同轴开式地热单井换热器性能分析

文章针对具有开式结构的深井换热器(Open-loop Borehole Heat Exchanger,OBHE)建立了数值模型,研究了其在建筑供暖利用方面的性能表现。将OBHE与闭环结构的深井换热器(Deep Borehole Heat Exchanger,DBHE)进行了对比。分析了OBHE在单个以及多个供暖季的性能表现,研究了包括井径、含水层渗透率、岩石导热系数在内的因素对OBHE换热性能的影响。结果表明:两种形式的地热单井的运行特性相似,单井的换热功率以及出口水温都随着时间逐渐下降;OBHE在第1和第10个供暖季的平均换热功率分别为485.2 kW和451.3 k W,而DBHE则分别为431.2 kW和391.1 kW,OBHE的换热能力比DBHE更好;OBHE的换热功率和出口温度随着孔径、岩石导热系数、含水层渗透率的增大而增大。     

基于金属埋管含水层换热效率的物理模型试验

考虑到浅埋含水层渗透性高和热扩散能力强的特点,提出一种基于金属埋管的新型高效含水层换热技术,以实现地热能高效开发。通过室内模型试验研究金属埋管在不同渗透系数岩土材料中的换热特性以及温度响应特征。结果表明,加热功率恒定为80 W时,金属埋管的能效系数随岩土介质渗透系数的增大而增大,渗透系数为1.04×10^-3m/s的砾石能效系数较黏土提升72.31%,表明该技术在含水层具有极大的适用性。地温监测显示含水层渗透系数越大,其温度场温升速率越低,表明换热系统具有更好的换热可持续性。此外,加热功率对金属埋管能效系数影响较大,加热功率为100 W的换热器能效系数较50 W时提升73.08%,表明热激发对流换热效应在高负荷下更为显著。     

竖直埋管地热换热器钻孔内的传热分析

准三维模型为竖直埋管地热换热器的结构优化提供了较为精确的理论基础。利用准三维模型对竖直埋管地热换热器进行分析与研究得出，不同的行程布置对双U型埋管地热换热器的传热性能有较大影响。就钻孔内热阻的对比，双U型埋管比单U型埋管钻孔内的热阻低，因而双U型埋管地热换热器较单U型埋管地热换热器更为合理。     

地下水渗流条件下埋地换热器传热性能的实验研究

建立了根据地温分布确定地下水流动特性的简化数学模型。针对一个实际工程钻孔进行了地温分布测量,并测试了局部地下水渗流条件下埋地换热器的传热性能。结果表明,上述理论模型具有较好的预测精度。在该文实验条件下,地下水渗流对埋地换热器的传热性能具有明显的强化作用,埋地换热器的换热量平均提高11.35%。     

深层地埋管换热器换热性能模拟及稳定性研究

以深层地源热泵地埋管换热器为研究对象,对其换热特性进行数值模拟和实验研究。建立考虑轴向地温梯度的深层地埋管换热器传热模型并进行模拟计算,通过示范工程现场测试数据验证该模型的正确性。对深层地埋管换热器换热的性能稳定性进行研究,发现深层地埋管换热器连续长期运行及间歇长期运行下换热性能基本稳定。当按不同运停比运行时,岩土温度恢复效果良好。研究结果表明深层地源热泵有较好的换热性能及运行稳定性,为深层地热能的开发利用提供了新思路。     

土壤源地热换热器的传热性能分析及优化设计研究

文中对垂直U型埋管土壤源地热换热器的传热性能进行了分析,并优化设计计算了不同负荷下地热换热器的长度.首先根据土壤源地热换热器的传热性能分析,在满足工程实践的基础上,选择了IGSHPA模型简化下的传热分析方法计算传热热阻;然后利用一维导热和线热源模型,得到流体至管道内壁的对流换热热阻,塑料管壁的导热热阻,钻孔内部的导热热...     

