地源热泵桩基螺旋埋管换热器传热特性及其对桩基结构性能影响的研究

A pile heat exchanger (PHE) which utilizes the building foundation piles as part of the ground heat exchanger (GHE), becomes promising in the geothermal heat pump industry recently. The heat transfer process of the PHE with buried spiral coils is complex and quite different from the conventional vertical GHE. Inevitably,it is noteworthy that the continuous heat rejection/extraction of the PHE to/from the piles will cause significant temperature variations of piles and the surrounding soil, which can influence the mechanical behavior of the pile foundation.This project will investigate the 3-D transient heat transfer performance of the PHE by means of the theoretical, numerical and experimental methods, and propose a design method of the novel pile heat exchanger for engineering. To properly estimate the thermal effects of PHE on the pile foundation caused by temperature changes, this project will investigate the thermo-mechanical behavior of HE pile through a series of numerical analysis and experiments, on the bases of the results from the heat transfer simulation models.The stress variations of the pile with the temperature changes of the pile and sourrounding soil will be studied under the geothermal heat pump system operating conditions. Geotechnical design method of this new type of pile foundation will be proposed accordingly. Based on the thermal and thermo-mechanical analysis, design guideline will be developed to promote the applications of this new technology in building construction industry.

桩基埋管换热器是将换热管直接埋设于建筑物桩基内为地源热泵系统提供冷热源的一种新型地埋管换热器。桩内螺旋埋管与桩土间的复杂传热过程会对桩基力学性能产生一定影响。本项目拟采用理论分析、数值模拟与实验测试相结合的方法来研究桩基埋管换热器的传热与力学性能。首先分析埋管桩及周围岩土内的三维瞬态传热机理；得到桩基螺旋埋管换热器的非稳态温度场的解析解；并提出传热计算方法。进而从学科交叉角度，建立合理的热-结构耦合模型并模拟热泵系统长期运行条件下桩基及周围土壤内的受力状态的变化。通过理论分析和实验研究，揭示在热泵系统周期性冷热负荷作用下，桩截面内的应力变化规律与桩体可能产生的沉降与变形；探讨土壤周期性换热的温度变化对桩基承载力的影响以及埋管在受力条件下自身的安全性，得出内置埋管换热桩的力学性能变化规律，提出工程上适用的换热桩的力学设计方法和准则。研究成果将为桩基埋管换热器的推广应用提供理论依据与技术指导。

桩内螺旋埋管与桩土间的复杂传热过程会对桩基力学性能产生一定影响。本项目采用理论分析、数值模拟与实验测试相结合的方法研究了桩基埋管换热器的传热与力学性能。对埋管桩及周围岩土内的三维瞬态传热机理进行了深入的研究；得到了桩基螺旋埋管换热器的非稳态温度场的解析解；并提出传热计算方法。分析探讨了桩-土非均一热物性及地下水渗流对整个换热器的传热性能的影响；研究发现混凝土与土壤的导热系数与导温系数的比值是影响桩基埋管换热的重要参数；同时渗流速度是决定地下水流动对换热过程影响程度的重要因素。对桩内内置热负荷对混凝土-土界面的受力影响进行了探索研究。研究结果表明桩土温度的改变会引起混凝土-土界面性质的变化，从而影响其力学性质表现。