黏弹性流体基铜纳米流体流动与传热实验研究

成功建立了流体流动阻力和换热性能测试实验台,在45℃的流体温度下,对不同铜粒子体积分数和基液浓度的纳米流体在湍流状态下的对流换热特性和流动阻力进行了实验测量。实验结果表明:黏弹性流体基液中添加纳米粒子后,在降低对应基液减阻率的同时能明显增强传热性能.例如,将1.0%体积分数的铜纳米粒子添加到质量分数为6×10~(-4)的基液中所形成的黏弹性流体基纳米流体其综合性能指数K=0.47,表现了很好的传热强化和减阻性能.     

非牛顿纳米流体在螺旋隔板翅片管换热器与弓形隔板光滑管换热器壳程的换热性能对比

选用黄原胶作为非牛顿流体基液,碳纳米管为纳米添加粒子,实验研究了非牛顿纳米流体及其基液在螺旋隔板翅片管换热器和弓形隔板光滑管换热器的壳程冷却传热性能。实验结果表明,两台换热器的对流传热系数和压降均随雷诺数的升高而升高。与基液相比,非牛顿纳米流体在螺旋隔板翅片管换热器和弓形隔板光滑管换热器中的壳程传热系数分别提高31.3%和18.3%,其压降分别增大36.1%~52.6%和35.6%~41.4%。在雷诺数相同时,非牛顿纳米流体在螺旋隔板翅片管换热器的综合热性能为弓形隔板光滑管换热器的2.78~2.93倍。     

ZnO纳米流体对氨水降膜吸收影响的实验研究

通过配制不同浓度ZnO纳米流体的氨水溶液,研究了ZnO纳米流体氨溶液在降膜吸收器内定初始压力条件下对氨气的吸收效果。结果表明:不同浓度的纳米流体体现出对氨水降膜吸收强化的效果有所不同,随着ZnO纳米流体浓度的增大,对氨水降膜吸收的强化影响先增大后减小,且在浓度为0.1wt%时强化效果最佳。从实验还可看出,氨水基液浓度的增加使氨气的吸收量逐渐减小,但纳米流体的添加均可使吸收量得到不同程度的提高,纳米流体的加入,还可放缓由于氨水基液浓度增高引起的吸收速率降低的速度,且强化吸收的最佳纳米流体浓度不随氨水基液浓度的改变而改变。     

抛物槽式太阳能集热器内合成油基纳米流体传热特性的数值模拟

对Al₂O₃-合成油纳米流体在槽式太阳能集热管内的传热特性进行流体动力学数值模拟,重点考察纳米流体导热系数模型的影响。通过与管内Nusselt数半经验模型的预测结果对比,表明使用考虑布朗运动的纳米流体导热系数模型可较好地预测集热管内传热特性。研究表明纳米颗粒与流体基液的相对运动具有促进集热管内传热的作用。最后,定量研究纳米颗粒添加量对提高基础流体平均传热系数的影响,显示纳米流体在太阳能集热器中具有巨大应用潜力。     

碳纳米流体强化传热研究

在去离子水中分别添加了单壁、多壁碳纳米管材料,通过配比一定质量的亲水性分散剂,经超声波振荡试制了水基碳纳米流体。测试分析了不同碳纳米管材料质量分数下的纳米流体热导率和动力粘度等重要热物性参数。测试结果表明:碳纳米管粒子能够强化基液工质导热性能,随着其质量分数的增加,水基单壁、多壁碳纳米流体热导率均明显提高;单壁碳纳米流体粘度显著增加,多壁碳纳米流体粘度无明显变化,多壁碳管更适用于纳米流体强化传热。     

二氧化钛纳米流体的固液相变特性

采用“两步法”制备出不同浓度的TiO2-H2O纳米流体,选用紫外分光光度法对纳米流体的分散稳定性进行了评价,使用差示扫描量热仪对纳米流体以及基液一去离子水的固液相变过程进行了测试。实验结果表明：纳米流体的分散稳定性随着TiO2粒子质量浓度的增大先增强后减弱;添加纳米颗粒可以促进去离子水的异质成核,减小其固液相变过冷度;...     

