Thermo-mechanical Characteristics Of Pile-based Borehole Heat Exchanger Under Different Working Conditions

At present,a lot of researches have been carried out on the heat exchange efficiency of pile-base heat exchangers,and the temperature changes caused by heat and heat release during the cooling and heating process of ground source heat pump system are generated.The pile foundation tube heat exchanger(also known as the energy pile)has to bear the function of the conventional pile foundation while serving as the end heat exchange device of the soil source heat pump.This not only requires that the heat transfer performance should meet the air conditioning heating load demand,but also requires that the pile foundation stress changes caused by the intermittent alternating take-up and heat release process can not endanger the stability of the upper building structure.In this paper,the three-dimensional dynamic numerical simulation model of parallel double-U pile foundation tube heat exchanger is established by using Comsol and Abaqus software.The correctness of the established model and simulation results is verified by the field test results.The analysis gives the water temperature at the outlet of the buried pipe,the heat exchange capacity per unit pile depth,the dynamic temperature distribution along the pile body and the internal part of the pile foundation,the axial force distribution,the change of the side frictional resistance of the pile and the displacement of the pile body.Subsequently,the heat transfer performance and mechanical characteristics of the double-U-type pile-base heat exchangers with five different flow rates,four different pile length-diameter ratios,different pile-series soil types and different operation-stop ratios were further explored.And the heat transfer performance and mechanical characteristics of pile-buried tube heat exchangers under three different buried pipes.Finally,the influence of pile top restraint on the mechanical characteristics of energy piles is explored,and the outlet water temperature and unit pile depth heat transfer are revealed.Time-varying characteristics of dynamic performance parameters such as dynamic heat transfer performance and axial force and pile top displacement.The results show that the influence of the buried pipe form and the length-diameter ratio of the pile foundation on the heat transfer and mechanical properties of the pile-base heat exchanger is significant,and the influence of the flow velocity is weak.The larger the aspect ratio and the flow velocity,the higher the heat transfer capacity of the buried pipe.The additional pile axial force,pile top displacement and side friction resistance caused by temperature change also increase correspondingly;intermittent operation has a certain effect on soil temperature recovery around pile foundation.When the stopping time is relatively large,the heat transfer performance of the pile foundation is better.The pile top load has a significant influence on the axial force of the pile body.The axial force of the pile body is positively correlated with the pile top load;The larger the pile top load,the more the position of the pile body displacement is biased.