| At present,the shortage of energy resources and environmental pollution have severely restricted the sustainable and healthy development of our national economy.As a kind of abundant,stable and clean energy,geothermal energy has attracted more and more attention.People mainly use the ground source heat pump technology to extract geothermal energy,and the buried tube heat exchanger is the core component of the ground source heat pump system,and its heat exchange performance is directly related to the operating efficiency and economy of the entire system.With the further research,it is found that the flow of groundwater will change the composite heat transfer coefficient of the soil medium around the buried pipe,which leads to the increase of the temperature difference between the inlet and outlet of the buried pipe and the increase of the heat flow per well depth.However,unnecessary heat transfer will inevitably occur between the two pipes which is the so-called ‘thermal short circuit’ due to the close distance and the temperature difference between the inlet and outlet pipes during the heat exchange process,and the existence of groundwater may aggravate this phenomenon.Therefore,it is extremely important to take measures to weaken the thermal short-circuit phenomenon between the two pipes under seepage conditions,so as to improve the heat transfer performance.Based on the porous media model and the turbulence model,this paper establishes a threedimensional heat transfer model coupling of thermo-permeability between single U-shaped buried pipe heat exchanger and surrounding soil,the simulated heat exchanger outlet water temperature is compared with the data in the literature,and the algorithm verification is completed.The thermal short-circuit phenomenon under different working conditions is analyzed.Finally,the thermal short-circuit is weakened by installing a heat insulation board between two branch pipes or laying an insulation layer outside part of the outlet pipe.The geometric dimensions of the thermal insulation board and the thermal insulation layer and the installation position of the thermal insulation board are optimized.The specific work is as follows:(1)The U-shaped buried pipe heat exchanger and soil thermal seepage coupling heat transfer model is established to simulate and study the relationship between the thermal short circuit unbalance coefficient and the heat flux per well depth with the drilling depth,branch pipe spacing,backfill material thermal conductivity and seepage velocity under different pipe flow velocities.(2)In order to weaken the thermal short-circuit,reduce the water temperature at the outlet of the buried pipe heat exchanger and increase the heat flow per well,the heat insulation board is installed between the inlet and outlet water pipes,the thickness,width,height and installation position of the heat insulation boards are optimized in turn.(3)The insulation layer is laid outside part of the outlet pipe section to explore the relationship between the outlet water temperature of the buried pipe heat exchanger and the heat flow per well depth with the height and thickness of the insulation layer,so as to find the best value of the insulation layer height and thickness.The following important conclusions are obtained through research:(1)The flow rate of the working fluid in the pipe is constant,the greater the drilling depth,the greater the total heat flow of the buried pipe,and the more serious the thermal short circuit.The thermal short-circuit unbalance coefficient gradually decreases with the increase of the branch pipe spacing,while the heat flux per well depth gradually increases.As the thermal conductivity of the backfill material increases,the thermal short-circuit phenomenon gradually increases,but the heat flux per well depth is gradually increasing.The greater the seepage velocity,the greater the heat flux per well depth,and the greater the thermal short circuit imbalance coefficient.(2)The installation of heat insulation board can effectively weaken the thermal short-circuit phenomenon between the branch pipes,and also inhibit the heat dissipation of the water inlet pipe to the side of the water outlet pipe.The heat insulation board is not as thick as possible,when it is about 12 mm,the heat exchange effect is the best.When the width of the heat insulation board is 120 mm,the corresponding heat flow per well depth is the largest.The heat transfer performance is highest when the height of the heat shield is about 42% of the drilling depth.When the heat insulation board is installed at a position that moves 2mm to the side of the outlet pipe relative to the center of the two branch pipes,the maximum heat flow per well depth can reach 44.7084W/m.(3)It is effective to suppress the thermal short-circuit between two branch pipes by laying insulation layer,but at the same time it will also hinder the heat exchange between the outlet pipe and the surrounding soil.The insulation layer is not as high as possible,when the height is about 25% of the drilling depth,the maximum heat flow per well depth is 44.0516W/m.When the thickness of the insulation layer is about 4mm,the best heat exchange effect will be achieved. |
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