SiO2纳米流体强化换热的实验和仿真研究

为研究纳米流体稳定性并增强换热机理,在乙二醇/去离子水基液中,采用原液化学生长法制备了不同质量浓度（1%,2%,3%,4%和5%）的氧化硅-乙二醇/水纳米流体,通过Zeta电位测量和透射扫描电镜实验表征纳米流体的稳定性。实验测量并研究了温度和质量浓度对纳米流体的导热系数和粘度的影响。依据实测结果,利用格子玻尔兹曼方法对圆管内纳米流体的流动与换热特性进行数值模拟研究。结果表明：二氧化硅颗粒在基液中具有良好的稳定性;纳米流体的导热系数随温度和质量浓度的提高而增大;纳米流体的加入可以显著提高基液的对流换热系数,当质量浓度为5%时对流换热系数的提高幅度可达到25.5%。     

纳米流体黏度影响因素的试验研究

以SiO2纳米颗粒分别分散在去离子水(DW)、乙二醇(EG)以及两者的混合液中得到的纳米流体为研究对象,研究了颗粒大小、温度、基液成分和体积分数等因素对纳米流体黏度的影响.结果表明:在相同基液的条件下,随着颗粒粒径减小,流体黏度增加;基液中EG体积分数越大,黏度受温度影响越明显;黏度随体积分数变化的规律与修正的K-D模型吻合较好,但当体积分数超过1%时,由于纳米颗粒团聚程度不同,使得以混合液(EG体积分数为50%)为基液的纳米流体的黏度远大于预测值.     

水基石墨烯量子点纳米流体主要物理化学特性研究

在液体中添加固体纳米粒子是提高工质换热能力的一种有效途径,利用超声震荡法将石墨烯量子点扩散于去离子水中制备了水基石墨烯量子点纳米流体,研究其物理稳定性、流变性能、热力学性质、光学性能和腐蚀特性。结果表明：在实验选定的加入量范围内,制备的水基石墨烯量子点纳米流体具有较高的物理稳定性,石墨烯量子点的加入对基液黏度的影响不大;在水中添加少量的石墨烯量子点就能显著地提高流体对太阳辐射能的吸收率,并且显著提高基液的导热系数;与去离子水相比,水基石墨烯量子点纳米流体能加速铜的腐蚀,抑制碳钢的腐蚀,对不锈钢的腐蚀性与去离子水几乎相同。水基石墨烯量子点纳米流体的特性表明,它有潜力成为一种新的流体换热工质。     

石墨烯/丙酮纳米流体振荡热管传热特性研究

通过实验研究了不同质量浓度的石墨烯/丙酮纳米流体振荡热管不同充液率下的传热性能。结果表明,小充液率(45%)下,石墨烯/丙酮纳米流体振荡热管的热阻均小于纯工质丙酮,但烧干现象并没有得到明显改善;中等充液率(62%~70%)下,石墨烯/丙酮纳米流体振荡热管较纯工质丙酮来说不再发生烧干现象,纳米流体振荡热管的热阻随着加热功率的增加而明显降低,浓度为0.01%时具有较为明显的传热优势;大充液率(90%)下,石墨烯/丙酮纳米流体振荡热管的传热性能则普遍优于纯工质,且随着加热功率的增加,传热性能的优势更加明显。     

氧化石墨烯脉动热管传热性能实验研究

通过实验研究以氧化石墨烯分散液为工质的脉动热管的传热性能。实验采用1mg/m L的氧化石墨烯分散液,所得结果与以去离子水为工质的脉动热管传热性能进行比较发现:氧化石墨烯对以去离子水为工质的脉动热管传热性能具有强化作用,但是和脉动热管的加热功率密切相关。在加热功率低于20 W时,氧化石墨烯对脉动热管的强化作用较弱;当加热功率在30~60 W时,氧化石墨烯对脉动热管的强化作用较强,在3.71%~11.33%之间,且强化作用随加热功率的增大呈逐渐增强趋势;但随着功率继续增大,氧化石墨烯的强化作用逐渐减弱,当加热功率达到80 W后,热管传热性能减弱,原因可能是氧化石墨烯颗粒出现了沉降现象。     

