【作者】 赵强；

【导师】 陈宝明；

【作者基本信息】 山东大学 ， 动力工程及工程热物理， 2018， 博士

【摘要】 地埋管地源热泵技术是浅层地热能利用中一种主要形式,被誉为20世纪最为节能环保的供热空调技术。对使用桩基础和地源热泵的建筑物,使用能量桩换热器取代一部分钻孔换热器,是浅层地热能工程应用的一个趋势。螺旋埋管能量桩换热器作为一种新型的地埋管换热器近年来在能量桩的工程中逐渐得到应用,是能量桩地源热泵技术的研究热点之一。本文采用理论分析、数值模拟和实验研究相结合的方法,对螺旋埋管能量桩换热器的传热特性进行了系统研究。首先,搭建了螺旋埋管能量桩换热器模型化实验台,实施了螺旋埋管能量桩换热器不同工况的热响应实验,获得了热流负荷、温度响应和等效热阻等传热特性参数的变化规律。通过大量高精度温度探头的布置和恒定热功率放热实验获得了螺旋埋管能量桩换热器的热流负荷、循环水温度、换热管壁温、能量桩桩壁温度和岩土温度的变化规律,进一步得到了螺旋埋管能量桩换热器的等效总热阻和管中热阻、桩身热阻、岩土热阻等等效子热阻,分析了加热功率、回填介质和岩土介质热物性对螺旋埋管能量桩换热器的热流负荷、温度响应和等效热阻的影响。在恒定热功率下,能量桩换热器的热流负荷首先迅速上升,逐渐达到一个相对稳定值,然后随电压波动;在脉冲热功率下,加热器启动阶段热流负荷迅速增长;加热器停止时,热流负荷达到最大值;加热器停止后,热流负荷迅速降低,然后逐渐减少至最小值。管中等效热阻达准稳态时比等效总热阻小1-2个数量级;桩身等效热阻比等效总热阻小0.5-1个数量级;短时间运行时,桩身等效热阻在等效总热阻中占据最大份额;长时间运行时,岩土等效热阻在等效总热阻中占据最大份额。提高回填介质的导热系数和体积比热可使得桩身等效热阻、岩土等效热阻和等效总热阻有效降低,改善回填介质的热物理性质可有效提高能量桩换热器的传热效率;高导热率和高体积比热的岩土可使桩身等效热阻、岩土等效热阻和等效总热阻有效降低。其次,建立了可代替三维模型的螺旋埋管能量桩换热器的二维瞬态轴对称环管热源模型,分析了设计因素对螺旋埋管能量桩换热器短期等效热阻的影响。通过换热管管壁的热通量边界条件,环管热源模型将能量桩换热器的固体域和流体域联立实现耦合求解,仿真结果可同时提供各匝流体、管壁、桩身和周围岩土的温度响应。环管热源模型的计算速度很快,具备解决工程技术问题的实用性。环管热源模型的计算结果与三维数值模型对比分析表明,环管热源模型的模拟结果保持了三维传热模型的基本传热特性;提出以管长相对比的概念来评估二维数值模型的计算精度,在一定条件下环管热源模型可替代三维模型使用。利用环管热源模型分析了设计参数对螺旋埋管能量桩换热器短期等效热阻的影响,包括螺旋管的长径比、螺旋管的曲率、流体的雷诺数,以及回填介质与周围岩土的相对热扩散率。螺旋埋管能量桩换热器的等效总热阻随螺旋管长径比的减小而减小,小长径比有助于提高能量桩的传热性能;桩身、管壁和流体三项等效子热阻随螺旋管曲率的增加而减小,原因应是曲率的增加导致管道传热面积和工作流体热容量增加;桩身等效热阻随着回填材料热扩散率的增大而减小,当回填介质的热扩散率小于周围岩土的热扩散率时,提高回填介质的热扩散率可有效降低能量桩换热器的桩身等效热阻和等效总热阻;螺旋埋管能量桩换热器的等效总热阻随循环流体雷诺数的增加仅略有下降,提高管内循环流体的对流换热强度对提高螺旋埋管能量桩换热器的传热性能作用不大。然后,基于三维数值传热模型的能量桩温度响应精确计算,分析了埋管类型和螺距对能量桩传热特性的影响,给出了螺旋埋管能量桩换热器循环流体平均温度的G函数,分析了螺距对热泵COP的影响。考虑了循环流体的轴向传热,基于三维固体导热模型和一维非等温管道流传热模型建立了能量桩换热器的三维瞬态数值传热复合模型。三维数值传热模型在桩身传热、岩土传热和流体传热三方面均比较完好的保持了能量桩换热器的三维传热特征。由于将管中流体和管壁传热作为一维模型进行处理,在保证换热管轴向热量传递的条件下,所建立的三维数值传热几何模型减少了计算网格数目,提高了计算资源的利用效率;采用线圈热源模型的计算结果验证了三维数值传热模型的长期岩土温度响应,采用现场热响应实验验证了三维数值传热模型的流体短时温度响应,结果表明本文所建立的三维瞬态数值传热模型是有效且可行的。其次,建立了 U形管、W形管和螺旋埋管这三种不同埋管类型能量桩换热器的几何模型,采用三维数值传热模型进行求解,获得了恒热流工况下U形管、W形管能量桩换热器和四种不同螺距的螺旋埋管能量桩换热器的等效热阻、温度响应和桩壁面传热率;先对比分析了 U形管、W形管和螺旋埋管换热器恒热流工况下对能量桩换热器传热特性的影响;又对比分析了恒热流条件下螺距对螺旋埋管能量桩换热器传热特性的影响。结果表明对于形状和回填介质一致的能量桩,不同几何形式的换热管对能量桩和岩土之间的传热率没有显著的影响,但是较长管路和较大换热面积导致换热管中循环流体的热容量较大、温变较小,这对提高地源热泵系统的能效比是有利的;即换热管的管长和换热面积较大的影响了循环水的温度响应、桩身的温度均匀度和桩身的局部传热率,但是对桩身总传热率和桩身平均温度几乎没有影响。基于数值计算结果,拟合给出了四种螺距螺旋埋管换热器循环流体温度响应的G函数,并基于叠加原理和G函数法建立了热泵COP的计算方法,计算了四种螺距在不同影响因素下的热泵COP,分析了螺旋埋管能量桩换热器的螺距在不同影响因素下对热泵COP的影响规律。螺距小的螺旋埋管能量桩换热器传热效率高、适用性较好;螺距小的能量桩换热器允许采用低浓度的防冻液作为循环流体,对减小能量桩换热器循环流体的温变和提高系统能效比有益;在不同影响因素(岩土初始温度、负荷比、冷热负荷比和热冷负荷比)条件下,本文案例中相对螺距(SP=0.25,0.5,1.0,2.0)的增加导致热泵平均 COP 降低 0.77%-16.49%。最后,以济南市某大型地源热泵复合系统为依托,建立了全尺度螺旋埋管能量桩-钻孔换热器的现场热响应实验台,采用恒定热功率法分别对钻孔换热器和能量桩换热器实施了恒定功率热响应实验,又对能量桩-钻孔换热器复合系统实施了供热季运行热响应实验,对比研究了螺旋埋管能量桩换热器和双U形埋管钻孔换热器的恒定功率和运行热响应特性,分析了等效热阻、流体温度、传热率、热泵COP和管周混凝土温度的变化规律。结果表明:大径向尺寸的热源和高热物理性质的回填介质对提高地埋管换热器的热响应速率有利;螺旋埋管能量桩换热器的传热性能远好于双U型钻孔换热器;小长径比的换热器和每米深度内较长的换热管有助于提高地埋管换热器的换热效率;螺旋埋管能量桩换热器对负荷的适应能力强于双U形埋管钻孔换热器;螺旋埋管能量桩换热器的瞬态等效热阻远小于双U形埋管钻孔换热器。螺旋埋管能量桩换热器的热响应速率要远快于双U形埋管钻孔换热器。另外,供热季运行实验中,钻孔换热器和能量桩换热器的循环水温度和延米传热率随供热运行呈现波动趋势;流体温度和延米传热率的振幅和波动频率可能决定于热泵机组的制热量和建筑物的热负荷。流体温度和传热速率的幅值和工作频率应受热泵控制参数的制约。本文实验条件下,恒定功率热响应实验结果表明,双U形埋管钻孔换热器的平均等效热阻约为螺旋埋管能量桩换热器的5.65倍;能量桩换热器的等效热阻减小率的变化速率约为钻孔换热器的9倍。本文工程案例的冬季供热季运行热响应实验结果表明,有效深度为27米的能量桩换热器提取的热量比有效深度为90米的钻孔换热器提取的热量略高;在准稳态运行期间,单位长度能量桩换热器的峰值传热率约为钻孔换热器的3.65倍,而能量桩换热器的每米管长传热率(dn25)约为钻孔换热器(dn32)的1.32倍。螺旋埋管能量桩换热器的管周混凝土温度随热泵取热负荷的减小而增大,随热泵取热负荷的增大而减小。供热运行期间,管周混凝土的最低温度比初始土壤温度低5.05℃,供热结束时管周混凝土温度相比初始温度降低3.24℃。在运行后期的间歇性运行期间,经过一夜的恢复期,管周混凝土温度可恢复0.65℃左右。当供热期结束,再经过15天的恢复期,管周混凝土温度只比初始温度降低 1.7℃。本文采用实验方法定量获得了螺旋埋管能量桩换热器的等效总热阻和等效子热阻的变化规律,为数值传热模型的简化和传热特性的分析提供了依据。建立的二维瞬态轴对称环管热源模型准确度高、实用性强,为螺旋埋管能量桩换热器的技术分析和工程设计提供了一个实用性工具。拟合给出了螺旋埋管能量桩换热器循环流体平均温度的G函数,优化了能量桩换热器的工程设计方法。获得了螺旋埋管能量桩换热器的恒定功率热响应特性和供热运行热响应特性规律,对能量桩换热器传热模型改进、工程设计和运行策略优化有指导意义。更多还原

