Numerical Simulation Of Heat Transfer And Thermal Response Radius Of Energy Piles

As a new type of pile-based ground source heat pump technology,energy pile can not only bear the load from superstructure,but also exchange heat with shallow geothermal energy.However,with the development and utilization of renewable energy,the thermal environment of geotechnical engineering caused by temperature changes in the process of environmental protection is also highly valued by domestic and foreign scholars.Therefore,the research on the heat transfer characteristics of energy piles has important theoretical,engineering practical value and practical significance.The main purpose of this paper is to establish a discretized numerical model based on the analytical model of heat transfer in the pile foundation.The distribution characteristics of the temperature field of the surrounding rock of the energy pile are analyzed,and the influence factors of heat transfer performance of the energy pile is studied.Based on the infinite long line heat source model,the calculation method of thermal response radius of energy pile under single load is proposed,In view of the fact that the heat transfer power per meter of the energy pile is a function of time in practical engineering,a numerical model for heat transfer of the energy pile under variable load is established,and a simplified calculation method for the thermal response radius of the whole life cycle of the energy pile is proposed.The completed work results mainly include:(1)The three-dimensional heat transfer numerical model of energy piles under constant heating power(TRT)conditions is established.The relationship between the excess temperature of the pile wall and the distribution of excess temperature along the radial direction and depth direction under different heating times is discussed.The results show that the excess temperature in the rock and soil is the largest at the pile wall,exponentially decays with the increase of the distance from the pile wall,and increases with the increase of the running time.The air temperature on the soil surface only affects the temperature distribution near the soil surface and the influence on the temperature distribution of rock and soil increases with time.With the increase of running time,the excess temperature of the pile bottom soil is also affected by the vertical heat transfer of the pile bottom.Comparing the calculation results of four heat source analytical models(ILS,FLS,ICS,SCS)and three-dimensional numerical models,the calculation results of the numerical model and the analytical solution trend are basically consistent under the long-term effect,and the correctness of the numerical model is verified.(2)The three-dimensional numerical model of heat transfer of energy piles under the condition of constant inlet temperature(TPT)was established,and the influences of running time,pile length,pile diameter,inlet water temperature,fluid velocity in the pipe,distance between branch pipes,soil mass around the pile,backfilling material of pile body,number and form of buried pipes on heat transfer performance of energy piles were studied respectively.The results show that the heat transfer power of the energy pile fluctuates greatly in a short period of time,but tends to be stable with the extension of running time.The heat transfer power per meter of the energy pile is inversely proportional to the length of the pile,directly proportional to the diameter of the pile,directly proportional to the water temperature at the inlet,directly proportional to the fluid velocity in the pipe,and directly proportional to the distance between the branch pipes.The higher the thermal conductivity is,the higher the heat transfer power per meter is.Among the single U,double U and triple U energy piles,the heat transfer power per meter of double U energy piles is the largest,while that of triple U energy piles is the smallest.Among the single U type,W type and spiral type energy piles,the heat transfer power per meter of W type energy pile is the highest,followed by spiral type energy pile,and the heat transfer power per meter of single U type energy pile is the lowest.(3)According to the condition of pile-embedded heat exchanger in pile group in engineering,the temperature of pile wall of pile-embedded heat exchanger in pile group is calculated by superposition principle,and the vertical distance between adjacent pile center lines is defined when the thermal response radius of pile-based buried heat exchanger is the excess temperature influence coefficient caused by other piles.Based on a lot of calculation and analysis,the formula of thermal response radius of pile-based buried pipe heat exchanger under single working condition is fitted by least square method.The results show that for the single pile,single row pile,double row pile and multiple row pile layout,the maximum relative error between the results of the fitting formula and the analytical solution of the infinite long line heat source model is 4.62%,4.45%,3.77% and 3.32%,respectively,indicating that the method has good calculation accuracy and meets the engineering requirements.(4)In view of the fact that the heat transfer power per meter is a function of time during the operation of the actual energy pile system,a three-dimensional heat transfer numerical model of the energy pile under dynamic load is established,and the influence of different load distribution forms,peak load,operation time and the whole life cycle on the thermal response radius of the energy pile is analyzed.Results show that the load distribution form and the size of the peak load only affect rock mass on the temperature and its gradient,the thermal conductivity of the rock mass transfer area and operation time influence the temperature,the amount of total life cycle of cold and hot load can cancel each other out,radius of thermal response calculation according to thermal response under working condition of single load radius calculation method.(5)According to the arrangement mode of single pile,single row pile,double row pile and multiple row pile,the variation relationship of thermal response radius of energy pile with time under different pile diameters and different geotechnical mass thermal diffusivity is drawn,which provides convenience for engineering application.