石墨烯-铜纳米流体液滴蒸发特性的实验研究

文中针对纯石墨烯、纯铜纳米流体液滴以及石墨烯-铜混合纳米流体液滴在铜基底表面的蒸发特性开展了实验研究,分析了纳米粒子质量分数、石墨烯与铜配比对液滴蒸发过程中接触角和接触直径动态演化以及蒸干后粒子沉积形貌的影响。结果表明:纯石墨烯纳米流体液滴的初始平衡接触角大于纯铜纳米流体液滴、蒸发过程平均接触角小于纯铜纳米流体液滴;纯石墨烯纳米流体液滴蒸干后粒子密集地堆积在中心和边缘位置、边缘形成沉积环、中心形成粒子均布图案,纯铜纳米流体液滴蒸干后形成明显的咖啡环。在混合纳米流体液滴中,随着石墨烯含量的增加,初始平衡接触角增大、蒸发过程平均接触角减小、液滴蒸发速率增大、液滴蒸干后铜基底表面中心区域粒子密度增大。     

石墨烯量子点GQDs纳米流体导热系数模型

综合考虑布朗运动、纳米液膜层、粒子簇、微尺寸效应等多种因素的影响,建立了石墨烯量子点(graphene quantum dot, GQDs)强化基液导热系数计算模型,使用Hot Disk热常数分析仪测量去离子水质量分数分别为0.002%、0.004%、0.006%、0.008%、0.010%的GQDs纳米流体的热导率进行验证,并且用预测模型对更高温度和更高质量分数GQDs纳米流体的导热系数进行了预测。研究表明：模型预测误差不超过2.5%,准确度较高,可以很好地预测不同质量分数GQDs纳米流体在不同温度下的导热系数;GQDs纳米流体由于布朗运动引起的类似对流换热的作用提升了导热系数;而GQDs的添加比例并非越大越好,添加比例过高反而会产生沉降效果,抑制导热系数的提升。     

建立余热回收中纳米工质的导热系数预测模型

采用两步法制备质量分数为1%的Cu/Al2O3-H2O/EG混合纳米流体。首先,研究其导热系数随温度和基液混合比的变化情况。然后,根据多项式回归理论建立Cu/Al2O3-H2O/EG混合纳米流体的导热系数预测模型。实验结果表明,纳米流体的稳定性随乙二醇含量的增大而增强,由于不同种类粒子间的分子吸附力不同,导致相同种类粒子容易结合形成团聚体,而Cu粒子与Al2O3粒子的团聚体则较少。导热系数随着温度的升高非线性升高,随基液中水含量的增大而下降。根据实验数据,拟合了导热系数与温度及基液混合比的多项式预测模型,回归系数R2达0. 998,精度较高可以很好地预测Cu/Al2O3-H2O/EG混合纳米流体的导热系数。该模型可以指导工程应用。     

CuO-ZnO混合纳米流体导热系数影响分析

采用"两步法"制备了质量分数分别为1.0%,2.0%,3.0%,5.0%的CuO-ZnO混合纳米流体,制备过程中不添加分散剂。混合纳米流体选用乙二醇与去离子水作为基液,二者质量比(φv=EG:DW)分别为20:80,40:60,50:50,60:40和80:20,CuO与ZnO的质量比为温度范围从25℃到60℃,研究了不同比例基液、温度和质量分数对纳米流体导热系数的影响。结果表明:导热系数随混合纳米流体质量分数和温度的升高而增大。在温度为60℃时,质量分数为5.0%,基液比例φv为20:80的混合纳米流体导热系数增幅最大为26.1%。混合纳米流体的导热系数随乙二醇比例的增加而降低。实验还发现混合纳米流体导热系数与基液比例呈线性关系。     