【Abstract】 Ground source heat pump technology is a major form for the utilization of shallow geothermal energy.It is known as the most energy-saving and environmentally-friendly for space heating and cooling in the 20th century.Energy piles are an attractive and cost-effective solution to partly replace borehole heat exchangers for buildings with pile foundations and ground-coupled heat pump systems,which is a trend for the engineering applications of shallow geothermal energy.As a new type of ground heat exchangers,spiral-tube heat exchangers in energy pile have been used in energy pile projects in recent years.It is one of the hot spots for energy pile ground source heat pump system.In this work,the research method of combining theoretical analysis,numerical simulation and experimental research is used to systematically study spiral-tube heat exchangers in energy pile.Firstly,a laboratory experimental bench for the heat transfer characteristics of a spiral-tube heat exchanger in energy pile with was built.Based on the thermal response tests by using the constant heating power method,heat flow,fluid temperature,tube wall temperatures,pile wall temperatures and soil temperatures were obtained.Thetemperature response of an energy pile heat exchanger was acquired by the arrangement of a large number of high-precision temperature probes.The total equivalent thermal resistance of energy piles and equivalent sub thermal resistances were obtained through data processing and investigated deeply.At the constant heating power,the heat flow of energy pile first rapidly rises,gradually reaches a relatively stable value,and then fluctuates with the voltage.With the pulsed heating power,the heat flow increases rapidly in the start-up phase of heater;when the heater is stopped,the heat flow reaches a maximum value;after the heater stops,the heat flow decreases rapidly and then gradually decreases to the minimum value.When the fluid-pipe thermal resistance is at quasi-steady state,it is 1-2 orders of magnitude smaller than the total thermal resistance;the pile thermal resistance is 0.5-1 orders of magnitude smaller than the total thermal resistance.When operating is lasting for a short time,the pile thermal resistance occupies the largest share of the total thermal resistance.When operating is lasting for a long time,the soil thermal resistance occupies the largest share of the total thermal resistance.Increasing the thermal conductivity and volumetric specific heat of grout medium can effectively reduce pile thermal resistance,soil thermal resistance and the total thermal resistance.Therefore,improving the thermal physical properties of grout medium can effectively improve the heat transfer efficiency of energy pile heat exchangers.The soil with high thermal conductivity and high volumetric specific heat can effectively reduce pile thermal resistance,soil thermal resistance and total thermal resistance.Secondly,a two-dimensional axisymmetric model(the ring-tube heat source model),which could replace 3D numerical heat transfer model under certain conditions,was developed with the coupled boundary conditions between fluid and pipe and solved by using the finite element method.The simulation by means of the ring-tube heat source model could provide the temperature responses of working fluid,pipe,pile and soil.The fluid temperature comparison between