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

doi:10.11949/j.issn.0438-1157.20181470

Abstract

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

摘要：

桩基埋管换热器（能量桩）作为土壤源热泵的末端换热装置同时需承担常规桩基功能。因此，除其换热性能应满足空调供暖负荷需求之外，其间歇交替从周围土壤取、放热所引起的桩基应力变化亦不应危及上部建筑结构的稳定性。为了深刻揭示能量桩的热-力学特征，联合使用Comsol和Abaqus软件建立了并联双U形桩基埋管换热器的三维动态数值仿真模型，利用现场实测结果验证了仿真结果的正确性，分析了桩基内部的动态温度分布、轴力分布以及桩身位移状况；进一步探究了四种不同桩基长径比、流速情况下的双U形埋管以及三种不同埋管形式的桩基换热器的换热性能和力学特征，揭示了出口水温和单位桩深换热量等动态换热性能以及桩身轴力和桩顶位移等力学性能参数的时变特性。结果表明埋管形式和桩基长径比对桩基埋管换热器换热和力学性能的影响较显著，流速的影响较弱；长径比和流速越大，埋管的换热能力越大，但进、出口水温温差也越大，由温度变化所引起的附加桩身轴力、桩顶位移以及侧摩阻力也相应增大。

关键词: 计算流体力学, 热力学, 传热, 实验验证, 数值模拟

CLC Number:

TU 473

Shuang ZHANG, Lei ZHAO, Lin GAO, Hua LIU. Exploration on thermo-mechanical characteristics of energy piles with double-U pipes buried in parallel by means of numerical simulations[J]. CIESC Journal, 2019, 70(5): 1750-1760.

张爽, 赵蕾, 高林, 刘华. 并联双U形桩基埋管换热器热-力学特征的数值仿真研究[J]. 化工学报, 2019, 70(5): 1750-1760.

Figures/Tables 16

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· $m^{- 1} \cdot$ K^-1)	1.92
桩基热膨胀系数/℃^-1	10^-5
管壁热导率/(W· $m^{- 1} \cdot$ K^-1)	0.42
管壁比热容/(J· $k g^{- 1} \cdot$ 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· $k g^{- 1} \cdot$ K^-1)	837
管壁密度/(kg·m^-3)	1100
土壤热导率/(W· $m^{- 1} \cdot$ K^-1)	1.87
土壤比热容/(J· $k g^{- 1} \cdot$ K^-1)	1200
土壤泊松比	0.33
土壤膨胀角/(°)	0

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· $m^{- 1} \cdot$ K^-1)	1.92
桩基热膨胀系数/℃^-1	10^-5
管壁热导率/(W· $m^{- 1} \cdot$ K^-1)	0.42
管壁比热容/(J· $k g^{- 1} \cdot$ 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· $k g^{- 1} \cdot$ K^-1)	837
管壁密度/(kg·m^-3)	1100
土壤热导率/(W· $m^{- 1} \cdot$ K^-1)	1.87
土壤比热容/(J· $k g^{- 1} \cdot$ 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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