纳米流体流动传热性能的实验与模拟研究

为了研究高温冷却条件下,γ-Al2O3-PG90纳米流体作为冷却介质在一车用机油冷却器内的流动传热性能,采用三维k-ε湍流模型,应用块结构网格生成技巧,融合流固耦合研究方法和薄壳导热模型数值模拟了纳米流体的性能,进行了基液与纳米流体的性能对比计算,分析了纳米粒子体积分数对性能的影响,考察了纳米流体物性预测模型的普适性,并研究了将纳米流体视为单相流体进行性能分析的可行性,通过实验测试得到了性能数据.研究发现:与基液相比,纳米流体强化换热效果明显,流动阻力有所增加,随着纳米粒子体积分数的增加,传热性能提高,流动阻力增加,说明该物性预测模型不能普适,当纳米粒子体积分数大于3%时,将纳米流体视为单相流体的性能研究结果与实验数据偏差较大,可能原因是单相流体流动无法反映较多的粒子之间的相互作用.     

对流换热可控温差场分布原则

应用火积概念导出了对流换热过程的火积耗散表达式,进一步基于火积耗散极值原理讨论了换热器N股冷、热流在不同情况下参与换热的优化。研究表明,若参与换热的任何冷、热流之间的温差可以独立调控,在总换热量一定寻求火积耗散最小或在总火积耗散一定的条件下寻求换热量最大,则整个换热系统冷、热流之间的温差分布均匀时换热最优;若参与换热的任何冷、热流体之间的换热量或火积损耗可独立确定,则换热冷热流之间的温差分别保持各自的均匀温差分布时换热最优;若在任何冷、热流之间存在可能换热的情况下,无论是总换热量一定时寻求火积耗散最小,还是总火积耗散一定时寻求换热量最大,整个换热系统不同的冷热流换热的优化温差并不是同一个常数。     

Study on the performance of TEG with heat transfer enhancement using graphene-water nanofluid for a TEG cooling system

Improvement of the heat transfer effect of cold side of a thermoelectric generator(TEG) is one of the approaches to enhance the performance of the TEG systems.As a new type of heat transfer media,nanofluids can enhance the heat transfer performance of working liquid significantly.In this study,the performance of a commercial TEG with graphene-water(GW) nanofluid as coolants in a minichannel heat exchanger is investigated experimentally at low temperatures.The results show that the output power of TEG increases with the flow rate under 950 mL/min.However,the fluid flow rate has no influence on the output power of TEG with higher flow rate(larger than 950 mL/min) when the heat transfer dynamic balance state of the system is reached.The optimal concentration and flow rate of nanofluid are 0.1 wt%and 950 mL/min,respectively.At the optimal conditions,the improved voltage,output power and conversion efficiency with GW nanofluid applied in the cooling system are increased by11.29%,21.55%and 3.5%in comparison with those with only water applied,respectively.     

Kerosene-alumina nanofluid flow and heat transfer for cooling application

Kerosene-alumina nanofluid flow and heat transfer in the presence of magnetic field are studied. The basic partial differential equations are reduced to ordinary differential equations which are solved semi analytically using differential transformation method. Velocity and temperature profiles as well as the skin friction coefficient and the Nusselt number are determined analytically. The influence of pertinent parameters such as magnetic parameter, nanofluid volume fraction, viscosity parameter and Eckert number on the flow and heat transfer characteristics is discussed. Results indicate that skin friction coefficient decreases with increase of magnetic parameter, nanofluid volume fraction and viscosity parameter. Nusselt number increases with increase of magnetic parameter and nanofluid volume fraction while it decreases with increase of Eckert number and viscosity parameter.     

螺旋槽管脉冲流强化对流换热数值及场协同分析

通过数值模拟计算研究了在螺旋槽管内通入脉冲流后的强化换热机理.数值计算结果表明,脉冲流动能引起出口压力呈周期波动,波动幅度随脉冲流频率的增大而增大;脉冲流动能够使流体在螺旋槽管管壁附近产生漩涡,并出现周期性的生成、漂移和脱落;由于漩涡的作用,增强了流体的径向扰动和相对扰动;脉冲流动能够改善速度场与温度梯度场之间的协同程度,从而起到强化传热的效果.     

螺旋槽管强化传热数值模拟及其场协同分析

利用Fluent软件分别对光管、单头螺旋槽管及双头螺旋槽管传热及管内流动情况进行模拟,得到了湍流状态下管内流场分布云图,并从场协同理论出发,分析了螺旋槽管强化传热机理.仿真结果显示,在低雷诺数条件下,螺旋槽使管内产生较多的二次流,速度在横截面上的分量增加,努塞尔数保持为光管的1.6~2.2倍,换热性能优于光管,并且双头螺旋槽管的换热性能要优于单头螺旋槽管.但随着雷诺数的增加,场协同角逐渐接近90°,努塞尔数增加趋势变缓,换热性能增加量变缓,而且当雷诺数较高时,螺旋槽管的阻力系数急剧上升,换热性能不及光管.     

