Theoretical And Experimental Study On Heat Transfer Performance And Thermo-mechanical Characteristics Of Energy Piles

By combining the buried pipe heat exchanger with the building pile foundation,the energy pile not only realizes the function of bearing the building load,but also meets the heating and cooling load requirements of the building.The application of this technology can effectively solve the traditional drilling The bottleneck in the application of buried heat exchangers is very suitable for my country’s national conditions.However,during the operation of the energy pile,temperature changes will generate thermal stress in the pile body,and the thermal stress acting on the pile foundation will have a certain impact on its structural safety.So far,there is no comprehensive evaluation method for the influence of pile body thermal stress on structural safety,which largely hinders the further popularization and application of energy piles.For this reason,this paper studies the influence of different parameters on the heat transfer performance and thermodynamic characteristics of energy piles through model experiments and numerical simulations.In terms of model experiments,an energy pile model experiment platform was built to carry out experimental research on the effects of different inlet water temperatures,alternating cooling and heating cycles,different pile top loads,and thermal coupling on the thermodynamic characteristics of energy piles.The results show that increasing the inlet temperature in summer conditions and decreasing the inlet water temperature in winter conditions will improve the heat transfer performance of the energy pile to a certain extent,but at the same time it will further increase the accumulation of cold and heat in the pile body,and the increase in heat exchange rate It will become smaller and smaller;there is no unrecoverable pile top settlement under summer conditions,and unrecoverable pile foundation settlement under winter conditions,and the lower the inlet temperature,the greater the settlement;under winter and summer conditions,The pile stresses show a trend of first increasing and then decreasing with the increase of depth,and reach the maximum in the middle of the pile.The heat transfer per unit pile depth in the process of alternating cooling and heating cycles decreases with the increase of the number of cycles.The energy pile will produce irreversible displacement settlement under the action of cooling and heating cycles,and the displacement settlement will increase with the increase of the number of cycles.The soil pressure at the pile tip will increase with the increase of the number of refrigeration cycles,and decrease with the increase of the number of heating cycles.In the winter and summer working conditions,the thermal and mechanical coupling effects all aggravated the downward settlement and displacement of the pile foundation to a certain extent Under summer conditions,thermal coupling further increases the stress of the pile;under winter conditions,thermal coupling reduces the stress of the pile to a certain extent.In terms of numerical simulation,the multi-physics software COMSOL was used to establish a numerical model of the energy pile,and the length of the pile foundation,the diameter of the pile foundation,the thermal conductivity of the pile foundation material,the type of soil,the groundwater seepage,and the effect of long-period operation on the energy in the winter and summer conditions were discussed.The influence law of pile heat transfer performance and mechanical characteristics.The results show that the larger the length and diameter of the pile foundation,the stronger its heat storage(cold)capacity,so that more heat(cold)is stored inside the pile,which enhances the heat exchange effect of the energy pile to a certain extent.However,it is not possible to increase the heat transfer by increasing the length and diameter of the pile foundation without limitation;the greater the length and diameter of the pile foundation in winter and summer conditions,the greater the change in the pile top displacement;in summer conditions,the greater the pile foundation length and diameter are As the depth increases,the axial force of the pile body first increases and then decreases.The maximum value is taken near the middle of the pile foundation,and the maximum point is taken as the boundary.The axial force of the pile body is in the upper and lower parts of the pile foundation.The distribution trend of the half part is opposite:when in the upper part of the pile foundation,the axial force of the pile body decreases with the increase of the length and diameter of the pile foundation.When in the lower part of the pile foundation,the axial force of the pile body decreases with the increase of the pile foundation.The increase in length and diameter increases.Similar laws can be obtained in winter conditions.Increasing the thermal conductivity of the pile foundation material can greatly improve the heat transfer efficiency of the energy pile,but at the same time it will also produce greater displacement and axial force.In winter and summer conditions,the heat transfer per unit pile depth is the largest in rock and soil,followed by clay,and sand is the smallest.In winter and summer conditions,at the same depth,the axial force of the pile surrounded by rock and soil is the largest,followed by clay,and sand is the smallest.With the increase of seepage velocity,the heat transfer per unit pile depth increases in winter and summer,and the change of pile top displacement decreases.Under summer conditions,the axial force of the pile body decreases with the increase of the seepage velocity.In winter conditions,the axial force of the pile body increases with the increase of the seepage velocity in the upper half of the pile foundation,and the axial force of the pile body decreases with the increase of the seepage velocity in the lower half of the pile foundation.During long-period operation,as the number of operating days increases,the heat transfer per unit pile depth in winter and summer decreases;the displacement of the pile top increases,and an unrecoverable displacement settlement is produced at the end of the operation;the pile shaft axis under summer conditions The force gradually increases,the axial force of the upper part of the pile foundation gradually decreases in winter conditions,and the axial force of the lower part of the pile foundation gradually increases.This article aims to explore the influence of different influencing factors on the heat transfer performance and thermodynamic characteristics of energy piles in order to provide theoretical support for further in-depth research and engineering applications.