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。     

低体积分数水基SiO2纳米流体沸腾换热关键物性参数研究

在不添加任何分散剂和改变pH值的情况下,通过两步法将比表面积为150 m~2/g的气相SiO_2纳米颗粒制备成均匀稳定、透明度高、分散性能好的纳米流体。并对该功能性纳米流体进行了导热系数、黏度、表面张力和壁面接触角的测量。低体积分数下,功能性纳米流体较基液的导热系数几乎没有变化,但黏度却有较大改变。传统固液两相混合物黏度模型不再适用功能性纳米流体的计算,其主要原因是传统公式低估了分子间作用力对纳米流体黏度的影响。因此,建立了功能性纳米流体的黏度经验公式。由于纳米颗粒的存在提高了沸腾表面的粗糙度,从而使纳米流体的壁面湿润性能大大提高。实验结果表明,纳米流体的黏性和壁面接触角是沸腾换热发生骤变的关键。     

纳米流体在车用机油冷却器中的强化换热试验研究

为了探求新型冷却介质--纳米流体的换热效果,制备了不同粒子体积分数的氧化铝有机纳米流体,并在车用机油冷却器中进行了换热性能的试验研究.研究结果表明:添加纳米粒子能够有效提高纳米流体基础液体的换热能力,且换热能力随着粒子体积分数的增加 而增高.在不同温度和温差条件下,粒子体积分数为5%的纳米流体的传热量和换热系数均超过常规冷却介质(水和防冻液).纳米流体的黏度和流动阻力亦随着粒子体积分数增加而增加.当冷、热介质的进口温差不变时,提高冷却介质的进口温度能在明显增强换热能力的同时大幅度降低流动阻力,并且纳米流体换热能力的增幅要高于防冻液和基础液体.     

纳米流体Rayleigh-Bénard细胞流的格子-Boltzmann模拟

采用热格子-Boltzmann方法,对两平行平板间纳米流体的Rayleigh-Bénard细胞流现象及其影响因素进行数值模拟研究,模型计算结果与方腔内纳米流体的自然对流实验结果吻合较好。讨论了纳米颗粒种类(Al2O3、Cu和SiO2)、体积分数(1%～4%)等因素对细胞流流动和传热的影响。研究发现平板间纳米流体的Rayleigh-Bénard细胞流的流动和传热行为明显异于纯流体,同时发现纳米流体的热导率、黏度和颗粒的热运动共同决定细胞流的流场和温度场分布的规律:热导率越大、布朗运动的越强,会减小流体温度的非均匀性,削弱对流作用,使其细胞流对涡个数减小;黏度增大也会抑制细胞流的发展。     

Cu-H_2O纳米流体流动与强化换热的数值模拟研究

模拟纳米流体在三维管道中的流动和强化传热过程,运用数值计算方法研究纳米流体的流动特性和传热机理,探究不同纳米颗粒体积分数和不同纳米颗粒大小在不同雷诺数(Re)下对纳米流体的流动和传热特性的影响。基于DPM模型对纳米流体在圆管中的对流换热进行了数值模拟研究,研究结果表明,在一定范围内,每增加0.5%的体积分数,纳米流体的传热性能平均增强7.82%。随着纳米颗粒的减小,纳米流体的传热系数不断增加。     

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。     

活塞内冷油腔中纳米流体振荡流动与传热特性影响因素研究

采用计算流体力学的方法,研究了分别含Al₂O₃、Cu O、Si O₂的3种纳米机油在纳米颗粒体积分数为1%、3%、5%时相对于传统机油的振荡传热能力和机油在油腔内流动的规律。结果表明,纳米颗粒的加入改变了流体的物性参数,纳米流体的传热效果比传统机油更好,且内冷油腔的传热系数随着纳米流体体积分数的增加而增加,但对内冷油腔内瞬态机油的瞬态分布和充油率的影响不大;纳米流体的黏度、密度、导热系数、比热容都能影响内冷油腔的传热性能,密度的增加会使流体对壁面的冲击作用更强,从而增强油腔的传热能力;在纳米颗粒体积分数为5%时CuO纳米机油的传热系数比Al₂O₃、SiO₂纳米机油分别高8.2%和14.6%。     