Convective heat transfer and flow characteristics of Cu-water nanofluid

An experimental system is built to investigate convective heat transfer and flow characteristics of the nanofluid in a tube. Both the convective heat transfer coefficient and friction factor of Cu-water nanofluid for the laminar and turbulent flow are measured. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on the heat transfer and flow characteristics are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the convective heat transfer coefficient of the base fluid and show that the friction factor of the sample nanofluid with the low volume fraction of nanoparticles is almost not changed. Compared with the base fluid, for example, the convective heat transfer coefficient is increased about 60% for the nanofluid with 2.0 vol% Cu nanoparticles at the same Reynolds number. Considering the factors affecting the convective heat transfer coefficient of the nanofluid, a new convective heat transfer correl     

碳纳米流体特性及其在重力热管内的传热

采用阿拉伯胶辅助分散法制备了一种新型的纳米流体--碳纳米管 水纳米流体,并以其为工质在以铜为管壳的一种新型重力热管内进行了传热实验研究.制备和实验结果表明,碳纳米管 水纳米流体的导热系数比基液水约增加了30％～50％,导热能力明显增强;这种纳米流体黏度低,流动性好,制备过程简单,可用于大规模生产;纳米流体重力热管加热段沸腾换热弱化了纳米流体的稳定性对热管传热性能的影响,实现了纳米流体与重力热管的优势互补;以该纳米流体为工质的重力热管的内热阻比水小,并且随着传输功率的提高,热管总热阻逐渐减小,传热性能逐渐提高.     

管壳式换热器瞬态换热性能分析

通过理论分析，分别以壳程流体、管程流体、管内外壁为研究对象，建立了管壳式换热器瞬态换热数学模型．模型中很好地解决了顺流和逆流同时存在的问题．存此基础上借助于仿真手段对管壳式换热器的瞬态换热性能进行分析，为了减小线性化方法带来的误差以及提高计算的准确度，仿真计算时所采用的数学模型未进行线性化处理，而直接采用非线性化方程进行程序编写．仿真结果与换热器实际运行数据吻合很好，说明所建立的数学模型可用于同类换热器瞬态换热性能分析．通过对换热器瞬态换热性能的分析，町清楚地了解换热器的动态换热特性，为换热器设计及控制提供理论依据．     

冷凝器开缝肋片换热特性场协同原理分析

针对冷凝器强化传热,采用开缝肋片提高换热效率是一种有效的手段.通过对平直肋片和2种开缝肋片空气侧传热与压降的数值模拟得出,开缝肋片的传热性能远高于平直肋片,与均匀开缝肋片相比,后部区域局部开缝肋片的性能更好.应用场协同原理,对数值模拟得到两肋片之间流体的温度场、速度场和压降沿程变化进行了分析.结果表明,开缝肋片有效强化传热的根本原因是肋片开缝改善了速度与温度梯度的协同性,在场协同性较差的位置开缝会得到更好的换热效率.     

多旋静态混合器强化对流传质场协同分析

为了研究静态混合器强化高粘流体混沌对流传质机理,通过积分中值角定量评价雷诺数和混合构件结构对静态混合器内速度场与压力场协同程度的影响。研究结果表明:静态混合器内速度场与压力场的协同角随着雷诺数的增加而降低;当Re17.8时,随着截面内叶片个数的增加,多旋静态混合器的协同角逐渐增大;FKSM构件较KSM提高速度场与压力场的协同程度达5.9%~11.9%,且其概率密度分布更加集中,最大概率对应的协同角数值比KSM高2.3%~3.5%,旋流叶片的切割分流作用致使轴向相邻2组混合元件过渡处协同角数值较大。     

A general theoretical principle for single-phase convection heat transfer enhancement

The main methods of single-phase convection heat transfer enhancement are analyzed in this paper, and the unity of contradiction between heat transfer enhancement and energy consumption (or exergy destruction) is expounded. The thermodynamic relationship between heat (or exergy) transfer efficiency and energy consumption (or exergy destruction) as well as driving forces is established, and a general theoretical principle for single-phase convection heat transfer enhancement is further obtained. The principle shows that temperature gradient field distribution and velocity field distribution constrain each other, and that the optimum heat transfer efficiency can be obtained when they are synergetic. If the level of the synergy of temperature gradient field distribution with velocity field distribution is determined, the relative uniform temperature gradient is required, and vice versa. The principle also shows the relationship of relative temperature gradient with specific heat and coefficient of heat conductivity. The deduced results can be used as a theoretical guidance for single-phase convection heat transfer enhancement and optimum design of heat exchangers.     