连续与间歇运行工况下地埋管换热器的换热特性研究

为了研究地埋管换热器在变负荷下连续运行与间歇运行的换热性能,基于有限长线热源渗流模型建立竖直地埋管钻孔外准三维非稳态传热模型,应用叠加原理计算钻孔群中钻孔壁温度场及地埋管内流体温度场。在存在地下水水平渗流的情况下,研究了变负荷连续运行模式下不同渗流速度、不同运行工况及不同钻孔位置对地埋管换热器的换热性能的影响。结果表明,在连续运行模式下,边缘位置的钻孔及较大的渗流速度能够增强地埋管换热器的换热性能;在间歇运行模式下,地埋管换热器的运行份额越小,其制冷效果越好,反之,制冷效果越差;在间歇运行工况下,土壤温度能在系统间歇期内得到一定程度的恢复,从而更好地提高地热能的利用率。     

紧邻的双井中深层套管式地埋管换热器换热特性研究北大核心CSCD

目前中深层地热能受到了广泛的关注,文章基于有限差分法建立双管中深层换热器数值模型,并通过FLUENT软件对模型进行校验。利用双管模型研究了埋管间距、岩土热物性等因素对双管换热器换热性能的影响。计算结果表明:增大埋管间距可以有效缓解地下岩土冷量堆积的现象,埋管间距较小时,埋管中心点处温度更低;当埋管间距大于50 m时,双管换热器的换热特性与单管近似;当埋管深度增加时,可以适当减小埋管间距;岩土导热系数较大时,需要加大埋管间距;岩土体积比热对换热器影响较大,实际工程中需要根据岩土体积比热大小调整埋管间距。     

地下水渗流与地源热泵热量运移耦合模拟

为准确模拟不同地下水径流条件下地源热泵系统地埋管换热区地温场变化特征、系统换热性能变化规律以及由此而产生的热量堆积效应等问题,以同济大学生态园试验场地为研究对象,采用理论分析及数值模拟计算为主要手段,建立了水流、介质、热量三场耦合的数值模拟模型.结果表明:随着地下水渗流速度的增大,热量逐渐向地下水流动方向偏移,粉砂层扩...     

基于表皮效应的单井循环地下换热系统地下水渗流方程的统一分析

单井循环地下换热系统有循环单井、抽灌同井和填砾抽灌同井3种形式。由于上述3种热源井结构不同,导致地下水渗流数学模型繁多、复杂,并且缺乏统一的理论公式,难以应用于实际工程。为解决上述问题,文章利用表皮效应对渗透系数进行统一,获得3种热源井井壁处渗透系数转换关系式。文章以填砾抽灌同井为例,建立地下水渗流方程和边界条件,推导降深方程,并利用渗透系数转换关系式得到其它热源井的降深方程。研究结果表明:填砾抽灌同井地下水渗流方程的解析解与数值解之间的平均误差为2.0%;填砾抽灌同井地下水渗流方程可以解决填砾抽灌同井影响下的稳态、无越流承压含水层的地下水流动问题,并为实际工程应用提供理论基础。     

套管式中深层地埋管换热器传热建模及取热分析

为了研究无干扰换热条件下,中深层地热能的实际取热性能,文章通过数值模拟方法模拟计算了套管式中深层地埋管换热器的名义取热量。模拟结果表明,套管式中深层地埋管换热器的名义取热量随着钻孔深度、大地热流、循环水流量、当地大气年平均温度的增加而增加。套管式中深层地埋管换热器周围土层的地质条件分布也影响着中深层地埋管换热器的名义取热量,具体表现为浅层土层的导热系数越小,中深层地埋管换热器的名义取热量越大;深层土层的导热系数越大,中深层地埋管换热器的名义取热量也越大。通过调整地埋管换热器的相关参数,并选择合适的地埋管埋设地点等优化措施,可使套管式中深层地埋管换热器达到可观的名义取热量。     

夏季间歇运行工况下相变温度对相变回填地埋管换热器传热性能的影响

为探究相变温度对相变材料回填地埋管换热器传热性能的影响,建立管内流体换热、回填区域相变换热及土壤换热的三维耦合传热数值模型,利用焓-多孔介质模型对相变区域相变问题进行处理,研究夏季间歇运行工况下不同相变温度回填材料对埋管换热器传热性能的影响。结果表明:添加PCM,可有效提高换热量,短期内缓解埋管周围热积聚,利用相变温度18℃的PCM回填,单位井深换热量至少比普通材料回填提高49.54%;在间歇运行初期,换热量随相变温度的升高逐渐减小,低相变温度的PCM可明显改善埋管换热量,但随着时间的进行,较高相变温度PCM回填对换热器换热量的改善效果优于前期低相变温度。此外,在运行期间,不同相变温度的PCM表现出不同的熔化、凝固特性,当PCM的熔化、凝固过程交替进行时,可减缓土壤温度在运行期间内波动幅度。     