石墨烯纳米流体在PV/T系统中的分频性能分析

文章对质量浓度分别为0.02‰,0.05‰,0.1‰的水基石墨烯纳米流体的分频特性进行了研究。采用双光程法测试了光谱透过率,可理论分析石墨烯纳米流体的分频性能。在90 min模拟光照的条件下,分析不同浓度石墨烯纳米流体和水作为分频液时分频型PV/T系统集热效率、电效率和综合效率的变化情况。结果表明：0.02‰石墨烯纳米流体的光谱透过率最高,在400～900 nm的单晶硅电池较佳响应波段,其透过率为66.8%;将取热温度设定为40℃,0.1‰纳米流体达到40℃时,热效率最高达39.7%;水作为PV/T系统分频液时,PV电池输出效率最高为10.9%;同时,利用性能评价函数MF(The Merit Function)来评估分频液对PV/T系统综合性能的影响,当达到取热温度40℃时,0.1‰石墨烯分频液瞬时MF值最高达1.85。     

Performance investigation on a thermal energy storage integrated solar collector system using nanofluid

The effect of Fe nanofluid on the performance enhancement on solar water heater integrated with thermal energy storage system is investigated experimentally. A 0.5% wt fraction of Fe nanoparticle was synthesized with the mixture of water/propylene‐glycol base fluid. The experimental implementation utilized 40‐nm‐size Fe nanoparticle, 15 ° collector tilt angle, and 1.5 kg/min mass flow rate heat‐transfer fluid circulation. The system efficiency reached 59.5% and 50.5% for with and without nanofluid. The water tank temperature was increased by 13 °C during night mode. The average water tank temperature at night mode was 47.5 °C, while the average ambient temperature was 26 °C. The Fe nanofluid improved the system working duration during night mode by an average of 5 h. The techno‐economic analysis results showed a yearly estimated cost savings of 28.5% using the Fe nanofluids as heat transfer fluid. The embodied energy emission rate, collector size, and weight can be reduced by 9.5% using nanofluids. Copyright © 2016 John Wiley & Sons, Ltd.     

Thermo-physical properties of water based SiC nanofluids for heat transfer applications

The aim of current paper consists in the fabrication, characterization and preparation of water based on SiC nanofluids and the experimental investigation of their thermo-physical properties. Thermal conductivity, viscosity and surface tension of SiC/water nanofluids were measured for two two weight concentrations of nanoparticles 0.5 and 1.0 wt% respectively, within the range 20 °C to 50 °C. Concerning the thermal-properties of studied nanofluids, the experimental results show that the thermal conductivity increases with the increasing both of the weight concentration of the nanoparticles and temperature. Also, the dynamic viscosity of the SiC/water nanofluids increases with increasing nanoparticles concentration and decreases with the increasing temperature. Furthermore, the surface tension of studied nanofluids increases with the increase of the weight concentrations of the nanoparticles, but the results show that at a concentration of the nanoparticles of 0.5 wt%, the surface tension was lower than the surface tension of the water, while at 1.0 wt% nanoparticles, the surface tension of the nanofluids was close to the surface tension of the water. Measurements were compared with experimental data available in literature and theoretical models. Finally, SiC/water nanofluids were used as working fluid inside of the two-phase closed thermosyphon in order to study of the heat transfer from point of view both of operating temperature and the nanoparticles concentration.     

