Research On The Thermal Transfixion Characteristics Of Multi-stage Recharging Forced External Circulation Standing Column Well

With the development of and utilization of clean and efficient geothermal energy,ground source heat pump has developed rapidly.Forced external circulation standing column well is a form of a single well circulating underground heat exchange system in the groundwater source heat pump.Forced external circulation standing column well(FECSCW)has gained universal attention with the characteristics of high efficiency,energy saving and environmental protection.It is widely used in practical projects.But the recharging issue and thermal transfixion appear in FECSCW with the accumulation.Its efficiency gradually decreases.Researcher improves well-making process.Multi-stage recharging forced external circulation standing column well(MRFECSCW)has appeared as a new type of filling and recharging in the same well.However,the research on the MRFECSCW is still in its initial stage.Therefore,combining with the underground geologic conditions of the FECSCW,the influence of the porous media in the underground aquifer on the seepage and the breakdown of the aquitard are taken as the basic points in this paper.The thermal transfixion characteristics of multi-stage recharging in forced external circulation standing column well are explored through experiments and numerical simulations.Firstly,the experimental table is built in this subject according on the Darcy law.The permeability coefficient and porosity of porous media was tested.The test results show that the porosity of the porous medium is 39.42%,and the seepage coefficient is 0.253 cm/s.The result provides the basal data for simulation.The tensing and pressing universal machine is used to compress cement mortar.A relationship of the force of the cement mortar specimen with time variation is obtained.A relationship of the strain with time variation is obtained.A relationship of the force with displacement variation is obtained.The pressure peak of the specimen is provided under different ratios.The complete failure point of the specimen and other parameters are provided.Combining the aquitard fracturing experiment,The COMSOL model is established.The two-dimensional and three-dimensional strain distribution diagrams are obtained from the model calculation.The preliminary condition of strain is estimated in the aquitard with a similar proportion.The effect of thermal transfixion in MRFECSCW is analyzed.Finally combined with Darcy’s experiment and aquitard fracturing experiment,The mathematical model of one-stage,two-stage and three-stage recharging forced external circulation standing column well is established.By the numerical simulation analysis of the flow field and temperature field and compared with other single well models,the temperature of the pumping water at the 9 min of the three-stage recharge type forced external circulation standing column well is 0.8 ℃ higher than that of the first-stage recharged pumping at the same time.Meanwhile analyzing the flow field,multi-stage recharge can slow down the phenomenon of thermal transfixion,improve the rate of return water and increase return water.It solved the problem of recharge.By researching on the thermal transfixion characteristics of multi-stage recharging in forced external circulation standing column well,it is verified that Multi-stage recharge can increase the amount of recharge and slow down the phenomenon of thermal transfixion in certain extent.It has cetain guidance for piratical engineering.It promotes the development of ground source heat pump and accords with the background of energy conservation and environmental protection.