Numerical study on the effects of design parameters on the heat transfer performance of coaxial deep borehole heat exchanger

Deep borehole heat exchanger (DBHE) is attracting attention intensively owing to much more geothermal extraction, higher efficiency for heat pumps, and lesser land demand compared with shallow borehole heat exchanger. DBHE is usually dipped into several thousand meters in the subsurface, having a complicated heat transfer with surrounding rock–soil. However, the heat transfer characteristics below surface under different conditions are rarely studied. In this study, a numerical model considering the comprehensive effects of geothermal gradients and heat loss from inner pipe was proposed. The model was validated with experimental data and Beier analytical solution. Based on the model, the effects of primary design parameters on the heat transfer performance below surface along the pipe were investigated. The results indicate that temperature at pipe bottom increases with inlet flow rate decreasing, while the heat load cannot be extracted fully to the surface because of the heat loss of inner pipe. When the inlet flow rates decrease from 41.39 to 4.52 m³/h, the heat loss ratio increases from 25.5% to 63.7%. It is an effective way of insulating inner pipe to reduce heat loss under low inlet flow rates. Increasing the velocity in inner pipe by lessening the inner pipe diameter can also decline the heat loss well. While by this way, the increasing pumping power resulting from the higher velocity in inner pipe has to be considered. This study is significant to effective optimization of DBHE and energy conservation of buildings.     

中深层地热Ｕ型井地埋管换热器的试验研究

地热能作为一种非碳基能源,具有储量丰富、清洁可再生等特点,开发利用地热能有助于碳达峰的实现。在中深层地源热泵领域,我国主要以单井同轴管为主,而相对高效的中深层地热U型井地埋管案例屈指可数。为了了解中深层地热U型井地埋管换热性能及井下换热参数变化,完成了新型的Ｕ型井地埋管换热器工程,并在此基础上进行了实验研究。首先,开展了地温测量,确定了研究区的地层温度,根据热储的物性条件选取了水平井段及对接位置;其次,分析空载循环试验工况下循环水的流量及井下温度的变化情况,研究了负载工况下供回水温度、流量、换热量、不同井段对换热的贡献率、井下温度的动态变化、U型井的恢复能力等因素。实验结果表明,中深层Ｕ型井地埋管换热器井底温度会随运行时间增长而降低,流量大且回水温度较低的情况下,换热器的换热量比较高,最高为1336.8kW;回水井对换热量的增加有限,每百米增加0.12℃,实际工程中可以考虑减小口径,降低建设费用。U型井地埋管换热器的地温恢复能力较强,停止运行24h左右井底温度与初始温度差为-13℃。研究结果有助于研究人员对中深层Ｕ型井地埋管换热器有更进一步的认识,从而推动中深层地热能的健康可持续发展。     

Back‐analyses of in‐situ thermal response test (TRT) for evaluating ground thermal conductivity

In this study, a series of computational fluid dynamics (CFD) numerical analyses was performed in order to evaluate the performance of six full‐scale closed‐loop vertical ground heat exchangers constructed in a test bed located in Wonju, South Korea. The high‐density polyethylene pipe, borehole grouting and surrounding ground formation were modeled using FLUENT, a finite‐volume method program, for analyzing the heat transfer process of the system. Two user‐defined functions accounting for the difference in the temperatures of the circulating inflow and outflow fluid and the variation of the surrounding ground temperature with depth were adopted in the FLUENT model. The relevant thermal properties of materials measured in laboratory were used in the numerical analyses to compare the thermal efficiency of various types of the heat exchangers installed in the test bed. The numerical simulations provide verification for the in‐situ thermal response test (TRT) results. The numerical analysis with the ground thermal conductivity of 4.0 W/m?K yielded by the back‐analysis was in better agreement with the in‐situ TRT result than with the ground thermal conductivity of 3.0 W/m?K. From the results of CFD back‐analyses, the effective thermal conductivities estimated from both the in‐situ TRT and numerical analysis are smaller than the ground thermal conductivity (=4.0 W/m?K) that is input in the numerical model because of the intrinsic limitation of the line source model that simplifies a borehole assemblage as an infinitely long line source in the homogeneous material. However, the discrepancy between the ground thermal conductivity and the effective thermal conductivity from the in‐situ TRT decreases when borehole resistance decreases with a new three pipe‐type heat exchanger leads to less thermal interference between the inlet and outlet pipes than the conventional U‐loop type heat exchanger. Copyright © 2012 John Wiley & Sons, Ltd.     

