Research On Heat Transfer Performance Of Ground Source Heat Pump Underground Vertical Tube Heat Exchanger

Nowadays,energy shortage,air pollution and global warming have become serious problems facing mankind.In recent years,shallow geothermal energy has attracted the attention of many countries due to its rich reserves,environmental protection,high efficiency,energy saving,stability and reliability.However,there are still many problems in the use of shallow geothermal energy in the bedrock area of southwest China:(1)the initial investment is high and the area is large;(2)the summer cooling load is greater than the winter heat load,which leads to a reduction in heat transfer performance.In view of the above problems,this paper relies on the actual office building in Chongqing as the experimental basis,and studies the improvement of the heat transfer performance of the buried pipe from three aspects: buried pipe structure,buried pipe material and backfill material.First of all,this paper makes a statistical analysis and description of the climatic characteristics,hydrogeology,thermal parameters of rock and soil body and initial temperature field of rock and soil body.Based on the actual engineering,a new type of buried pipe with elliptical cross section is proposed.,Using GAMBIT to establish a layered heat transfer model,and using traditional circular buried tube thermal response test(TRT)experimental data to verify the accuracy of the model,and then use FLUENT software to conduct heat exchange effects of different buried tube heat exchangers Based on the analysis,the heat transfer performance was evaluated in terms of outlet water temperature,heat transfer volume per unit well depth,energy efficiency coefficient,pressure drop,thermal performance potential,and borehole thermal resistance,and compared with traditional circular buried pipes.The results show that the outlet water temperature of the elliptical buried tube heat exchanger is lower than that of the circular one under all working conditions.After 24 hours of continuous operation,the energy efficiency coefficient of the elliptical buried tube heat exchanger is comparable to that of the circular buried tube The heat exchanger is improved by 13.4%～ 20.4%.In addition,the heat transfer effect of the elliptical buried tubes in the row is better than that of the parallel.The thermal interference of the adjacent inlet and outlet pipes in the borehole is round buried pipe in order from strong to weak.Then,through the FLUENT numerical simulation study,the effects of steel materials,aluminum materials and PE materials on the heat exchange performance of the buried tube heat exchanger are compared,and the total thermal resistance between the steel tubes,aluminum tubes and PE tubes and the surrounding soil is analyzed.The cost and feasibility of steel pipe in practical engineering application are analyzed.The results show that the heat transfer per unit depth of aluminum tube and steel tube heat exchangers is better than that of heating seasons during the cooling season,the energy efficiency coefficient and the total thermal resistance of the borehole are better than the PE pipe,and the borehole temperature is higher than the PE pipe It shows that the heat exchange performance of aluminum pipes and steel pipes is better than that of PE pipes.In addition,the economics of aluminum pipes and steel pipes are similar to that of PE pipes,but from the consideration of the tight land use and corrosion resistance of cities,it is better to use aluminum pipes.Finally,a new backfill material,namely kaolin,was proposed to improve the heat transfer performance of the buried tube heat exchanger.The FLUENT software was used to simulate and analyze the original soil backfill and kaolin backfill intermittently for a cooling season and heating season.The hourly heat exchange rate of single well,outlet water temperature,heat exchange energy system coefficient,and soil temperature recovery rate were compared with the continuous operation mode.The results show that after the cooling season,the average excess temperature of the borehole wall surface of each soil layer is higher than that of the original soil backfill,and the rate of soil temperature recovery is lower than that of the original soil backfill.With the increase of operating time,the energy efficiency coefficient during the backfilling of the original soil decreased from 22.71% to 19.63%,and the energy efficiency coefficient during the backfilling of kaolin decreased from 25.68% to 22.06%.About 2.6% higher.