the simulation by using the proposed model and the test data indicates that the ring-tube heat source model is valid and feasible.The comparison between the proposed model and 3D numerical model indicates that the simulation with the ring-tube model maintains the basic heat transfer characteristics of 3D heat transfer model.The equivalent sub thermal resistances were obtained,including fluid thermal resistance,pipe thermal resistance,pile thermal resistance and soil thermal resistance.The soil thermal resistance and the pile thermal resistances increase with time when energy pile is undertaking a heat flux.The pipe thermal resistance and fluid thermal resistances can be considered as stable when Fo>0.002.The stable fluid thermal resistance is 1-2 orders of magnitude smaller than the total thermal resistance when Fo>0.01.The stable pipe thermal resistance is one orders of magnitude smaller than the total thermal resistance when Fo>1.Furthermore,the parametric analysis on the short-term thermal resistances of spiral-rube heat exchangers in energy piles was performed,including the relative height of spiral-tube,the curvature of spiral-tube,Reynolds number of working fluid and the relative thermal diffusivity.Both pile thermal resistance and pipe thermal resistance increases with an increase of the relative height of piles.The large radial scale heat source and small length-to-radius ratio helps to improve the heat transfer performance of energy piles.The pile,pipe and fluid thermal resistance decrease with an increase of the curvature of spiral-tube.It should be due to that an increase of curvature lead to an increase of both the pipe heat transfer area and heat capacity amount of working fluid.The pile thermal resistance decreases with an increase of grout thermal diffusivity.Improving the thermal diffusivity of backfilled materials can significantly reduce the pile thermal resistance and total thermal resistance for energy piles,especially when the thermal diffusivity of backfilled materials is less than that of soil.The total thermal resistance only decreases slightly with an increase of Reynolds numbers of working fluid.Enhancing the convective heat transfer of working fluid in pipes helps little for improving heat transfer performance of energy pile.Thirdly,based on the accurate temperature response of energy pile heat exchangers by using transient 3D heat transfer model,the influence of the type or the pitch of heat exchange tube on the heat transfer charactiritics of energy piles.G-functions of average fuid temperagure are fitted from the simulation and used to calculate COPs of heat pump.The influence of spiral pitch was analyzed under different design conditions.A transient 3D heat transfer model was established in combination with heat conduction model in solids and non-isothermal pipe flow model.The long-term temperature response of the 3D model is verificated with the ring-coil heat source model while the short-term temperature of the 3D model is verificated with thermal response test.An in-situ test has been performed so as to obtain temperature response profiles of a spiral-tube energy pile heat exchanger.The thermal behavior in the temperature response test have been predicted by using the established model.The simulated results are in good agreement with the experimental results.A comparative analysis of heat transfer processes for three types of heat exchangers(U-tube,W-tube and spiral-tube)in foundation piles was conducted.The changes in