Numerical and experimental study of transient conjugate heat transfer in helical closed-loop geothermal heat exchangers for application of thermal energy storage in backfilled mine stopes

The geothermal potential available from deep underground mines has yet to be utilized. However, stope-coupled heat exchangers (SCHE) are aiming to take advantage of the unused low-grade geothermal energy. Backfilled stopes provide a unique opportunity to install nonlinear heat exchangers, as the geometry is not limited to the shape of a borehole. Helical pipes deliver superior fluid mixing and heat exchange compared to straight pipes, due to the effect of the secondary flow within the helical pipe. The helical closed-loop geothermal heat exchanger enables the backfilled stopes of the mine to be repurposed as thermal energy storage units. This article delves into the experimental results from a unique state-of-the-art laboratory scale helical closed-loop heat exchanger with varying thermophysical parameters. Additionally, a novel conjugate numerical model is developed and its results are validated against the base case of the experimental studies. Additionally, the numerical model is validated in a spatial-temporal sense with thermocouple data from the experimental rig. The numerical model is also applied to a helical SCHE situated within a backfilled stope for the first time. The results of the numerical model suggest that the pumping rate through the SCHE has a significant effect on the heat exchange rate and the overall energy transfer between the SCHE and the backfill. Additionally, the temperature contours from the numerical model suggest that a decreased pitch/helical diameter will increase the storage capacity of the helical SCHE. Overall, an average of 2.5 MW can be stored over the first 4 days of geothermal charging with the investigated full-scale SCHE, boasting a pseudo-steady-state storage rate of 1.7 MW.     

地源热泵竖直埋管的有限长线热源模型

对地热换热器竖直埋管的非稳态传热模型进行了分析讨论。采用虚拟热源和格林函数法给出了半无限大介质中有限长线热源产生的非稳态温度场的解析解表达式。与稳态温度场的解进行比较,讨论了温度场达到名义上的“稳态”所需的时间,同时对于达到稳态时的温度场也进行了分析,指出了现行教科书中关于该问题的错误,提出了稳态时两个地热换热器孔壁代表性温度的定义,并对两者进行了比较,进而给出了可供工程应用的简化计算公式,并对两者进行了比较,进而给出了可供工程应用的简化计算公式。基于以上分析,进一步讨论了全年冷热负荷不平衡对地热换热器长期性能的影响。     

中深层地热单井循环系统传热强化方法研究

针对中深层地热单井循环系统井内热贯通导致的换热功率低的问题,提出一种内管末端变径的井下传热强化方法,并建立数值模型,利用FLUENT进行为期30 d的模拟计算。结果表明,采用内管末端变径的方式能有效增强地下水“互动”,充分利用含水层的高温来提高单井换热功率。将井下换热分为导热区和采灌区两部分,随着封堵比例的增加,抽水中的含水层补给占比增加,且采灌区换热功率在系统换热功率中的占比逐渐增加。当封堵比例增大到100%时,采灌区换热功率达到导热区的1.76倍,井口出水温度可基本稳定在58℃,系统换热功率稳定在约995.46 kW,相较于内管等径系统,换热功率可提高84.71%。同时,单井循环系统仅导热区的延米换热量就可达到154.23～216.89 W/m,超过了闭式同轴套管换热系统稳定运行的最高延米换热功率,而系统换热功率可达到闭式系统的3.57～6.60倍,在单井换热系统中具有显著优势。