Study On Heat Transfer Performance Of Single Geothermal Well Heat Exchanger

Direct extraction of geothermal water is the main method of geothermal energy development at present,and there are problems such as unbalanced harvesting and irrigation,high power consumption,and surface chemical pollution.However,the heat extraction power of a single well is low.The material parameters,operating parameters,and thermal properties of the heat exchanger are the key factors that affect the heat transfer performance of a single geothermal well.In this paper,through numerical simulation calculation and experimental research,the heat transfer characteristics of coaxial casing,helical tube and heat pipe heat exchanger are respectively carried out.Establishing a three-dimensional coaxial casing heat transfer model and discussing influences of operating parameters,material parameters,reservoir groundwater flow velocity and rock mass physical parameters on the temperature distribution of geothermal water along the tube and the heat extraction power of the system.Reducing the injection temperature can increase the heat exchange temperature difference with the reservoir,and increase the heat absorption temperature rise and heat extraction power of geothermal water.When the injection flow rate was increased from 2 kg/s to 16 kg/s,the flow rate increased,the heat exchange time between the geothermal water and the reservoir decreased,and the outlet temperature decreased from 40.1℃ to 21.2℃,but the heat transfer coefficient increased by 219.3 W/(m~2·℃),the heating power is increased from210 k W to 417.8 k W.Proposing a helical tube heat exchanger placed in the wellbore and flushed with geothermal water and to achieve the purpose of strengthening heat transfer by convection in the wellbore.A numerical calculation model of a three-dimensional helical tube heat exchanger is established to reveal the influence of working fluid and flow on endothermic load and heat transfer coefficient,the heat transfer capacity of the spiral tube under different structures and operating parameters was explored and analyzed.The flow rate of geothermal water outside the tube is increased from 4 kg/s to 20 kg/s,the flow rate is increased,the convective heat transfer coefficient is increased by 80.6%,and the heat extraction power can be increased by 18.1%.When the helical tube spacing decreases from 500 mm to 160 mm,the heat exchange area increases and the heat taking power of spiral tube increases by 23.4%.However,too small spacing is easy to cause thermal interference in pipeline,and the heat taking power per unit pipe length decreases by52.3W/m.When R134a and R22 are injected in the state of undercooling,the heat absorption load of the working fluid is 25.1 k W and 26.2 k W higher than that of water when the inlet flow rate is 0.2 kg/s,and when the flow rate of the working fluid of R22 is increased to 1.5 kg/s,the endothermic load can be increased to 313.9 k WBuilding a gravity heat pipe experimental platform for geothermal development,and explored the influence of the heating temperature,cooling water flow and temperature in the evaporation section on the heat transfer coefficient of the heat pipe,the heat absorption of the system and other performance characteristics.The research shows that increasing the heating temperature of the evaporation section can increase the heat transfer power of the heat pipe and the heat transfer coefficient of the evaporation section.Increasing the cooling water flow rate can increase the heat transfer power of the heat pipe,but the convective heat transfer coefficient of the evaporation section increases first and then decreases with the increase of the flow rate.With the thickening of the liquid film,the heat transfer coefficient of the condensation section is reduced by 630.7 W/(m~2·℃),while the evaporation of the working fluid is affected,and the heat transfer coefficient of the evaporation section is reduced by 456.3 W/(m~2·℃).