| As a natural resource with huge reserves on the earth,geothermal energy has become one of the renewable energy resources that cannot be ignored in today's world.China is the world's largest energy consumer and the second largest economy.The use of geothermal energy as a renewable resource is crucial to China's sustainable development.Although some research has been considering groundwater seepage of the underground heat exchanger heat transfer performance impact,but the existing research assumed either completely saturated homogeneous porous media for the underground rock and soil layer material,either in a rock layer can be divided into underground water level of saturated soil and unsaturated soil,few studies have considered the layered geological conditions and underground part of the formation as a thermal seepage coupling aquifer.Therefore,this paper makes a further study on the efficiency improvement,structure optimization and arrangement of boreholes of buried pipe heat exchangers under the complex geological environment and the corresponding seepage condition.First somewhere in chongqing source heat pump system is based on practical engineering geological conditions,using Gambit software establishing multi-layer geologic and thermal seepage coupling under the condition of three dimensional heat transfer model,to the actual working condition parameter setting model in Fluent software initial conditions,and the model calculated value of export water temperature and thermal physical property test buried pipe water temperature,the comparison between the accuracy of the model was verified.The homogeneous model,layered heat transfer model,pure heat conduction model and thermal osmosis coupling model were established to compare and analyze the influence of different working conditions on the heat transfer performance of buried pipe.The results show that the heat transfer per well depth of the homogeneous model is 0.89%higher than that of the layer model.The heat transfer per unit well depth in the heat permeability coupling model is 10.74%higher than that in the pure heat conduction model.Under the condition of soil layers and the coupling of the thermal penetration,the seepage velocity and flow direction of heat transfer performance of buried heat exchanger,the results show that with the increase of flow velocity,heat transfer performance of heat exchanger is always increase,when the flow velocity is less than1.5 x 10-5m/s,the seepage direction for the return pipe to the inlet pipe heat transfer performance of the best;When the seepage velocity is no less than 1.5×10-5m/s,the seepage direction is perpendicular to the axis of the inlet/return pipe,and the heat transfer performance is the best.Then the heat exchanger of aluminum-plastic composite pipe is used to improve the heat transfer efficiency of buried pipe.The results show that under the same operation mode and seepage condition,the heat exchange of aluminum-plastic composite pipe heat exchanger is more improved than that of PE pipe,and different operation modes have different effects on the performance evaluation of buried pipe heat exchange under different seepage condition.During continuous operation,the best seepage direction is from the return pipe to the inlet pipe when the seepage velocity is no more than 1.5×10-5m/s.Otherwise,it is perpendicular to the axis of the inlet pipe.During intermittent operation,the best seepage direction of the aluminum-plastic composite pipe at 6 different seepage velocities is the direction from the return pipe to the inlet pipe.Considering the heat transfer performance and economy of aluminum-plastic composite buried pipe,it is more reasonable to adopt the design method of aluminum-plastic composite pipe with the 2nd,3rd,4th and 5th layers.Then,based on the optimal seepage direction and the optimal composite depth of aluminum-plastic composite pipe,the section structure of u-shaped buried pipe was changed,and the u-shaped flat tube section heat exchanger was proposed.The section size of the branch pipe is optimized when the center distance and cross-sectional area of the branch pipe are kept constant.The results show that if the section flatness of flat tube is reduced(or the section height of flat tube is increased),the heat transfer per unit well depth of u-type flat tube heat exchanger is greater,and the heat transfer performance of u-type flat tube heat exchanger under the same seepage condition is 21.84%higher than that of PE tube.Finally,based on the optimal working condition of u-tube heat exchanger obtained in the previous paper,the heat transfer models of three different drill hole arrangement modes are established to study the influence of drill hole arrangement modes with different aspect ratio on the heat transfer performance of the pipe group.The results show that the overall average heat transfer of the pipe group increases first and then decreases with the decrease of aspect ratio.When the aspect ratio is 1:1.5,the heat transfer per unit well depth of the pipe group is the highest,the average thermal resistance of borehole is the smallest,and the thermal short-circuit loss coefficient of rock and soil is the smaller. |
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