Heat Transfer and Pressure Drop Characteristics of Dilute Alumina–Water Nanofluids in a Pipe at Different Power Inputs

This work addresses the effect of temperature on the thermophysical properties (i.e., density, viscosity, thermal conductivity, and specific heat capacity) of alumina–water nanofluid over a wide temperature range (25°C–75°C). Low concentrations (0–0.5% v/v) of alumina nanoparticles (40 nm size) in distilled water were used in this study. The pressure drop and the effective heat transfer coefficient of nanofluids were also estimated for different power inputs and at different flow rates corresponding to Reynolds numbers in the range of 1500–6000. The trends in variation of thermophysical properties of nanofluids with temperature were similar to that of water, owing to their low concentrations. However, the density, viscosity, and thermal conductivity of nanofluids increased, while the specific heat capacity decreased with increasing the nanoparticle concentration. The convective heat transfer coefficient of the nanofluid and the pressure drop along the test section increased with increasing the particle concentration and flow rate of nanofluid. Results showed that the heat transfer coefficient increases, while the pressure drop decreases slightly with increasing the power input. This is because of the fact that increasing power input to heater increases the bulk mean temperature of nanofluids, resulting in a decreased viscosity. The prepared nanofluids were found to be more effective under turbulent flow than in transition flow.     

Thermal Physics and Critical Heat Flux Characteristics of Al2O3–H2O Nanofluids

This study investigates the influence of the thermal physics of nanofluids on the critical heat flux (CHF) of nanofluids. Thermal physics tests of nanoparticle concentrations ranged from 0 to 1 g/L. Pool boiling experiments were performed using electrically heated NiCr metal wire under atmospheric pressure. The results show that there was no obvious change for viscosity and a maximum enhancement of about 5 to 7% for thermal conductivity and surface tension with the addition of nanoparticles into pure water. Consistently with other nanofluid studies, this study found that a significant enhancement in CHF could be achieved at modest nanoparticle concentrations (<0.1 g/L by Al₂O₃ nanoparticle concentration). Compared to the CHF of pure water, an enhancement of 113% over that of nanofluids was found. Scanning electron microscope photos showed there was a nanoparticle layer formed on the heating surface for nanofluid boiling. The bubble growth was photographed by a camera. The coating layer makes the nucleation of vapor bubbles easily formed. Thus, the addition of nanoparticles has active effects on the CHF.     

Molecular dynamics simulation on the effect of nanoparticles on the heat transfer characteristics of pool boiling

Molecular dynamics simulation was performed to investigate pool boiling of nanofluids on the metal wall. Nanoparticles were placed near the wall. Results showed that with the addition of nanoparticles the fluid temperature, net evaporation number and heat flux were increased, indicating that the boiling heat transfer was enhanced. In addition, the nanoparticles were able to move around the wall disorderly but did not move with the fluid. The effects of heated temperature and nanoparticle size on the boiling heat transfer were also investigated. By increasing heated temperature and nanoparticle size, the boiling heat transfer enhancement increased.     

Effect of particle concentration,temperature and surfactant on surface tension of nanofluids

Heat transfer performance with nanofluids depends on the thermo physical properties of the suspension. Surface tension is an important property for heat transfer calculation. In this paper, various parameters that effect on the surface tension of nanofluids such as nanofluid preparation method, effect of volume fraction, temperature, and surfactants on nanofluids have been studied. Additionally, precise assessments on the theoretical correlations related to the surface tension of nanofluids have also been included. Based on the existing experimental results, surface tension augments respectively with volume fraction intensification. Surface tension of nanofluids decreases accordingly with the increase of temperature and surfactant concentration. Nevertheless, there have been some contradictory results on the effect of volume fraction and surfactant on surface tension of nanofluids.     

Effect of nanofluids on the thermal performance of a flat micro heat pipe with a rectangular grooved wick

In this paper, the effect of water-based Al₂O₃ nanofluids as working fluid on the thermal performance of a flat micro-heat pipe with a rectangular grooved wick is investigated. For the purpose, the axial variations of the wall temperature, the evaporation and condensation rates are considered by solving the one-dimensional conduction equation for the wall and the augmented Young–Laplace equation for the phase change process. In particular, the thermophysical properties of nanofluids as well as the surface characteristics formed by nanoparticles such as a thin porous coating are considered. From the comparison of the thermal performance using both DI water and nanofluids, it is found that the thin porous coating layer formed by nanoparticles suspended in nanofluids is a key effect of the heat transfer enhancement for the heat pipe using nanofluids. Also, the effects of the volume fraction and the size of nanoparticles on the thermal performance are studied. The results shows the feasibility of enhancing the thermal performance up to 100% although water-based Al₂O₃ nanofluids with the concentration less than 1.0% is used as working fluid. Finally, it is shown that the thermal resistance of the nanofluid heat pipe tends to decrease with increasing the nanoparticle size, which corresponds to the previous experimental results.     

