily,high performance,lower cost on system maintenance etc.Vertical heat exchanger is mainly divided into single U-tube heat exchanger and double U-tube heat exchanger.At present,single U-tube heat exchanger is widely used and research on heat exchanger is mainly about single U-tube.Recently, application of double U-tube heat exchanger rises for its better heat transfer performance and higher utilization efficiency of borehole.In this paper,the model of fluid in the U-tube and underground soil around single U-tube heat exchanger under coupled thermal conduction and groundwater seepage were set up.Soil was considered as the even,isotropy and saturated porous medium,and the seepage of groundwater had three-dimensional characteristic.Soil was layered along the depth direction, and the thermal property change was taken into consideration.The Fluent software was used to carry out the numerical simulation of soil temperature field of ground heat exchanger with burial depth 50m after 20 days continuous operation on summer conditions.The creative points of this paper are:â‘ the thermal property change along the depth direction was taken into consideration,which make the model more close to the reality;â‘¡the seepage of groundwater had three-dimensional characteristic,and heat transfer in soil along the depth direction was considered.Compared with the model used by early researchers,in which the soil was considered as single type or the seepage had two-dimensional characteristic,it's an essential progress.In this paper,the soil temperature field and heat transfer performance with different inlet fluid temperatures,backsoil and inlet velocities were studied.The operation with different parameters was studied,that has reference value for chosen of operating parameters of ground-source heat pump.The model of fluid in the U-tube and underground soil around double U-tube heat exchanger was constructed in the same way,and the soil temperature field and heat transfer performance with different inlet fluid temperatures,backsoil and inlet velocities were studied. The performance of single U-tube heat exchanger and double U-tube heat exchanger was compared based on temperature difference between inlet and outlet and heat transfer rate.It can be concluded that the double U-tube heat exchanger offered a better performance than single U-tube heat exchanger.