基于分段法的同轴套管换热器解析模型研究

为方便同轴套管换热器的优化设计,提出一种基于分段法的同轴套管换热器解析模型.在解析模型中,根据"热阻理论"和"线热源理论"分别构建钻孔内与钻孔外换热模型,并基于"分段法"和"时间迭加理论"组合构建准三维同轴套管换热器瞬态解析模型.将解析模型计算结果与试验结果、数值分析结果进行对比,验证解析模型的合理性.采用解析模型,研...     

Effects of flow direction and thermal short-circuiting on the performance of small coaxial ground heat exchangers

The effects of flow direction and thermal short-circuiting on the performance of small-size coaxial ground heat exchangers, currently used in Northern Italy, are studied by finite-element simulations, performed through the software package COMSOL Multiphysics 3.4 (©Comsol, Inc.). The real 2-D axisymmetric unsteady heat conduction and convection problem is considered, both for winter and for summer working conditions. The flow in the outer annular passage is laminar in winter and turbulent in summer. The distribution of the fluid bulk temperature in the inner circular tube is determined by means of the weak form boundary condition available in COMSOL Multiphysics; the forced-convection heat transfer in the outer annular passage is simulated directly. Two Small Coaxial Ground Heat Exchangers (SCGHEs) with the same length (20 m) but different cross-sections are examined; moreover, two values of the ground thermal conductivity, as well as two materials for the inner tube wall are considered. The results point out that the annulus-in flow direction (fluid inlet in the outer annular passage) is more efficient than the center-in flow direction (fluid inlet in the inner circular tube) and that, on account of the small length, the effect of thermal short-circuiting is not important for SCGHEs, especially if the annulus-in flow direction is employed.     

Melting and solidification performance in two horizontal shell-and-tube heat exchangers with different structures

Recently, a critical problem of latent heat thermal energy storage remains the low thermal conductivity of phase change materials (PCMs), which can lead to low heat transfer rate. Structural optimization design is an effective solution for this problem. In this work, two horizontal shell-and-tube heat exchangers (HEs) with one inner tube (n = 1) and four inner tubes (n = 4) are designed keeping the same amount of PCM and water flow rate, and their melting and solidification thermal performance and heat transfer characteristics are compared. The results show that in comparison with one-inner-tube HE, the temperature of detected points are affected by both upper and lower inner tubes for four-inner-tube HE, thus the differences in phase change process appear. In addition, the phase change time reduction rates are 34.1%, 33.39%, 28.82% at T_in (inlet water temperature) = 75°C, 80°C, 85°C during charging process, and 17.2%, 27.69%, 36.67% at T_in = 10°C, 15°C, 20°C during discharging process, respectively. In comparison with the one-inner-tube HE, the theoretical efficiency of four-inner-tube HE is increased from 75.88% to 90.34%. Although more friction loss should be paid by four-inner-tube HE, a lower energy consumption and a higher heat-energy ratio are achieved. Based on the results of this study, the amount of cumulative heat per energy consumption is 1.52 × 10⁸ and 2.88 × 10⁸ for one-inner-tube and four-inner-tube HE, respectively.     

相变贮热管管内强化传热的数值研究

以管壳式相变换热器为例,研究了肋片对增强贮热管管内传热的影响。建立了内部设置肋片的贮热管放热过程的数理模型,分析比较了不同的肋片参数,如肋片布置密度、长度及厚度等因素对贮热管放热效果的影响,并通过瞬态传热过程的计算,确定了与多种肋片布置方式所对应的相变贮热管的有效导热系数。     

超临界压力下航空煤油在竖直管内的传热研究

对竖直上升管内超临界压力下航空煤油的传热特性进行了实验研究。分析了不同质量流量、热流密度、压力和进口温度对超临界压力下航空煤油传热特性的影响。实验结果表明,提高质量流量或进口温度均使煤油传热效果变好。而热流密度对流体传热的影响主要在于改变了流体和壁面温度,热流密度越大,传热系数越高。压力对煤油传热影响不大,一般情况下,提高压力会恶化传热。超临界状态下,煤油物性变化很大,因此对煤油的传输和热力学性质的准确计算是研究超临界压力下传热现象的关键。利用拓展的对比态法来计算煤油的密度和传输特性,如黏度、热导率等。给出了煤油在超临界压力下的传热关联式,其计算值和实验值吻合良好。     