多孔泡沫内纳米流体非平衡传热及体积分数分布

研究了纳米流体在金属泡沫内的对流换热,建立了局部非热平衡数学模型,得到了金属泡沫内纳米流体速度、温度和纳米颗粒体积分数分布,分析了纳米流体和金属泡沫的强化换热效果。当使用纳米流体或在通道内填充金属泡沫时,截面速度和温度变得更均匀。随着纳米颗粒体积分数的增大,努塞尔数先增大然后又逐渐减小,即存在一个合适的体积分数能使换热效果达到最好;当金属泡沫孔隙率增加时努塞尔数也会减小,有利于换热的进行。纳米流体和金属泡沫对换热具有明显强化作用,但压降随纳米颗粒体积分数增大而急剧增大。此外,还考虑了布朗扩散和热泳扩散等因素的影响。     

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%。     

多壁碳纳米管-乙二醇纳米流体集热性能研究

为研究多壁碳纳米管-乙二醇纳米流体(MWCNTs-EG)的集热能力,采用单层玻璃管作为集热管,对6种不同体积分数、不同颗粒粒径的MWCNTs-EG流体和水进行了闷晒集热实验。测试表明,相同体积分数的MWCNTs-EG流体的集热温度随着颗粒粒径的增大而增大,其瞬时集热量最高可达54.2k J,是水集热量的2.4倍。其中,MWCNTs-EG(30~50nm,0.06vol%)的温升最为明显,最大平均值是53.8℃,温升为22.7℃。此外,MWCNTs-EG的体积分数、颗粒粒径以及其团聚沉降均能影响着纳米流体的集热效果。     

Experimental investigation on the thermal conductivity and shear viscosity of viscoelastic-fluid-based nanofluids

Viscoelastic-fluid-based nanofluids with dispersion of copper (Cu) nanoparticles in viscoelastic surfactant solution (aqueous solution of cetyltrimethylammonium chloride/sodium salicylate) were prepared. A comparative study of thermal conductivity and viscosity between viscoelastic-fluid-based Cu nanofluids and distilled water based nanofluids was then performed experimentally. Different concentrations of viscoelastic base fluid and volume fraction of Cu nanoparticles were matched in order to check their influences on fluid’s thermal conductivity and viscosity. The experimental results show that the viscoelastic-fluid-based Cu nanofluids have a higher thermal conductivity than viscoelastic base fluid, and its thermal conductivity increases with increasing temperature and increasing particle volume fraction. Furthermore, the viscoelastic-fluid-based Cu nanofluid shows a non-Newtonian behavior in its viscosity, and the viscosity increases with the increase of Cu nanoparticle concentration and decrease of temperature.     

Thermal Conductivity and Viscosity of Nanofluids Having Nanoencapsulated Phase Change Material

In this study, the thermal conductivity and viscosity of nanofluids, composed of a base fluid and nanoencapsulated phase change material (NEPCM), were investigated experimentally. The NEPCM was prepared by the encapsulation of n-nonadecane as phase change material with diethylenetriamine and toluene-2,4-diisocyanate using interfacial polymerization method. The NEPCM was characterized using scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and differential scanning calorimetry (DSC) analyses. In the preparation of the nanofluids containing NEPCM, two different base fluids, water and ethylene glycol (EG), were used. The concentration of NEPCM and the working temperature were selected as the main parameters. It was found that the viscosity of the nanofluids decreases with increasing temperature and increases with increasing solid concentration. The viscosity was also expressed as a function of the solid concentration and temperature. The thermal conductivity of the nanofluids was found to increase with increasing temperature. Thermal conductivity exhibited an increasing tendency with increasing solid concentration, but the changes in thermal conductivity according to base fluid are in the range of uncertainty of the measurement for both nanofluids with a solid volumetric fraction lower than 1.68%.     

