Experimental And Numerical Study On Heat Transfer Performance And Thermo-mechanical Coupling Characteristics Of Phase Change Concrete Energy Pile

Energy pile is a kind of economical and efficient building energy-saving technology which combines the ground heat exchanger in the traditional ground source heat pump with the building pile foundation.Under the current situation of energy shortage and environmental pollution,energy pile technology has attracted much attention due to its advantages such as energy saving,environmental protection,high efficiency,and saving land area.However,in actual operation,the heat exchange capacity of traditional energy piles will be greatly limited by the pile length and the number of piles.At the same time,the pile body will be deformed to a certain extent due to thermal expansion and contraction due to temperature changes,which will affect the heat exchange capacity and bearing performance of the energy pile and endanger its safe and efficient operation.To this end,a phase change concrete energy pile composed of phase change material(PCM)as a part of the traditional energy pile filling material is proposed.The characteristics of PCM’s phase change latent heat and small temperature change during the phase change process are used to improve the energy storage and heat transfer characteristics of traditional energy piles,and at the same time reduce the deformation of energy piles during heat exchange.However,there are few studies on the thermodynamic characteristics of phase change concrete energy piles under the thermo-mechanical coupling effect.To this end,the effects of phase transition process,different operating modes,and the physical properties and structural parameters of the pile on the thermodynamic characteristics of phase change concrete energy piles are studied through model experiments and numerical simulationsIn terms of model experiments,traditional and phase-change concrete energy pile model test benches were set up to carry out experimental studies on the effects of phase change process,inlet water temperature,different intermittent ratios,and pile top loads on the thermodynamic characteristics of phase change concrete energy piles under exothermic conditions.The results show that the addition of PCM in the energy pile not only can effectively reduce the temperature rise of the pile body and the soil around the pile,thereby weakening the deformation of the pile body due to thermal expansion,the thermal stress of the pile body and the soil pressure at the tip of the pile,but also improve its heat exchange performance.The higher the inlet water temperature,the greater the heat exchange capacity,but the larger the temperature rise of the pile body and the soil around the pile,which will cause the pile top displacement,the pile body thermal stress and the pile tip soil pressure to be greater.Under different intermittent ratios,the longer the running time,the larger the total heat transfer,but the smaller the average heat transfer time,the lower the soil temperature recovery rate around the pile.The residual displacement at the top of the pile,the pile body thermal stress and soil pressure at the tip of the pile will also increase.Increasing the pile top load will increase the cumulative settlement of the pile top,the pile body stress,and the soil pressure at the tip of the pileIn terms of numerical simulation,the numerical models of traditional and phase-change concrete energy piles were established by using FLUENT and ABAQUS software.The effects of phase transition process,thermal conductivity,phase transition temperature,latent heat of phase transition,distance between buried pipe branches and length-to-diameter ratio of the pile body on the thermodynamic characteristics of phase change concrete energy piles under exothermic conditions were discussed.The results show that compared with traditional energy piles,phase change concrete energy piles can improve heat exchange capacity,reduce the range of thermal influence,reduce the change in displacement of the pile body,the axial force of the pile body,and the frictional force on the side of the pile.At the same time,increasing the thermal conductivity of phase change concrete can significantly increase the heat exchange capacity,but it will also lead to a larger range of soil thermal influence and an increase in the displacement of the pile body and the axial force of the pile body.The use of lower phase change temperature and larger phase change latent heat under exothermic conditions is beneficial to improve the heat transfer performance of phase change concrete energy piles,reduce the range of soil thermal influence,reduce the change of pile displacement and the axial force of the pile.With the increase of the distance between the U-shaped piles buried pipe legs,the heat transfer capacity and the soil thermal influence range gradually increase,but the axial force of the pile body decreases and the change of the displacement of the pile body shows a trend of increasing first and then decreasing.The larger the length-to-diameter ratio of the pile body,the greater the total heat exchange capacity of the phase change concrete energy pile,but the heat exchange capacity per unit pile depth may not necessarily be greater.In addition,the pile body deformation caused by the temperature change is greater.The purpose of this paper is to explore the phase change process and the influence of different factors on the thermodynamic characteristics of phase change concrete energy piles,in order to provide theoretical guidance and technical support for its further research and engineering applications.