水基ZnO纳米流体电导和热导性能研究

利用水热法生成了形状规则、粒径均匀的球形ZnO纳米颗粒, 并超声分散于水中, 制备得到稳定的水基ZnO纳米流体. 实验测量水基ZnO纳米流体在体积分数和温度变化时的电导率, 并测试室温下水基ZnO纳米流体在不同体积分数下的热导率. 实验结果表明, ZnO纳米颗粒的添加较大地提高了基液(纯水)的热导率和电导率, 水基ZnO纳米流体的电导率随纳米颗粒体积分数增加呈非线性增加关系, 而电导率随温度变化呈现出拟线性关系; 纳米流体的热导率与纳米颗粒体积分数增加呈近似线性增加关系. 本文在经典Maxwell热导模型和布朗动力学理论的基础上, 同时考虑了吸附层、团聚体和布朗运动等因素对热导率的影响, 提出了热导率修正模型.将修正模型预测值与实验值对比, 结果表明修正模型可以较为准确地计算出纳米流体的热导率. 关键词： 水热法 电导率 热导率 热导模型     

微细通道纳米与非纳米流体传热和压降对比研究

分别以0.2%、0.5%、1%质量分数的Al2O3-H2O纳米流体和去离子水为实验工质,在高2mm,宽1mm的矩形微细通道内进行纳米流体与非纳米流体两相沸腾传热和压降对比研究。实验结果表明:增加质量通量对两种工质换热系数影响都较小,但增加热流密度可提高换热系数;在相同工况下,与水基液相比,采用Al2O3-H2O纳米流体换热系数明显增大,且随着纳米流体质量分数的增加而增加,对于该实验换热系数可提高8%～17%;随着纳米颗粒质量分数和质量通量的增加,两相摩擦压降显著增大。     

Al2O3-H2O纳米流体的相变蓄冷实验研究

通过在水介质中添加纳米氧化铝粒子,研制了一种新型传热冷却工质一氧化铝-水纳米流体,粒径分布显示了加入分散剂的悬浮液具有很好的分散性,同时对Al2O3-H2O纳米流体和水溶液的结冰过程进行了实时观察.结果表明,加入Al2O3纳米粒子后,流体的结冰速率得到了很大提高,结冰时间大大缩短,DSC测试结果得到0.1%Al2O3-H2O纳米流体的相变潜热值为313.1 J/g.     

基于基液连续假设的大体系Cu-H2O纳米流体输运特性的模拟研究

分子动力学模拟是研究纳米流体的输运特性的重要手段, 但计算量庞大. 为研究能体现流动传热过程的大体系纳米流体的输运特性, 本文对基液采用连续介质假设, 将基液的势能拟合在纳米团簇的势能中, 大幅度减小了计算量, 使得大体系输运特性的模拟成为可能, 且模拟结果与多组实验结果吻合较好. 采用此方法模拟研究了速度梯度剪切对Cu-H₂O纳米流体颗粒聚集过程和聚集特性的影响, 进而对Cu-H₂O纳米流体在流动传热过程中的热导率和黏度进行了模拟计算, 定量揭示了宏观流动传热过程中不同的速度梯度、速度、平均温度和温度梯度对于Cu-H₂O纳米流体热导率和黏度的影响.     

Intriguingly high convective heat transfer enhancement of nanofluid coolants in laminar flows

We reported on investigation of the convective heat transfer enhancement of nanofluids as coolants in laminar flows inside a circular copper tube with constant wall temperature. Nanofluids containing Al₂O₃, ZnO, TiO₂, and MgO nanoparticles were prepared with a mixture of 55 vol.% distilled water and 45 vol.% ethylene glycol as base fluid. It was found that the heat transfer behaviors of the nanofluids were highly depended on the volume fraction, average size, species of the suspended nanoparticles and the flow conditions. MgO, Al₂O₃, and ZnO nanofluids exhibited superior enhancements of heat transfer coefficient, with the highest enhancement up to 252% at a Reynolds number of 1000 for MgO nanofluid. Our results demonstrated that these oxide nanofluids might be promising alternatives for conventional coolants.     

Effect of Brownian motion on flow and heat transfer of nanofluids over a backward-facing step with and without adiabatic square cylinder

A mathematical model to predict large enhancement of thermal conductivity of nanofluids by considering the Brownian motion is proposed. The effect of the Brownian motion on the flow and heat transfer characteristics is examined. The computations were done for various types of nanoparticles such as CuO, Al₂O₃, and ZnO dispersed in a base fluid (water), volume fraction of nanoparticles ? in the range of 1 % to 6 % at a fixed Reynolds number Re = 450 and nanoparticle diameter d_np = 30 nm. Our results demonstrate that Brownian motion could be an important factor that enhances the thermal conductivity of nanofluids. Nanofluid of Al₂O₃ is observed to have the highest Nusselt number Nu among other nanofluids types, while nanofluid of ZnO nanoparticles has the lowest Nu. Effects of the square cylinder on heat transfer characteristics are significant with considering Brownian motion. Enhancement in the maximum value of Nu of 29 % and 26 % are obtained at the lower and the upper walls of the channel, respectively, by considering the Brownian effects, with square cylinder, compared with that in the case without considering the Brownian motion. On the other hand, results show a marked improvement in heat transfer compared to the base fluid, this improvement is more pronounced on the upper wall for higher ?.     