Heat transfer enhancement in microchannel heat sinks using nanofluids

Heat transfer enhancement in a 3-D microchannel heat sink (MCHS) using nanofluids is investigated by a numerical study. The addition of nanoparticles to the coolant fluid changes its thermophysical properties in ways that are closely related to the type of nanoparticle, base fluid, particle volume fraction, particle size, and pumping power. The calculations in this work suggest that the best heat transfer enhancement can be obtained by using a system with an Al₂O₃–water nanofluid-cooled MCHS. Moreover, using base fluids with lower dynamic viscosity (such as water) and substrate materials with high thermal conductivity enhance the thermal performance of the MCHS. The results also show that as the particle volume fraction of the nanofluid increases, the thermal resistance first decreases and then increases. The lowest thermal resistance can be obtained by properly adjusting the volume fraction and pumping power under given geometric conditions. For a moderate range of particle sizes, the MCHS yields better performance when nanofluids with smaller nanoparticles are used. Furthermore, the overall thermal resistance of the MCHS is reduced significantly by increasing the pumping power. The heat transfer performance of Al₂O₃–water and diamond–water nanofluids was 21.6% better than that of pure water. The results reported here may facilitate improvements in the thermal performance of MCHSs.     

Experimental determination of thermal conductivity of three nanofluids and development of new correlations

Experimental investigations have been carried out for determining the thermal conductivity of three nanofluids containing aluminum oxide, copper oxide and zinc oxide nanoparticles dispersed in a base fluid of 60:40 (by mass) ethylene glycol and water mixture. Particle volumetric concentration tested was up to 10% and the temperature range of the experiments was from 298 to 363 K. The results show an increase in the thermal conductivity of nanofluids compared to the base fluids with an increasing volumetric concentration of nanoparticles. The thermal conductivity also increases substantially with an increase in temperature. Several existing models for thermal conductivity were compared with the experimental data obtained from these nanofluids, and they do not exhibit good agreement. Therefore, a model was developed, which is a refinement of an existing model, which incorporates the classical Maxwell model and the Brownian motion effect to account for the thermal conductivity of nanofluids as a function of temperature, particle volumetric concentration, the properties of nanoparticles, and the base fluid, which agrees well with the experimental data.     

Simultaneous Measurement of Thermal Conductivity,Thermal Diffusivity,and Specific Heat of Nanofluids

Effective thermal conductivity, effective thermal diffusivity, and effective specific heat of nanofluids were simultaneously measured by using a transient double hot-wire technique. Several types of nanofluids were prepared by suspending different volume percentages (1 to 5%) of titanium dioxide (TiO₂), aluminum oxide (Al₂O₃), and aluminum (Al) nanoparticles in ethylene glycol and engine oil. While effective specific heats of these nanofluids decrease substantially with nanoparticle volume fraction, the enhanced effective thermal conductivity and effective thermal diffusivity were found to increase significantly with increasing volumetric loading of these nanoparticles. The increments of the effective thermal diffusivity of nanofluids were slightly larger than their effective thermal conductivity values. Predictions of the effective specific heats of nanofluids by the volume fraction mixture rule-based model showed fairly good agreement (within 7%) with the experimental results. Besides particle volume fraction, particle material, particle shape and the type of base fluid were identified to have influence on these properties of nanofluids. Both the calibration results of the base fluids (system accurate to ≤2.7%) and uncertainty analysis (uncertainty ≤2.1%) indicate high accuracy of using the double hot-wire method to simultaneously measure the effective thermal conductivity, effective thermal diffusivity, and specific heat of nanofluids.     

Buoyancy-driven heat transfer of water-based Al2O3 nanofluids in a rectangular cavity

In this paper, thermal characteristics of natural convection in a rectangular cavity heated from below with water-based nanofluids containing alumina (Al₂O₃ nanofluids) are theoretically investigated with Jang and Choi’s model for predicting the effective thermal conductivity of nanofluids and various models for the effective viscosity. To validate theoretical results, we compare theoretical results with experimental results presented by Putra et al. It is shown that the experimental results are put between a theoretical line derived from Jang and Choi’s model and Einstein’s model and a theoretical line from Jang and Choi’s model and Pak and Cho’s correlation. In addition, the effects of the volume fraction, the size of nanoparticles, and the average temperature of nanofluids on natural convective instability and heat transfer characteristics of water-based Al₂O₃ nanofluids in a rectangular cavity heated from below are theoretically presented. Based on the results, this paper shows that water-based Al₂O₃ nanofluids is more stable than base fluid in a rectangular cavity heated from below as the volume fraction of nanoparticles increases, the size of nanoparticles decreases, or the average temperature of nanofluids increases. Finally, we theoretically show that the ratio of heat transfer coefficient of nanofluids to that of base fluid is decreased as the size of nanoparticles increases, or the average temperature of nanofluids is decreased.     

