Numerical Studies On Heat Transfer Characteristics Of Pile Heat Exchanger With Spiral-shaped Pipes

The pile foundation heat exchangers rely on the thermo-active ground structures of the building,which has the characteristics of large pile diameter and buried in shallow depths below the ground surface,and has more obvious advantages than the traditional borehole heat exchangers,such as smaller areas,saving the time and expenses associated to the drilling holes and stable performance of heat exchanger and so on,conforms to our country’s sustainable development strategy.Compared with the pile foundation heat exchangers with the U-shaped pipes or W-shaped pipes,the pile foundation heat exchangers with the spiral-shaped pipes not only has longer pipe length and more heat transfer areas in the same pile foundation,but also can prevent the gas from concentrating on the top of the pipe.However,the heat transfer mechanism of the pile foundation heat exchangers with the spiral-shaped pipes is different from that of the conventional borehole tube heat exchanger and has complex geometrical structures,and the research on its heat transfer performance is not sufficient.Therefore,we firstly established a 3-D dynamic simulation model to study the heat transfer processes of a pile foundation heat exchanger in which double spiral pipes and the return risers were buried in parallel.And the numerical simulation results were validated by the data obtained in an in-situ test and analytical solution of the solid cylindrical heat source model.Then,it is known that buried pipe geometry of heat exchangers has an important role in heat transfer between the circulating fluid and the ground,under the conditions of the same pipe materials,the same pipe dimensions and the same thermal physical properties and flow parameters,we established full-scale 3-D numerical models for three energy piles with single spiral pipe,double spiral pipes and double spiral pipes in parallel buried in each,and simulated unsteady state heat transfer processes of above three pile foundation heat exchangers on the ground.The results show that the heat transfer effect of the pile foundation heat exchangers have a difference as various buried pipe geometry,pile foundation heat exchangers with the double spiral shaped pipe is best,pile foundation heat exchangers with the single spiral shape is better than double spiral type in parallel one.Then,the effect of inlet flow rate,inlet water temperature,soil initial temperature,pipe diameter and pitch size on heat transfer performance of the heat exchanger was simulated which based on pile foundation heat exchangers with the single spiral shaped pipe,and the thermal change regularities of each design parameter was obtained.In addition,according to the load characteristics of building air conditioning project,three kinds of on-off ratio are selected,and the heat transfer performance of the pile foundation heat exchangers with the spiral-shaped pipes under different intermittent operating modes is discussed,and the temperature variation regularities at different locations on the pile wall and in the soil surrounding along the pile depth and in radial directions are analyzed.The index of pile wall temperature recovery percentage is proposed to quantify the temperature recovery degree under different intermittent operational modes.Finally,to understand the effect of groundwater advection on pile foundation heat exchangers with the double spiral shaped,established its simulation model considering thermal conduction and groundwater advection.The heat transfer effect and the surrounding soil temperature field under groundwater advection is influenced by parameters such as the velocity of fluid flow and the soil kinds.Furthermore,the index of the heat transfer rate per unit pipe length is proposed to quantify the heat transfer effect of groundwater advection.The results of this paper provide a good reference value for the actual engineering design and application of the pile foundation heat exchangers with the spiral-shaped pipes.