Evaluating the Optical Properties of TiO2 Nanofluid for a Direct Absorption Solar Collector

Recent studies specify that designated nanofluids may increase the proficiency of direct absorption solar thermal collectors. To determine the efficiency of nanofluids in solar applications, their capability to change light energy to thermal energy must be identified (i.e., the absorption spectrum of the solar material). In view of that, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (200– 1100 nm). In the first decade of nanofluid research, most of the focus was on measuring and modeling the fundamental thermophysical properties of nanofluids (i.e., thermal conductivity, density, viscosity, and convection coefficients). Lately, considerable focus is given to the fundamental optical properties of nanofluids. However, the effect of particle size, shape, and volume fraction of nanoparticles as well as alternation of the base fluids, which can significantly affect scattering and absorption, have not been addressed to date in the literature. In this study, the effects of size and concentration of TiO₂ nanoparticles on the extinction coefficient were analyzed using the Rayleigh approach. The results show that smaller particle size (<20 nm) has a nominal effect on the optical properties of nanofluids. Volume fraction is linearly proportionate to the extinction coefficient. Considering a nanoparticle size of 20 nm, almost 0% transmissivity is obtained for wavelengths ranging from 200 to 300 nm. However, a sudden increase of 71% in transmissivity is noted from 400 nm, gradually increasing to 88% and becoming similar to that of water at 900 nm. Promising results are observed for volume fractions below 0.1%.     

A new empirical correlating equation for calculating effective viscosity of nanofluids

In this paper, an empirical correlation for the nanofluid viscosity is proposed. The new equation for the nanofluid effective dynamic viscosity, normalized by the dynamic viscosity of the base liquid, is derived from a wide selection of experimental data available in the literature. This correlation presented the viscosity of the nanofluid as a function of the base fluid viscosity, nanoparticle volume fraction, nanoparticle diameter, nanoparticle temperature, and mass density of the base fluid. The new correlation was evaluated against 898 experimental data for the viscosities of nanofluid collected from the literature. The experimental data included different working nanofluids, such as alumina, Iron, and silica, where the diameter of nanoparticles was ranging between 10 and 350 nm, suspended in water, propylene glycol, and kerosene. The predicted results were then compared with many other published experimental results for different nanofluids and very good concordance between these results was observed. In general, this correlation has higher accuracy and precision.     

The Viscosity of Nanofluids: A Review of the Theoretical,Empirical, and Numerical Models

The enhanced thermal characteristics of nanofluids have made it one of the most raplidly growing research areas in the last decade. Numerous researches have shown the merits of nanofluids in heat transfer equipment. However, one of the problems is the increase in viscosity due to the suspension of nanoparticles. This viscosity increase is not desirable in the industry, especially when it involves flow, such as in heat exchanger or microchannel applications where lowering pressure drop and pumping power are of significance. In this regard, a critical review of the theoretical, empirical, and numerical models for effective viscosity of nanofluids is presented. Furthermore, different parameters affecting the viscosity of nanofluids such as nanoparticle volume fraction, size, shape, temperature, pH, and shearing rate are reviewed. Other properties such as nanofluid stability and magnetorheological characteristics of some nanofluids are also reviewed. The important parameters influencing viscosity of nanofluids are temperature, nanoparticle volume fraction, size, shape, pH, and shearing rate. Regarding the composite of nanofluids, which can consist of different fluid bases and different nanoparticles, different accurate correlations for different nanofluids need to be developed. Finally, there is a lack of investigation into the stability of different nanofluids when the viscosity is the target point.