temperature and equivalent thermal resistances as well as the heat transfer rate on the surface of energy piles were analyzed so as to conduct a thermal performance comparison.Numerical results show firstly that the average external thermal resistance almost the same and is independent of tube types of energy piles,and secondly that the spiral-tube heat exchanger gives the lowest temperatures in working fluid and the minimum internal thermal resistances under the same initial conditions and boundary conditions.In particular,the exhausted heat between the energy piles and the ground is not much difference for the three heat exchangers.In addition,the change distributions in temperature in pile for the spiral-tube heat exchanger are uniformly distributed compared with the other two heat exchangers.Therefore,the spiral-tube heat changer gives the better thermal performance than the other two heat exchangers in the context of long-term and short-term thermal load due to the longer pipeline,larger heat capacity of working fluid and geometrical arrangement form.Then the 3D numerical model was used to study the influence of spiral pitch on the thermal behaviors of a spiral-tube heat exchanger in energy piles.The heat transfer process of spiral-tube energy piles with four kinds of pitch(SP=0.25,0.5,1.0,2.0)was simulated and analyzed.The total thermal resistance and the sub thermal resistances were obtained for the four piles.The change in fluid temperature and internal thermal resistance diminishes but the change is narrowed down as spiral pitch decreases.The total heat transfer rate on the surfaces of piles is almost same for the four piles,and the remained heat in both the pile body and working fluid should be almost as much.However,the spiral-tube with a small pitch could provide a better uniform distribution for both the temperature and heat flux in pile.The G-functions of fluid temperature(gf)for the four piles were proposed,and the fluid temperature and COP were simulated based on superposition method and G-functions.As spiral pitch decreases,the difference between the extreme fluid temperatures diminishes and the mean COP increases for both heating and cooling.A spiral-tube energy pile with a small pitch is energy-efficient and has a better applicability.An increase of spiral pitch(SP=0.25,0.5,1.0,2.0 sequentially)results in a reduction of the mean COPs from 0.77%to 16.49%according to the influential factors(the undisturbed ground temperature and the different air conditioning loads).A decrease of spiral pitch could improve the energy efficiency and the applicability of a ground-coupled heat pump system by taking water or anti-freeze fluid with a low concentration to be the working fluid.Finally,based on a large-scale ground source heat pump system in Jinan,an on-site thermal response test bench for full-scale energy piles and borehole heat exchangers was established.The thermal behavior of ground heat exchangers under contant heating poower and operation is meaningful for system design and energy efficient of GCHP systems.Four thermal performance tests by means of the constant heating power method were conducted to compare the thermal behavior of spiral-tube energy piles and double U-tube borehole heat exchangers under constant heating power.An operation test for space heating were carried out based on the ground-coupled heat pump system to compare the operation thermal behavior of spiral-tube energy piles and double U-tube borehole heat exchangers.Equivalent