Exploration On Thermal And Mechanical Characteristics Of Energy Piles Buried With Spiral-shaped Pipes

The development of soil-source heat pumps is restricted by the problems of small heat transfer area and large investment cost of drilling holes in traditional buried pipe heat exchangers.Energy pile(pile foundation heat exchangers),which is based on the construction pile foundation,has the characteristics of large diameter of pile,shallow buried depth,small occupation area,and saving drilling cost,and can better promote the utilization of geothermal energy.Compared with single-U type,double-U type and W type buried pipe energy pile,the spiral buried pipe energy pile with the same pile depth can be embedded with longer buried heat transfer pipes and larger heat transfer area.However,the energy exchange between the energy pile and the soil around the pile leads to the change of the temperature of the pile,and the constraint of the rock and soil will hinder the deformation of the pile and thus cause the thermal stress of the pile,resulting in the change of bearing capacity and settlement characteristics of the pile foundation.In view of the need of energy pile to meet both the structural load and heat transfer needs,but the current research on pile foundation spiral buried pipe mainly focuses on the heat transfer performance,the bearing characteristics and settlement characteristics of its pile are less studied.In this paper,a 3-D numerical simulation model of energy pile buried with double spiral coils in parallel is firstly established,and the field test is simulated by simulation software COMSOL Multiphysics.The comparison between the simulation result and the test result shows that the simulation result of the numerical model is correct.Since different types of buried pipe heat exchangers have an important impact on the heattransfer performance of energy piles,the bearing and settlement characteristics of each type of buried pipe energy piles are also different.Under the same conditions of inlet water temperature,inlet velocity,pipe diameter,pipe material and pile diameter,3-D simulation models of energy piles buried with double spiral coils in parallel and double spiral coils in series and W-type were established respectively to investigate the heat transfer performance and bearing characteristics.The results show that the heat transfer effect of energy piles buried with double spiral coils in series is better,and the additional temperature load of the pile body is the minimum,while the additional temperature load of the W type buried pipe is the maximum.Then,the influence of inlet velocity,coil pitch,pile length and tip constraint on heat transfer performance and load bearing performance of energy pile buried with double spiral coils in series was investigated as an example.The results show that the velocity,pitch and pile length have important influence on the heat transfer performance,but the influence on the bearing characteristics of the pile is only reflected in the numerical value,and the pile top and pile end constraints will change the distribution of the axial force,temperature-induced loading and the displacement of the pile section.Finally,combined with the intermittent operation characteristics of ground source heat pump,three on-off ratios(1:2,1:1 and 2:1)were selected to simulate the multiple thermal or cold cycles of the spiral buried pipe energy pile with a period of 24 h within 5 days,so as to explore the heat transfer performance and settlement accumulation during intermittent operation.The results show that compared with the continuous operation mode,the intermittent operation mode can better alleviate the pile soil heat accumulation,and the heat transfer effect is significantly improved.With the increase of the stop-operation ratio,the phenomenon of heat accumulation becomes more and more serious,the heat transfer effect decreases,and the settlement accumulation of pile top becomes larger and larger.Although the settlement accumulation of pile top under 5 times of heat cycle or cold cycle is small,it is worthy of further study whether the settlement accumulation of pile top settlement will affect the safety of pile foundation under long-term operation of energy pile.