ZnO对不同流态下相变微胶囊悬浮液对流换热特性影响研究*

相变微胶囊悬浮液（MPCS）可作为热交换介质和储热流体,但其导热率较低导致其应用受到一定的限制。以水为基液使用相变微胶囊（MPCM）制备MPCS,加入氧化锌（ZnO）颗粒以提高MPCS导热率。使用旋转流变仪、差式热量扫描仪、导热仪分别测定了MPCS的黏度、相变潜热和导热系数等物理性质。设计并搭建了试验台,在内径6 mm的圆管中,使用水、MPCS以及ZnO@MPCS在层流和湍流下进行强制对流换热实验,通过对比其换热情况分析ZnO对MPCS换热特性的影响。结果表明：加入ZnO的MPCS具有良好的储热性和导热性,1%ZnO@5%MPCS导热系数较5%MPCS提高了17.9%。层流条件下MPCS的平均局部换热系数低于水,1%ZnO@5%MPCS平均局部换热系数比水高6.5%;湍流时,1%ZnO@5%MPCS在相同质量流量和功率下的平均局部换热系数相较于水提高了15.7%。     

基于线热源模型的地下岩土热物性测试方法

提出了一种用于竖直埋管与周围岩土的传热分析模型。该方法回避了其它模型对钻孔中埋管的具体位置、上升管和下降管之间的距离以及埋管和回填材料的物性等参数的要求,从而可以消除上述参数测量带来的误差。在现场钻孔,埋设U型埋管,通过测量施加的加热功率、埋管中循环水流量、埋管出入口水温随时间变化,利用该模型并结合最优化估计方法,确定了某地源热泵空调系统工地地下岩土的热物性参数,检验证实了该方法的实用性和可靠性。     

Experimental performance evaluation of a closed‐loop vertical ground source heat pump in the heating mode using energy analysis method

An experimental study is performed to determine the performance of a ground source heat pump (GSHP) system in the heating mode in the city of Erzurum, Turkey. The GSHP system using R‐134a as refrigerant has a single U‐tube ground heat exchanger (GHE) made of polyethylene pipe with a 16 mm inside diameter. The GHE was placed in a vertical borehole with 55 m depth and 203.2 mm diameter. The average coefficients of performance (COP) of the GSHP system and heat pump in heating mode are calculated as 2.09 and 2.57, respectively. The heat extraction rate per meter of the borehole is determined as 33.60 W m^?1. Considering the current gas and electric prices in Erzurum city, the equivalent COP of the GSHP system should be 2.92 for the same energy cost comparing with natural gas. The virgin ground in Erzurum basin has high permeability and low thermal conductivity. In order to improve the thermal efficiency of GHE and thus improve COP of a GSHP in the basin, the borehole should be backfilled with sand as low‐cost backfill material and a 1 to 2 m thick surface plug of clay should be inserted. Copyright © 2007 John Wiley & Sons, Ltd.     

水平管壳式相变蓄热装置的强化传热研究

基于焓法模型对水平管壳式相变蓄热装置热性能的增强进行研究,首先分析蓄热过程中传统管壳式装置内材料的传热及流动机理;然后引入椭圆元素并对比椭圆内管及外壳的强化传热效果;最后对热源温度、相变材料导热系数及初始温度对装置热性能的作用规律进行探讨。结果显示,椭圆外壳的强化传热效果优于内管,同等条件下,长短轴之比为2的椭圆外壳可使蓄热时间缩短53.5%。热源温度升高,椭圆外壳的强化传热效果进一步增强,相变材料的导热系数及初始温度对装置热性能的影响较小。     

多管型套管式换热器传热与流阻性能试验研究

多管型套管式换热器是在大尺寸外管的内部布置多根内管所构成的换热设备 ,与单根内管的套管式换热器相比 ,流量大幅增加 ,选用螺纹内管和管间折流板可以强化传热。对于多管型套管式换热器的传热性能试验 ,采用修正威尔逊法进行试验计算 ,得出了两种多管型套管式换热器的传热与流动阻力性能试验结果。     

热响应测试在土壤热交换器设计中的应用

分析了土壤热交换器系统的影响因素以及设计与施工中存在的问题,介绍了自主研制的移动式地源热响应测试装置原理与构成。针对天津市某地源热泵项目,阐述了热响应测试的方法与步骤,得到了项目所在地的无干扰地温以及地埋管系统的供回水温度响应曲线。利用线源理论,得到了地埋管换热器钻孔的导热系数及热阻,分析了测试装置与环境的热损失和热增益、测试时间、供电稳定性、无干扰地温、不同深度土壤热导率的变化以及地下水流动对热响应测试造成的影响。测试结论对于指导土壤热交换器设计与施工具有一定的参考价值。     