Studies on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>, CuO,and TiO<Subscript>2</Subscript> water-based nanofluids: A comparative approach in laminar and turbulent flow

The Prandtl number, Reynolds number and Nusselt number are functions of thermophysical properties of nanofluids, and these numbers strongly influence the convective heat transfer coefficient. The thermophysical properties vary with volumetric concentration of nanofluids. Therefore, a comprehensive analysis was performed to evaluate the effects on the performance of nanofluids due to variations of density, specific heat, thermal conductivity and viscosity, which are functions of nanoparticle volume concentration. Three metallic oxides, aluminum oxide (Al₂O₃), copper oxide (CuO), and titanium dioxide (TiO₂), dispersed in water as the base fluid were studied. A convenient figure of merit, known as the Mouromtseff number, is used as a base of comparisonfor laminar and turbulent flows. The results indicated that the considered nanofluids can successfully replace water in specific applications for a single-phase forced convection flow in a tube.     

氧化处理的蓝宝石基片上沉积的ZnO/MgO多量子阱的结构及光学性质研究

采用射频反应磁控溅射的方法,在经过氧化处理的Al₂O₃（0001）基片上制备了具有良好调制结构的ZnO/MgO多层膜量子阱.利用X射线反射率测量、X射线衍射分析、电子探针显微分析、原子力显微镜、透射光谱以及光致发光光谱等表征技术,研究了ZnO/MgO多量子阱的结构、表面形貌和光致发光等特性.XRD以及扫描的结果表明多层膜样品具有高c轴择优取向并且与蓝宝石基片有良好的外延关系.通过X射线反射率测量的结果得到多量子阱的调制周期,结合电子 关键词： ZnO/MgO 多量子阱 反应磁控溅射 变温光谱     

Experimental investigation and simulation of flow boiling of nanofluids in different flow directions

In this work, the flow boiling of TiO₂/water and Al₂O₃/water nanofluids was investigated experimentally and simulated with two phases. Experimental results were obtained in two directions and compared together. The volume fraction and heat transfer coefficient obtained from the vertical tube were compared with those obtained from the horizontal tube. The results showed that the contours of vapor volume fraction in horizontal tube are completely different from the vertical tube, which is due to the buoyancy effect. Moreover, the effect of nanoparticles on both flow directions was almost the same, while heat transfer coefficient was not the same in these flow directions. Based on the experimental result, presence of nanoparticles in the base fluid cannot increase the heat transfer coefficient.     

Pumping power of nanofluids in a flowing system

Nanofluids have the potential to increase thermal conductivities and heat transfer coefficients compared to their base fluids. However, the addition of nanoparticles to a fluid also increases the viscosity and therefore increases the power required to pump the fluid through the system. When the benefit of the increased heat transfer is larger than the penalty of the increased pumping power, the nanofluid has the potential for commercial viability. The pumping power for nanofluids has been considered previously for flow in straight tubes. In this study, the pumping power was measured for nanofluids flowing in a complete system including straight tubing, elbows, and expansions. The objective was to determine the significance of two-phase flow effects on system performance. Two types of nanofluids were used in this study: a water-based nanofluid containing 2.0–8.0 vol% of 40-nm alumina nanoparticles, and a 50/50 ethylene glycol/water mixture-based nanofluid containing 2.2 vol% of 29-nm SiC nanoparticles. All experiments were performed in the turbulent flow region in the entire test system simulating features typically found in heat exchanger systems. Experimental results were compared to the pumping power calculated from a mathematical model of the system to evaluate the system effects. The pumping power results were also combined with the heat transfer enhancement to evaluate the viability of the two nanofluids.     

Numerical study of heat transfer enhancement of counter nanofluids flow in rectangular microchannel heat exchanger

This paper reports a numerical analysis of the performance of a counter-flow rectangular shaped microchannel heat exchanger (MCHE) using nanofluids as the working fluids. Finite volume method was used to solve the three-dimensional steady, laminar developing flow and conjugate heat transfer in aluminum MCHE. The nanofluids used were Ag, Al₂O₃, CuO, SiO₂, and TiO₂ and the performance was compared with water. The thermal, flow fields and performance of the MCHE were analyzed using different nanofluids, different Reynolds numbers and different nanoparticle concentrations. Temperature profile, heat transfer coefficient, pressure profile, and wall shear stress were obtained from the simulations and the performance was discussed in terms of heat transfer rate, pumping power, effectiveness, and performance index. Results indicated enhanced performance with the usage of nanofluids, and slight penalty in pressure drop. The increase in Reynolds number caused an increase in the heat transfer rate and a decrease in the overall bulk temperature of the cold fluid. The increase in nanoparticle concentration also yielded better performance at the expense of increased pressure drop.     

ICP-AES测定氧化铁球中SiO2等成分

研究了ICP-AES测定氧化铁球(铁皮)中的SiO2、CaO、MgO、Al2O3、P、TiO2等成分的快速分析方法,通过试验确立了样品溶解的最佳条件.实验结果表明,本法快速、简便、准确、可靠.