Numerical simulation on heat transfer performance of vertical U-tube with different borehole fill materials

Heat exchange performance of vertical U-tube heat exchanger was studied with two different borehole fill materials and CFD software. Borehole surface temperature and water temperature distribution were simulated on the condition of continuous operation for 8 h in winter with inlet water temperature being 10℃. The results show that there is no obvious difference on heat exchanger performance between the two different borehole fill materials.

Thermal performance of a single U-tube ground heat exchanger:A parametric study

In this research,the thermal performance of a single U-tube vertical ground heat exchanger is evaluated numerically as a function of the most influential flow parameters,namely,the soil porosity,volumetric heat capacity,and thermal conductivity of the backfill material,inlet volume flow rate,and inlet fluid temperature.The results are discussed in terms of the variations of the heat exchange rate,the effective thermal resistance,and the effectiveness of the ground heat exchanger.They show that the inlet volume flow rate,inlet fluid temperature,and backfill material thermal conductivity have significant effects on the thermal performance of the ground heat exchanger,such that by decreasing the inlet volume flow rate and increasing the backfill material thermal conductivity and inlet fluid temperature,the outlet fluid temperature decreases considerably.On the contrary,the soil porosity and backfill material volumetric heat capacity have negligible effects on the studied ground heat exchanger’s thermal performance.The lowest inlet fluid temperature reaches a the maximum effective thermal resistance of borehole and soil,and consequently the minimum heat transfer rate and effectiveness.Also,multilinear regression analyses are performed to determine the most feasible models able to predict the thermal properties of the single U-tube ground heat exchanger.

The Effect of Injected Air Bubble Size on the Thermal Performance of a Vertical Shell and Helical Coiled Tube Heat Exchanger

In the present study,the effect of injecting air bubble size on the thermal performance of a vertical counter-current shell and coiled tube heat exchanger is experimentally investigated.The experiments were accomplished in a cylindrical shape heat exchanger with a 50 cm height and 15 cm outer diameter.Copper coil with 3.939 m equivalent length and 0.6 cm outer diameter was used to carry the hot fluid(water).Four different cold fluid(shell side)flow rates(Q_(s)=2;4;6 and 8 LPM)Þunder laminar flow conditions(316≤Re≤1223),constant hot(coil side)flow rate fluid rates(Q_(h)=1 LPM),four different injected air flow rates(Q_(a)=0:5;1;1:5 and 2 LPM),invariant temperature difference(ΔT=20°C),and constant bubble’s number(1400)were tested.To demonstrate the effect of bubble size,a sparger with orifice diameters of 0.1,0.8,and 1.5 mm was manufactured and used in the study.The overall heat transfer coefficient(U),NTU,effectiveness,and pressure loss were invested.The experimental results clearly showed that the heat exchanger’s thermal efficiency significantly improved with increasing the shell side flow rate and the injected air flow rate.The maximum improvement in U,NTU,and effectiveness was 153%,153%,and 68%,respectively.The thermal performance of the heat exchanger was shown to be improved with increasing the bubble size.Although the latter finding agrees with recent CFD published results,more studies need to be confirmed.

Numerical simulation for thermal response test performance in closed-loop vertical ground heat exchanger

In this study, a series of numerical analyses was performed in order to evaluate the performance of full-scale closed-loop vertical ground heat exchangers constructed in Wonju, South Korea. The circulating HDPE pipe, borehole and surrounding ground formation were modeled using FLUENT, a finite-volume method (FVM) program, for analyzing the heat transfer process of the ground heat exchanger system. Two user-defined functions (UDFs) accounting for the difference in the temperature of the circulating inflow and outflow fluid and the variation of ground temperature with depth were adopted in the FLUENT modeling. The thermal conductivities of grouts (cement vs. bentonite) measured in laboratory were used as input values in the numerical analyses to compare the thermal efficiency of the cement and bentonite grouts used for installing the closed-loop vertical ground heat exchanger. A series of numerical analyses was carried out to simulate in-situ thermal response tests performed in the construction site. From the comparison between the in-situ thermal response test results and numerical simulations, the average thermal conductivity of the ground formation in the construction site is back-calculated as approximately 4 W/mK. This value can be used in evaluating the long-term performance of the closed-loop vertical ground heat ex changer.

