Research On Heat Transfer Mechanism And Engineering Application Of Energy Pile

Energy pile is a kind of energy technology for heating and refrigeration using geothermal resources.In the existing studies,there is a lack of systematic analysis on the influencing factors of heat transfer performance of energy pile.There are relatively few comparative studies on heat transfer performance in the two seasons of winter and summer.Based on the experimental results of soil thermophysical parameters measurement,a three-dimensional simulation analysis model of energy pile is established.The influence of related factors on energy pile heat transfer performance in summer and winter is systematically studied by using finite element method.Finally,the expected effect of the application of the energy pile in the project is analyzed.The main conclusions are as follows:1.The measurement of thermal properties of soil shows that the thermal conductivity and specific heat capacity of soil increase with the increase of moisture content.The influence of moisture content on the thermal conductivity of soil is greater than that of specific heat capacity.2.The numerical simulation analysis of the influence factors of energy pile in summer shows that: 1)With the increase of energy pile continuous running time,the heat transfer performance of energy pile gradually decreases and tends to be stable.2)The increase of velocity and inlet water temperature is conducive to heat transfer.However,excessive flow velocity increases energy consumption,and the overhigh inlet water temperature is not conducive to the operation of heat pump unit.The flow velocity of single U type energy pile should be controlled between 0.2-0.4m/s and inlet temperature should be controlled between 35℃-37℃.3)The heat transfer performance of steel pipe is better than that of HDPE tube,and the steel pipe can be used as the preferred heat transfer tube in the project.4)Compare the energy piles with different buried forms(single U,W,double U): The heat transfer performance index(heat transfer,temperature difference between inlet and outlet,operation efficiency)of the double U is the best when both of the section area of pipe and the flow rate is the same.When the section area and the flow velocity is the same,the operating efficiency of the single U is the highest,the temperature difference of the inlet and outlet of W is the largest,and the heat exchange of double U is the largest.When the section area of the pipe is different,while the flow rate and flow velocity are the same,the single U have the highest operating efficiency,the heat exchange and the temperature difference of the inlet and outlet of double U are the greatest.5)The increase of pipe diameter is conducive to the increase of heat transfer and operation efficiency,but the outlet water temperature increases,which is not conducive to the operation of heat pump units.The diameter of pipes should be controlled between 27-32 mm.6)With the increase of pile length,the total heat transfer and temperature difference between inlet and outlet linearly increase,and the operating efficiency gradually decreases with the increase of pile length.The pile diameter increased from 0.6m to 0.8m,and the performance index of each heat transfer is increased.When the pile diameter is greater than 0.8m,the impact is smaller.The influence of pile length on the heat transfer performance of energy pile is larger than that of pile diameter.7)The thermal conductivity of the soil around the pile or the larger specific heat capacity is more beneficial to heat transfer.3.The numerical simulation of the influence factors of the energy pile in the winter condition and the comparison on heat transfer performance in the two seasons of winter and summer show that: 1)The variation laws of heat transfer performance index of the energy pile in winter is the same as that in summer,but the heat transfer performance is lower than that in summer.2)The operation efficiency of the energy pile decreases with the increase of velocity,but the decrease in winter is larger than that in summer.3)The operation efficiency increases with the increase of heat transfer temperature difference,and the increase of operation efficiency caused by the increase of heat transfer temperature difference in winter is greater than that in summer.4.The pile foundation in actual engineering is set as an energy pile,the numerical simulation analysis shows that: The maximum load ratio of single energy pile can be 0.191% in summer and 0.171% in winter.When all pile foundations are used as energy piles,the maximum load ratio can be 75.6% in summer and 67.7% in winter.