同轴地埋管换热器岩土热响应试验研究

岩土热物理性质是影响地源热泵系统设计和运营的关键因素,对位于武汉市洪山区的2口不同深度的同轴地埋管换热孔分别进行48 h的热响应试验,并对同轴地埋管换热器内外管之间环形空间中的平均流体温度进行测试.根据同轴地埋管换热器的几何特性,以简便实用的方式测量同轴地埋管换热器环状空间传热流体的平均温度,结合同轴地埋管换热器钻孔热...     

Numerical study on coupled fluid flow and heat transfer process in parabolic trough solar collector tube

A unified two-dimensional numerical model was developed for the coupled heat transfer process in parabolic solar collector tube, which includes nature convection, forced convection, heat conduction and fluid-solid conjugate problem. The effects of Rayleigh number (Ra), tube diameter ratio and thermal conductivity of the tube wall on the heat transfer and fluid flow performance were numerically analyzed. The distributions of flow field, temperature field, local Nu and local temperature gradient were examined. The results show that when Ra is larger than 10⁵, the effects of nature convection must be taken into account. With the increase of tube diameter ratio, the Nusselt number in inner tube (Nu₁) increases and the Nusselt number in annuli space (Nu₂) decreases. With the increase of tube wall thermal conductivity, Nu₁ decreases and Nu₂ increases. When thermal conductivity is larger than 200 W/(m K), it would have little effects on Nu and average temperatures. Due to the effect of the nature convection, along the circumferential direction (from top to down), the temperature in the cross-section decreases and the temperature gradient on inner tube surface increases at first. Then, the temperature and temperature gradients would present a converse variation at θ near π. The local Nu on inner tube outer surface increases along circumferential direction until it reaches a maximum value then it decreases again.     

Effects of temperature-dependent viscosity on fully developed laminar forced convection in a curved duct

For most liquids the specific heat and thermal conductivity are almost independent from temperature, but the viscosity decreases significantly. A fully developed laminar water flow in a curved duct with temperature-dependent viscosity is analyzed in this work. The mass, momentum and energy conservation equations are numerically solved by the finite element method. Both heating and cooling of the water flow is studied. The secondary flow induced by the curvature effects increases the heat transfer rate in comparison with the straight ducts but the velocity and temperature profiles are distorted when the effects of temperature-varying viscosity are included. The Nusselt number obtained when the fluid is cooled with variable viscosity assumption are lower than the constant properties results due to the increase of the viscosity values at the inner points of the curved tube that reduces the secondary flow effect. The friction factor results also show a marked dependence on the viscosity variations in the coil tube cross-section.     

Thermal and hydraulic characteristics of nanofluid flow in a helically coiled tube heat exchanger

The effects of using different geometrical parameters with the combination of nanofluid on heat transfer and fluid flow characteristics in a helically coiled tube heat exchanger (HCTHE) are numerically investigated. A CuO nanoparticle with a diameter of 25 nm dispersed in water with a particle concentration of 4% was used as the working fluid. The three dimensional governing equations (continuity, momentum and energy) along with the boundary conditions are solved using the finite volume method (FVM). The mass flow rate of water in the annulus was kept constant and the nanofluid flow rate in the inner tube was varied. The effect of flow configuration (parallel and counter) was also examined in this study. The performance of the HCTHE was evaluated in terms of Nusselt number, heat transfer rate, pressure drop, effectiveness and performance index. The results reveal that certain geometrical parameters such as the helix radius and inner tube diameter do affect the performance of the HCTHE under laminar flow conditions. It is also found that counter-flow configuration produced better results as compared to parallel-flow configuration.     

The influence of the thermosiphon effect on the thermal response test

The issue of natural and forced groundwater movements, and its effect on the performance of ground heat exchangers is of great importance for the design and sizing of borehole thermal energy systems (BTESs). In Scandinavia groundwater filled boreholes in hard rock are commonly used. In such boreholes one or more intersecting fractures provide a path for groundwater flow between the borehole and the surrounding rock. An enhanced heat transport then occurs due to the induced convective water flow, driven by the volumetric expansion of heated water. Warm groundwater leaves through fractures in the upper part of the borehole while groundwater of ambient temperature enters the borehole through fractures at larger depths. This temperature driven flow is referred to as thermosiphon, and may cause considerable increase in the heat transport from groundwater filled boreholes. The thermosiphon effect is connected to thermal response tests, where the effective ground thermal conductivity is enhanced by this convective transport. Strong thermosiphon effects have frequently been observed in field measurements. The character of this effect is similar to that of artesian flow through boreholes.     

