Numerical And Experimental Study On Thermo-mechanical Characteristics Of Energy Piles In Seasonal Energy Storage Process

As the innovation and development of traditional drilled buried pipe heat exchanger,energy pile combines buried pipe heat exchanger with pile foundation of building,which can not only bear the load of building structure but also meet the demand of heating and cooling in the building in winter and summer.Therefore,energy pile technology can not only greatly save the land area,but also can save construction drilling and a series of initial investment costs,so it is favored by practical projects.In practical engineering,energy piles often run in the form of pile group arrangement across seasons.Different pile-soil property parameters and different operation modes of pile groups will affect the energy storage and thermo-mechanical characteristics of the whole energy pile system.At present,the research on single energy pile has been conducted thoroughly,but most of the research on energy pile group still focuses on the heat transfer and thermo-mechanical characteristics of a single refrigeration(heat)condition in a short term,and rarely considers the differences in long-term energy storage and thermo-mechanicalcharacteristics of pile group from the aspect of operation mode.Therefore,this paper establishes a model of energy pile group and a corresponding experimental platform to study the thermo-mechanical characteristics of pile group in the process of seasonal energy storage from both experimental and simulation perspectives.In terms of numerical simulation,on the basis of single pile model,Fluent and Abaqus were used to establish a 4×4 pile group model,and the influences of temperature loading mode,operation strategy,pile with different thermal conductivity placement and different soil specific heat capacity on thermo-mechanical characteristics of energy pile group in seasonal energy storage process were studied.The results show that although the intermittent period reduces the monthly heat storage(extraction),the annual storage and extraction ratio of the whole pile group system increases by 4.43%when the inlet water temperature is constant.At the same time,the existence of intermittent period is conducive to the recovery of pile temperature and alleviates the constraint of soil around the pile,which leads to the decrease of pile top displacement and axial force but the increase of the maximum dimensionless displacement of the whole system.When the dynamic water temperature runs throughout the year,the variation of soil temperature around the pile lags behind that of the inlet water temperature,and the peak time of per unit heat storage(extraction)of the pile is advanced correspondingly.In the pile group,the pile top displacement and axial force of pile body at each position change sinusoidal with the inlet water temperature.After one year of operation,there is irreversible settlement of the pile at each position,which is about 0.3 mm,but all the piles’ axial force basically restore to the initial state.The axial force of the running piles is increased by turning on part of the piles in the group compared with turning on all the piles.At the same time,turning on the eight side piles during the heat extraction period will extract more heat every month than turning on the four internal piles and the four corner piles,and the ratio of extraction and storage is also higher.The four operating modes of non-uniform intensity with internal and external zones have their own characteristics in terms of energy storage and thermo-mechanical characteristics,and should be selected according to the actual situation.When piles with different thermal conductivity exist in the pile group,the thermal conductivity of inner pile,side pile and corner pile from small to large is relative to that from large to small,which not only increases the heat storage of pile group,but also reduces the maximum dimensionless displacement of pile group system.When the specific heat capacity of surrounding soil increases,the heat storage of pile group increases but the storage and extraction ratio decreases.At the same time,the pile top displacement and axial force of pile body increase.In terms of model test,a 3×3 energy pile group test platform was built,and the thermo-mechanical characteristics of pile group in the process of seasonal energy storage under different start-stop ratio,different connection forms and different circulating fluid flow directions were experimentally studied on a reduced time scale.The experimental results show that with the end of the third intermittent heating,the heat extraction of start-stop ratio 6:6 is 35.42%and 15.60%more than that of start-stop ratio 4:8 and 5:7,respectively.However,with the increase of start-stop ratio,the storage and extraction ratio gradually decreases.The recovery degree of soil around pile becomes worse with the increase of start-stop ratio.Pile excess temperature,pile top displacement,pile tip soil pressure and pile axial force all increase with the increase of start-stop ratio.The more series number of peripheral piles,the smaller cold(heat)interference of peripheral piles to the central pile,the more heat storage(extraction)of the central pile,but the pile group overall heat storage(extraction)decreased,the soil around the pile to the central pile constraints decreased,pile axial force decreased.For the peripheral pile,the changes of pile excess temperature,pile top displacement and pile axial force are affected by the temperature load and the degree of cold(heat)interference.Under the experimental conditions,in the heat extraction stage,when the circulating fluid flow direction of the peripheral pile is opposite to that of the heat storage,the interference of the peripheral pile to the central pile is intensified,which reduces the heat extraction of the central pile,but intensifies the constraint of the soil around the pile on the central pile,and the axial force of the central pile increases;For peripheral piles,the increase of the maximum temperature difference between piles leads to the increase of the maximum difference of pile top displacement,but the decrease of the maximum difference of pile axial force.This paper aims to explore the influence of different factors on the thermo-mechanical characteristics of energy pile group in the process of seasonal energy storage,so as to provide some theoretical support for practical engineering applications of energy pile group.