驻人月球科研站围护结构传热性能分析

为探索由月球表面特殊光热环境导致的驻人月球科研站围护结构传热过程与地球建筑的差异性,基于月球表面太阳辐射模型,根据有限差分法建立围护结构的传热模型,对影响内表面温度的物性参数、朝向等因素进行模拟分析。结果表明：当防热层、隔热层和阻气层分别取20 mm Nextel BF-20、240 mm Pyrogel 6650、20 mm Kapton时,水平屋顶内表面温度的波动范围为16.8～22.4℃;厚度和导热系数是影响围护结构传热性能最重要的因素,综合考虑建造成本与隔热性能,应尽量降低围护结构的厚度与导热系数,采用低密度、高比热的材料;内表面的对流换热系数与外表面的发射率直接影响围护结构的边界换热量,可结合不同朝向的辐射特点与热舒适要求,个性化定制具有差异化热惰性和外表面发射率的材料。     

螺旋槽管内流动换热场协同分析

运用数值模拟结合场协同原理,对螺旋槽管内充分发展湍流的流动和换热进行了分析。模拟以空气为工质,管壁温度恒定,分别选取了4种不同的螺纹节距和螺纹高度组成的16组结构参数。讨论了螺纹凸起,以及螺纹节距、螺纹高度变化对螺旋槽管场协同性能和强化传热能力的影响。结果表明：利用场协同原理,可以解释结构参数的变化对螺旋槽管传热效果的影响,并能够对强化传热元件的结构优化提供指导。通过对比,模拟所得Nu数与实验所得Nu数关联公式的计算结果基本一致。     

Application of entransy dissipation extremum principle in radiative heat transfer optimization

The concepts of entransy flux and entransy dissipation in radiative heat transfer were introduced based on the analogy with heat conduction and heat convection processes. Entransy will be partially dissipated during the radiative heat transfer processes due to the irreversibility. The extremum principle of entransy dissipation was developed for optimizing radiative heat transfer processes. This principle states that for a fixed boundary temperature the radiative heat transfer is optimized when the entransy dissipation is maximized, while for a fixed boundary heat flux the radiative heat transfer process is optimized when the entransy dissipation is minimized. Finally, examples for the application of the entransy dissipation extremum principle are presented. Supported by the National Basic Research Program of China (“973” Project) (Grant No. 2007CB206901)     

凝固换热器中地表水紊流冻结与换热特性

为分析采集凝固热热泵系统凝固换热器的特性,需要掌握地表水紊流工况的相变换热规律.基于准稳态近似方法与假想等壁温模型,分析凝固换热器中紊流水的管内冻结特性,并采用定义的当量平均表面换热系数比Kh和潜热显热比KSL讨论紊流水冻结换热特性.结果表明:紊流时,管半径和沿水流流动方向距离对冻结特性的影响不明显,雷诺数、管壁温和进口水温影响较为明显;随着无量纲凝固时间增长,换热性能急剧下降,入口水温与管壁温度越低、管径越小、管长越大、雷诺数越小,换热特性越好.所得结果将为凝固换热器优化设计及热泵系统性能改善提供重要参考.     

丝网热管换热性能的实验研究

为研究纳米流体工质的丝网热管的换热性能,制作了简单的丝网热管传热性能测试装置.实验采用SiO2纳米流体为工质,对在不同的充液率、浓度、热管倾角以及不同纳米颗粒粒径情况下丝网热管的热阻进行了研究.研究表明:丝网热管在充液率为60%、质量分数为1%、倾角为30°以及纳米粒径为30 nm时,换热性能处于最佳.研究结果为SiO2纳米流体工质在热管散热器中的运用提供理论依据.     

内环肋管道强化换热的场协同分析

通过对内环肋管道的湍流流动和等壁温条件下的换热进行数值模拟,以场协同理论为指导,在所研究的几何结构和计算雷诺数范围内(肋高比H／D=0．04～0．12,肋距比P／日=5～30．Re=20000～70000),分析速度场与温度场的协同性,讨论其强化换热机制,说明了对于湍流流动,场协同原理也是适用的．     

武汉与重庆典型地质结构下的地埋管换热性能

地埋管地源热泵换热器的换热性能受到不同地质结构的影响。以武汉和重庆地区的典型地质构成为边界条件,建立了三维地埋管的单孔双U管换热模型,通过模型计算,获得了两种地质条件下的地埋管换热性能,以重庆地区的地源热泵热响应测试结果以及工程运行数据出发,对模型的计算结果进行了验证,结果表明,模型吻合度较好,可以应用于工程分析。以模型为条件,进行地质结构对换热性能的影响度分析,预测了两地地埋管地源热泵的换热性能并计算得到换热器的平均换热系数分别为武汉地区K₁=1.65（W/m·K）,重庆地区K₂=1.51（W/m·K）。     

Effects of bending on heat transfer performance of axial micro-grooved heat pipe

Heat pipe is always bent in the typical application of electronic heat dissipation at high heat flux, which greatly affects its heat transfer performance. The capillary limit of heat transport in the bent micro-grooved heat pipes was analyzed in the vapor pressure drop, the liquid pressure drop and the interaction of the vapor with wick fluid. The bent heat pipes were fabricated and tested from the bending angle, the bending position and the bending radius. The results show that temperature difference and thermal resistance increase while the heat transfer capacity of the heat pipe decreases, with the increase of the bending angles and the bending position closer to the vapor section. However, the effects of bending radius can be ignored. The result agrees well with the predicted equations.