Advances in ground heat exchangers for space heating and cooling:Review and perspectives

As a renewable energy source,geothermal energy has been widely used to provide space heating and cooling for buildings.The thermal performance of ground heat exchanger(GHE)is significant for the operating efficiency of the ground source heat pump(GSHP)systems.This paper presents a comprehensive review of developments and advances of three kinds of GHE,including vertical borehole GHE(VBGHE),Pile GHE(PGHE),and deep borehole GHE(DBGHE)which are currently popular in larger GSHP systems.Firstly,analytical models proposed to ana-lyze heat transfer process of VBGHE with different geological conditions are summarized,such as homogenous or heterogeneous ground,with or without groundwater advection.Numerical and short-time step models and measures to improve GHE thermal performance are also reviewed.Secondly,a summary of research advances in PGHE is provided,which includes the heat transfer models of PGHE,the effects of geometric structure,oper-ation modes,pile spacing,use of phase change material(PCM),thermal properties of PCM,thermo-mechanical behavior and/or thermal performance of PGHE.The effects of groundwater flow direction and velocity on PGHE are also summarized in brief.Lastly,models of three kinds of DBGHEs,i.e.,deep coaxial GHE(DCGHE),deep U-bend GHE(DUGHE)and super-long gravity heat pipe(SLGHP),are reviewed.The physical bases of the dif-ferent analytical models are elaborated and also their advantages and disadvantages are described.Advances in numerical modelling and improving numerical model calculation speed of DCBHE,DCBHE array,and DUBHE are summarized.The review provides a meaningful reference for the further study of GHEs.

Experimental evaluation of a“U”type earth-to-air heat exchanger planned for narrow installation space in warm climatic conditions

The thermal performance of a“U”type earth-to-air heat exchanger is presented in this experimental study.The device has a serial-connected vertical configuration.The wells where tubes were installed have a depth of fewer than 3 m and are separated every 1.5 m,using an installation area of 3m2.The experimentation was carried out in March in Morelos,Mexico when the environmental temperature reaches 35℃ during the day.The performance of the device was measured and compared to the requirements of an office for cooling purposes within a university campus to reproduce the space restrictions found in urbanized areas.By using a small land surface,it is feasible for urbanized areas.The air temperature inside the“U”type earth-to-air heat exchanger,the surrounding soil temperature,the airspeed,and the power consumed by the fan were measured.The air temperature and the fan’s power consumption data were obtained by modifying the airspeed in four constant values,from 1.3 m/s to 6.6 m/s.Results show that the device evaluated in this work has adequate thermal performance for cooling purposes compared to the requirements of an office.A decrease in air temperature was recorded in a range of 5.1℃ to 9.4℃.Over 70%of the total temperature difference was reached in the first well,where the average soil thermal disturbance at 5 cm was 2.8℃.The device achieved a maximum COP of 12.8 and a maximum effectiveness of 88.4%.With these results,it is concluded that the system is suitable for cooling purposes in areas with space restrictions.This work is novel since the dimensions available for installation in urbanized areas are considered and compared with the thermal requirements of an office.In addition to the fact that there are no published works with vertical heat exchangers connected in series.

折流板结构对立式热交换器物理场的影响

目的为分析并比较不同结构的折流板对热交换器温度场、压力场和流场的影响,提高热交换器的换热效果,采用ANSYS FLUENT软件对热交换器进行数值模拟。方法基于某工业园区分布式能源系统,针对系统内的立式热交换器进行数值模拟研究,并根据数值模拟研究结果对热交换器进行结构优化。结果折流板的开口夹角越小,空气出口温度越高,当折流板的开口夹角从180°减小至30°时,空气出口温度提升11.82%,但同时在热交换器出口处空气速度明显升高,导致外筒承受的冲击较大;折流板的间距越小,空气出口温度越高,当折流板的间距从220 mm减小至110 mm,空气出口温度提升14.99%,烟气通过内筒壁面与空气换热接触时间变短,且烟气速度在烟气出口明显上升;折流板的开口面积越小,空气出口温度越高,当折流板外圆直径从2170 mm增大至2770 mm,空气出口温度提升9.22%。结论对立式换热器进行结构优化时,应应适当减小折流板的开口夹角或减小折流板的开口面积,同时减小折流板的间距,使空气能够在热交换器内部进行充分地换热。