不同纳米流体水平管内沸腾流型的模拟

对以纯水为基液的不同纳米流体管内流动沸腾进行了模拟。利用UDF编程定义流体的沸腾相变源项,将其导入FLUENT软件中分别模拟这四种流体在水平管内的流动沸腾过程。得到了四种流体流动沸腾的速度云图,以及流型分布云图。对比分析了四种流体的速度分布云图、四种流体从初始状态沸腾到1s时的流型分布云图以及四种流体层状流流型的特点。结果表明,四种流体在水平管内沸腾都会出现泡状流、弹状流、层状流以及波状流四种流型,并且沿管长方向含气量逐渐增加。在相同的时间内,纳米流体的沸腾比纯水更剧烈,而且,不同的纳米流体沸腾程度也不同。在相同的体积分数下,纳米颗粒的导热系数越大,其对应的纳米流体沸腾越剧烈。说明纳米颗粒的导热系数是影响纳米流体沸腾传热的主要因素。     

Sequential estimation of borehole resistance and ground thermal properties through thermal response test

Borehole thermal resistance and ground thermal properties (thermal conductivity and heat capacity) are the key parameters to implement the ground source heat pump (GSHP), usually obtained by thermal response test. In this study, a novel sequential parameters estimation method for the above three parameters is proposed, and the sensitivity analysis by using a special correlation method is performed to decide the best estimation sequences. At first, the Spearman partial rank correlation coefficient was used to represent the correlation between the estimated thermal properties and fluid temperature for the line source model (ILS), then the estimation sequence for the three parameters could be determined by the correlation results. Lastly, with the estimation step, Monte Carlo method was adopted to determine the parameters replacing conventional iterative algorithms. In addition, the effect of value bounds and initial inputs as well as random samples was investigated. The results showed that compared to the other estimation steps, the estimation sequence following borehole resistance firstly, then thermal conductivity, heat capacity lastly could get the best precision with 4.5%, 0.4%, 1% respectively. Specially, the estimation precision for ground heat capacity could be promoted by the sequential estimation. Also, the effect of value bounds on estimation precision was nearly eliminated by the proposed method.     

Experimental performance of borehole heat exchangers and grouting materials for ground source heat pumps

The system performance of a ground source heat pump (HP) system is determined by the HP characteristics itself and by the thermal interaction between the ground and its borehole heat exchanger (BHE). BHE performance is strongly influenced by the thermal properties of the ground formation, grouting material, and BHE type. Experimental investigations on different BHE types and grouting materials were carried out in Belgium. Its performances were investigated with in situ thermal response tests to determine the thermal conductivity (λ) and borehole resistance (R_b). The line‐source method was used to analyze the results, and the tests showed the viability of the method. The main goal was to determine the thermal borehole resistance of BHEs, including the effect of the grouting material. The ground thermal conductivity was measured as 2.21 W m^?1 K^?1, a high value for the low fraction of water‐saturated sand and the high clay content at the test field. The borehole resistance for a standard coaxial tube with cement–bentonite grouting varied from 0.344 to 0.162 K W^?1 m for the double U‐tube with cement–bentonite mixture (52% reduction). Grouting material based on purely a cement–bentonite mixture results in a high thermal borehole resistance. Addition of sand to the mixture leads to a better performance. The use of thermally enhanced grouts did not improve the performance significantly in comparison with only a low‐cost grouting material as sand. Potential future applications are possible in our country using a mobile testing device, such as characteristics, standardization, quality control, and certification for drilling companies and ground source HP applications, and in situ research for larger systems. Copyright © 2011 John Wiley & Sons, Ltd.     

用于现场测量深层岩土导热系数的简化方法

为便于工程上实现应用现场测量确定实际介质的物性,采用一种简化的传热分析方法确定深层岩土导热系数。该方法不需要测量钻孔中埋管的具体位置、上升管和下降管之间的距离以及埋管和回填材料的特性等参数,可消除上述参数测量带来的误差。通过现场测量地下埋管回路的加热热流、回路循环水流量以及回路出入口水温度随时间的变化,利用简化分析和最优化估计方法,确定了某工地地下岩土的导热系数,检验证实了该方法的实用性和可靠性。