thermal resistances,fluid temperatures,heat transfer rates,COP and concrete temperature were analyzed.Equivalent thermal resistances of spiral-tube energy piles and double U-tube borehole heat exchangers were derived from the test data of thermal performance tests.The equivalent thermal resistance of spiral-tube energy piles is much less than that of double U-tube borehole heat exchangers.The thermal response of spiral-tube energy piles is more rapid than that of double U-tube borehole heat exchangers.The heat transfer performance of spiral-tube energy pile is much superior to that of double U-tube borehole heat exchangers.A larger radial scale of heat source,a small aspect of length to diameter and a long heat exchange tube is helpful to improve the heat transfer efficiency of ground heat exchangers.A fluctuating heat transfer rate and fluid temperature with variable amplitude for ground heat exchangers were observed from the operation test.The amplitudes and operating frequency of fluid temperature and heat transfer rate should be determined by the heat capacity of heat pump and thermal loads of building.The amplitudes and operating frequency of fluid temperature and heat transfer rate should be affected by the control parameter of heat pump.For the case in this work,the average equivalent thermal resistances of double U-tube borehole heat exchangers are about 5.65 times as much as that of spiral-tube energy piles.The heat extracted by the spiral-tube energy piles over the average depth of 27 m is a little higher than that by the double U-type borehole heat exchangers over the average depth of 90 m.Between the periods of pseudo steady status for space heating,the peak heat transfer rate per unit meter length of energy piles is about 3.65 times as much as that of borehole heat exchangers,while the peak heat transfer rate per meter length of pipe for energy piles(dn25)is about 1.32 times as much as that for borehole heat exchanger s(dn32).The concrete temperature for energy piles increases gradually while thermal load becomes small.The minimum concrete temperature is 5.05 ℃ lower than the initial soil temperature;when heating is over,the concrete temperature is reduced of 3.24℃.If the energy piles do not work for a night,the concrete temperature could be recovered about 0.65 ℃.After space heating is over and after a 15-days recovery period,the concrete temperature is 1.7 ℃ lower than the initial soil temperature.In this work,the equivalent sub thermal resistance of spiral-tube heat exchangers in energy pile and its evolution law were obtained by experimental means,which provide a theoretical basis for the simplification of numerical heat transfer model and the analysis of heat transfer characteristics.The ring-tube heat source model is accurate and is a practical tool for the analysis of technical problems and engineering design of spiral-tube energy piles.Based on the simulation with 3D heat transfer model,G-function of average temperature of working fluid is proposed which optimize the design method of spiral-tube heat exchangers in energy pile.In addition,the analysis of influences of buried pipe type and spiral pitch and the analysis of influence of pitch on COP of heat pump are meaningful for the engineering design of energy pile heat exchangers.The different time scale thermal response experiments of full-scale energy piles and borehole heat exchangers is significant for the engineering design and operation strategy of energy pile heat exchangers.更多还原