CIESC Journal ›› 2019, Vol. 70 ›› Issue (5): 1750-1760.doi: 10.11949/j.issn.0438-1157.20181470

• Fluid dynamics and transport phenomena • Previous Articles     Next Articles

Exploration on thermo-mechanical characteristics of energy piles with double-U pipes buried in parallel by means of numerical simulations

Shuang ZHANG1(),Lei ZHAO1(),Lin GAO1,Hua LIU2   

  1. 1. School of Building Services Science and Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, Shaanxi, China
    2. School of Civil Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, Shaanxi, China
  • Received:2018-12-12 Revised:2019-02-26 Online:2019-05-05 Published:2019-05-10
  • Contact: Lei ZHAO E-mail:944459447@qq.com;leizhao0308@hotmail.com

Abstract:

Pile-based borehole heat exchangers (energy piles) can be used as terminal heat transfer devices for ground source heat pumps, playing the role of conventional pile foundations at the same time. Thus, not only their heat transfer performances must be good enough to meet heating or cooling air conditioning demands, but also the stress changes caused by intermittent heat extraction and release alternatively from and to the soil surroundings should not endanger the stability of building structures above. To reveal the thermo-mechanical characteristics of energy piles sufficiently, the software of Comsol and Abaqus were implemented jointly to establish a three-dimensional dynamic numerical simulation model for an energy pile with double-U pipes buried in parallel. Simulation results were validated by the data obtained during an in-situ test. Dynamic temperature distributions inside pile body, axial force distributions and the displacement of the pile body were analyzed. The heat transfer performances and mechanical characteristics of four energy piles in different ratios of pile length-to-diameter with double-U pipes buried in parallel were studied under the conditions of different water flow rates, as well as those of energy piles with three different forms of buried pipes. The results show that the influence of the form of the buried pipes and that of the length-to-diameter ratio of the pile on their heat transfer and mechanical performance are significant, and the influence of the flow velocity is weak. The larger the length-diameter ratio and the flow velocity, the greater the heat transfer capacity, the larger the temperature difference between the inlet and outlet water. And the additional pile axial forces, pile top displacements and side frictional resistances caused by temperature changes increase as well accordingly.

Key words: computational fluid dynamics, thermodynamics, heat transfer, experimental verification, numerical simulation

CLC Number: 

  • TU 473

Fig.1

Geometry size of calculation domain of energy pile with double-U pipes buried in parallel pipe and surrounding soil simulated"

Fig.2

Heat exchanger in energy pile with double-U pipes buried in parallel and meshes generated interiorly"

Fig.3

Comparison of measured and simulated outlet water temperatures and pile top displacements during 72 h continuous heat rejection process"

Table 1

Related parameters measured at project site"

参数 数值
桩基弹性模量E/GPa 30
桩基热导率/(W· m - 1 · K-1) 1.92
桩基热膨胀系数/℃-1 10-5
管壁热导率/(W· m - 1 · K-1) 0.42
管壁比热容/(J· k g - 1 · K-1) 1465
土壤密度/(kg·m-3) 1930
土壤弹性模量E/GPa 0.015
土壤内摩擦角/(°) 31
桩基泊松比 0.2
桩基密度/(kg·m-3) 2500
桩基比热/(J· k g - 1 · K-1) 837
管壁密度/(kg·m-3) 1100
土壤热导率/(W· m - 1 · K-1) 1.87
土壤比热容/(J· k g - 1 · K-1) 1200
土壤泊松比 0.33
土壤膨胀角/(°) 0

Fig.4

Radial temperature rise of pile foundation at different time and radial temperature profiles at different depth at the 72th hour"

Fig.5

Temperature distributions along pile center, pile circumference and soil boundary at the 72th hour"

Fig.6

Outlet water temperatures and heat exchange rates per pile depth of double-U pipes during continuous 72 h heat rejection process"

Fig.7

Axial forces caused by temperature changes of pile with double-U pipes buried in parallel"

Fig.8

Axial forces at different depth caused by temperature changes of pile with double-U tubes buried in parallel"

Fig.9

Transient side frictional resistances along pile depth caused by temperature variations of pile with double-U pipes buried in parallel"

Fig.10

Transient outlet water temperatures and heat transfer rates in cases of different flow rates"

Fig.11

Distribution of axial force, displacements of pile body and side frictional resistances along pile depth in cases of different flow rates in heat rejection mode at the 72th hour"

Fig.12

Comparison of transient heat transfer performance of piles with different length to diameter ratios"

Fig.13

Axial force of pile body, displacement of pile body and distribution of lateral friction of piles under different length to diameter ratios in heat removal mode at the 72th hour"

Fig.14

Comparison of transient heat transfer performance of energy piles with different forms of pipes buried"

Fig.15

Distribution of axial forces, pile displacements and lateral frictional resistances of piles with different forms of buried pipes operating in heat removal mode at the 72th hour"

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