超高层建筑空调水系统竖向分区设计研究

结合厦门某超高层办公楼的工程设计实例,综合考虑设备、管道、附件承压及建筑竖向分区等影响因素,对比分析了单冷源站,一次中间换热空调水系统;单冷源站,二次中间换热空调水系统;双冷源站空调水系统这三种空调水系统竖向分区方案的初投资、运行费用及优缺点,得出适合的超高空调水系统竖向分区方案。

桩埋管与井埋管实验与数值模拟

建立了一套组合型地下埋管换热器地源热泵实验系统,针对该系统所采用的不同回填材料U型垂直埋管即沙石回填的U型井埋管和混凝土回填的U型桩埋管换热器,分别进行在不同进口温度和流量下的取热和排热实验,分析这两种埋管换热器的换热效果和性能.理论上,采用所建立的内热源埋地换热器理论模型和专业软件,对这两种埋管方式的周围土壤温度场进行数值模拟.经检验,理论计算与实验结果吻合较好.相同实验工况下,得到了U型桩埋管的换热效果和换热稳定性要优于U型井埋管.排热时,U型桩埋管比U型井埋管的单位井深换热量提高62.50/0,取热时,提高约160/0.

增强地源热泵竖直埋管地下换热器换热性能的研究

地下换热器换热性能直接影响到地源热泵系统的初投资和运行成本。地下换热器可分为单U型、双U型和 1+2型三类。采用非稳态法对膨润土、水泥 +砂两类回填材料进行了室内实验 ;在相同的边界条件下 ,采用扶正器和定位器施工可以保证下管 ,并能降低钻孔内热阻。因地制宜地选择埋管形式 ,采用稳定。

地源热泵地下垂直埋管换热器的试验研究

介绍了地源热泵的发展意义及前景， 参照国外地下埋管换热器岩土温度场的求解方法，采用线源理论及热阻网络分析方法建立了传热模型，提出了采用计算法确定大地初始温度并与实测值进行了对比。在建设的垂直埋管换热器试验装置上进行了单管、三管串联运行试验，为开发地源热泵技术提供了可供参考的数据。

烧结余热回收竖罐内料层传热过程数值计算

罐体料层内气固传热过程是决定罐式回收是否可行的主要因素之一.采用Comsol Multiphysics软件,基于稳定传热前提下,计算了罐体料层内气固传热过程,研究竖罐内的传热规律.研究结果表明:气固比和料层高度是影响料层气固传热的最主要因素,随着气料比增加,烧结矿和气体出口温度逐渐降低,气体所携带的值呈现出先增加后减小的趋势;随着料层高度增加,气料比逐渐下降,气体出口温度逐渐增加,气体携带的值呈现先增大后减小的趋势.针对国内某典型360m2烧结机,配套单罐体,料层高度6～7m,气料比为1 500～1 650m3/t;配套双罐体,料层高度5～6m,气料比为1 360～1 550m3/t.

土壤热源热泵系统垂直U形埋地换热器传热过程及合理间距的分析

利用传热理论 ,提出了土壤热源热泵垂直U形埋地换热器的等效换热器 ,并通过传热热阻分析 ,得到 :埋地换热器在土壤中的取热过程 ,热阻的主要成分是土壤和塑料管壁。在换热强度为单位管长 40W /m(单位埋孔长度 80W/m) ,类似上海地区的气象与地质条件下 ,对 5 0m深的埋地换热器孔中心间距至少为 5～ 6m ,才能保证换热器之间相互不受干扰。

竖直埋管地源热泵系统研究进展

介绍了竖直埋管地源热泵的国内外研究情况。理论研究主要是针对地埋管换热器进行模拟,准三维模型已经被广泛接受;实验研究相比理论研究要少一些,但也得出了一些指导性的结论;系统耦合利用和技术经济性的研究也有进展。目前我国的地源热泵应用处于起步阶段,但发展迅速,随着政府及群众的重视,这项技术将有广阔的发展空间。

