Steady-state Heat Transfer Of The End-bearing Energy Pile With Spiral Pipe

In recent years,as a green resource,geothermal energy has attracted more and more attention.Ground source heat pump(GSHP)is a new type of air conditioning technology which utilizes shallow geothermal resources to refrigerat ing and heating buildings.It is composed of water source heat pump and heat exchanger.In winter,the ground temperature is lower than that of the soil layer,so the heat exchanger transfers the heat from the soil layer to the building.In summer,the ground temperature is higher than that of the soil layer,and the heat in the building is transferred to the soil layer by the heat exchanger.However,the traditional ground source heat pump has some disadvantages,such as requiring a large amount of land for laying heat exchangers.Also the cost of extra drilling is relatively high.Aiming at the above disadvantages,some scholars combined the ground source heat pump with pile foundation and put forward the concept of energy pile.Energy pile is a new type heat exchanger combined with building structure.It forms a heat exchange mechanism with the earth by embedded heat exchange pipe in the pile.Energy pile can not only bear the load of the superstructure,but also bear part of the cold and heat load of the building.As for heat transfer of energy piles,most existing models considered energy piles as infinite or semi-infinite rods.So the influence of pile length and convection heat transfer between atmosphere and ground can be ignored.However,there is no analytical solution for heat transfer of energy piles with small slenderness.Therefore,based on the existing theories,this paper investigates the steady-state heat transfer of rock-socketed energy piles with spiral pipes.The main contents and conclusions obtained are as follows:(1)For steady state heat transfer calculation of multi-layer composite rock-socketed energy pile,considering the influence of convection heat transfer between atmosphere and ground,pile length and composite material of length,the governing equations for the heat conduction between the energy pile with multilayer composites and the surrounding soil are constructed.According to the continuity conditions of the temperature and the radial heat flux density at the interface of the multilayer composites,a semi-analytical solution for the steady-state temperature fields of the pile and soil is obtained in the light of the separate variable method.Along the diameter of buried pipe,the additional temperature field of pile-soil system gradually decays,and the attenuation amplitude gradually slows down.Especially,when the radial distance and pile diameter ratio are between 1 and 4,the additional temperature field of pile-soil system has the largest attenuation amplitude.The temperature field of pile-soil system in the range of zero to two is affected largely by the boundary effect.The heat transfer efficiency of energy piles can be effectively improved by increasing the radius of buried pipes and selecting pile materials with high thermal conductivity.(2)This paper presents a new calculation model for the steady-state heat transfer calculation of soil-pile-atmosphere heat conduction interaction rock-socketed energy pile.The effects of temperature of bedrock,convection heat transfer between atmosphere and ground,adiabatic treatment at pile tip,fluid velocity and fluid heat loss on heat transfer can be considered.Free thermal field of the soil is obtained firstly.Then considering the soil-pile interaction during operating of the energy pile,temperature field of the pile is calculated.The boundary value problem of the soil temperature field is transformed into determination of a series of unknown coefficient.Lastly,additional temperature field of the soil is obtained in light of the boundary conditions.Parametric study shows that at the same depth,the higher of convective heat transfer coefficients between the air and the ground,the lower of temperature field of system.In addition,there is a linear relationship between the amplification of the soil additional temperature and the heat transfer coefficients.The pile temperature and the soil additional temperature increase with the increasing of the heat conductivity coefficient and the flow velocity of the fluid.Moreover,the increasing rate of the soil additional temperature decreases with the increasing rate of heat conductivity coefficient of the fluid.The pile temperature and the soil additional temperature are not sensitive to heat conductivity coefficient of soil and convective heat transfer between air and ground.Boundary effect has significant influence on additional temperature of soil.This paper innovatively puts forward a new model of steady state heat transfer of energy pile,which can be used to calculate the temperature field of pile-soil system caused when energy pile with buried pipes in operation.The model has the advantages of clear physical concept,high computational efficiency and certain engineering application value.