The effect of particle size and base liquid on thermo-physical properties of ethylene and diethylene glycol based copper micro- and nanofluids

Nanofluid (NF) is a fluid containing nanometer-sized particles. The present work investigates, experimentally and theoretically, on fabrication and thermo-physical properties evaluation of ethylene glycol and diethylene glycol (EG/DEG) based nanofluids/microfluids (NFs/MFs) containing copper nanoparticles/microparticles (NPs/MPs) with focus on the effect of the particle size and the base liquid. A series of stable Cu NFs and MFs with various NP/MP concentration (1, 2 and 3 wt%) were fabricated by dispersing Cu NPs and Cu MPs in EG and DEG as the base liquids. The physicochemical properties of Cu NFs and MFs were analyzed by various techniques including X-Ray diffraction (XRD), Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Dynamic Light Scattering (DLS). The thermo-physical properties including thermal conductivity (TC) and viscosity of EG/DEG based Cu NFs/MFs were measured at 20 to 40 °C. The results for TC and viscosity of EG based Cu NF/MFs were compared to the same NFs/MFs with DEG base liquid with focus on the impact of the particle size as well as the base liquid. The experiments showed that EG based NFs/MFs exhibit more favorable characteristics than that of DEG based ones. Moreover, NFs with Cu NPs revealed higher TC than those MFs containing Cu MPs at the same particle concentration and temperature (effect of NP size). As the best result, a TC enhancement of ~ 4.7% was achieved for EG based NF with 3 wt% Cu NP while maximum increase in viscosity of ~ 1.8% was observed for the same NF at 20 °C. To compare the experimental results with the estimated values, Maxwell predictive correlation and Corcione model were employed while Einstein equations as well as Kriger-Dougherty correlation were applied for TC and viscosity of NFs/MFs, respectively.     

Convective Heat Transfer and Fluid Dynamic Characteristics of SiO2 Ethylene Glycol/Water Nanofluid

Nanofluids comprised of silicon dioxide (SiO₂) nanoparticles suspended in a 60:40 (% by weight) ethylene glycol and water (EG/water) mixture were investigated for their heat transfer and fluid dynamic performance. First, the rheological properties of different volume percents of SiO₂ nanofluids were investigated at varying temperatures. The effect of particle diameter (20 nm, 50 nm, 100 nm) on the viscosity of the fluid was investigated. Subsequent experiments were performed to investigate the convective heat transfer enhancement of nanofluids in the turbulent regime by using the viscosity values measured. The experimental system was first tested with EG/water mixture to establish agreement with the Dittus-Boelter equation for Nusselt number and with Blasius equation for friction factor. The increase in heat transfer coefficient due to nanofluids for various volume concentrations has been presented. Pressure loss was observed to increase with nanoparticle volume concentration. It was observed that an increase in particle diameter increased the heat transfer coefficient. Typical percentage increases of heat transfer coefficient and pressure loss at fixed Reynolds number are presented.     

Experimental Studies on Convective Heat Transfer and Pressure Drop Characteristics of Metal and Metal Oxide Nanofluids Under Turbulent Flow Regime

This study aims at achieving improved heat transfer potential for a tube-in-tube counterflow heat exchanger using silver/water (Ag) and copper oxide/water (CuO) nanofluids. Heat transfer and rheological characteristics of prepared nanofluids containing spherical nanoparticles of average size of 12–64 nm were experimentally investigated. Based on the experimental results, for increase in nanoparticles volume proportion up to 1.0%, convective heat transfer coefficient and Nusselt number were augmented by 52% and 47.5% for Ag nanofluids, and 27.6% and 24.3% for CuO nanofluids, respectively, while compared with base fluid. Also, at higher Reynolds number typically around 7500 the heat transfer potential was also increased considerably. However, increased particle loading eventually increased the density and dynamic viscosity of nanofluids, which increased frictional pressure drop from 4% to 22.2%. The increased heat transfer rate at increased particle concentration was attributed to the thermal conductivity and diffusion of nanoparticles in fluid medium, whereas increased pressure drop was largely influenced by density and dynamic viscosity of nanofluids. Furthermore, the new correlations developed in this study based on the experimental results revealed that the mean deviation between experimental and predicted Nusselt number varied from +6% to –12% and from +3% to –7%, respectively, for Ag and CuO nanofluids.     

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.