地源热泵地下环路的设计方法

介绍了地热换热器的形式 ,提出了竖直埋管地热换热器的设计方法及地下环路循环泵的选择方法。

地埋管热响应测试及数据分析方法

对上海某商务楼垂直地埋管换热器进行热响应测试,该商务楼占地面积大,在相隔300m两个地块分别钻三口测试井。本测试的特点是：地埋管换热器有效换热深度大,测试内容包括单U型及双U型地埋管,根据测试要求设定不同的循环流速。采用一维线热源模型对测试数据进行科学严谨的分析,本文将给出计算土壤导热系数、垂直地埋管换热器传热热阻及确定单位井深换热量方法。试验数据具有很强的对比性并能验证测试准确度。测试结果表明,该测试点的土壤导热系数为1.84~1.94 W/（m.K）;相同埋管形式与管径地埋管换热器,传热热阻相同。本测试中双U型32埋管热阻大于双U型25热阻,单U型32型埋管热阻最大。应用线热源模整理出计算给定进水温度下的单位井深换热量公式,并计算出不同埋管形式及循环水流量下的单位井深换热量。

地埋管换热器热短路及其对热物性测试影响分析

为了更准确地建立垂直地埋管换热器钻孔内传热模型,采用模拟分析的方法探讨了不同钻孔、支管和布置形式的冷热支管热量回流情况,并拟合得到了热短路热阻的表达式。建立了三维数值模型,分析了管内流体流速、埋深对热短路的影响,流速越小,热短路损失率越大和单位管长换热量越小,然而流速过大,热短路损失率减少并不明显且能耗加大;增加埋深可以增大埋管进出口的温差,但冷热支管间的热损失也大大增加。对比了流体积分平均温度与几何平均温度的差别,由于忽略了热短路的影响,往往线性几何平均温度值偏大。借助于试验平台,分析了流速变化和埋深变化对土壤平均传热系数测试的影响,结果表明:流速越小,热短路损失率越大,土壤平均传热系数越小,即埋管的换热能力也越低;埋管的深度应综合换热量要求、热短路损失、投资而确定。

基于层换热理论的竖直地埋管换热器设计方法

建立了地源热泵竖直地埋管换热器的三维传热温度场数学模型,模拟计算了不同季节不同工况下地埋管换热器内的水温分布。提出了层换热理论,竖直地埋管换热器及其周围岩土可以分为三个换热层——饱和换热层、换热层、未换热层。通过实测验证了该层换热理论。介绍了地埋管换热器埋深的确定、出水管的保温及流量的确定等。

不同方式地下埋管换热器的实验研究

针对不同方式地下埋管换热器建立实验系统 ,并分别对砂石回填的单U型和双U型井埋管进行夏季排热和冬季取热实验。整理分析了冬、夏季节不同运行工况下的实验数据 ,在此基础上 ,以单位管长换热量为评价指标对不同埋管方式的换热性能进行分析比较 ,得出排热工况和取热工况下 ,单U型埋管单位管长换热量高于双U型。同时对埋管换热器的流量和进口水温对其换热性能的影响进行了研究 ,结果表明 ,随着流量的增大 ,排热工况与取热工况下 ,单位管长换热量均增加 ;随着进口水温的升高 ,单位管长排热量增大 。

模式搜索算法在地埋管换热器热阻确定中的应用

为了准确确定土壤源热泵系统地埋管换热器的热物性参数和热阻,该文以地埋管换热器线热源模型为基础,结合土壤的热响应试验,提出了一种应用模式搜索算法确定土壤综合导热系数和地埋管换热器热阻的方法。搜索算法实施时,热阻作为一个待定参数,不必考虑换热器的各项物理参数,降低了运算工作量。试验实测结果表明,应用模式搜索算法后该文试验条件下确定的土壤综合导热系数的相对误差为1.42%,热阻的相对误差为1.73%;通过与其他方法比较,模式搜索算法确定的参数的相对误差较小,算法精度较高,可靠性较高;该研究结果为土壤源热泵系统的设计与应用提供参考。