固相纳米复合材料的导热系数预测

研究了分散剂和纳米颗粒对固相纳米复合材料导热系数的影响。假设纳米颗粒在纳米流体中均匀分布,构建了一个考虑纳米颗粒尺度和分散介质影响的物理分析模型;并由此利用最小热阻力法则和比等效导热系数相等法则,建立了一个固相纳米复合材料的导热系数理论模型。计算结果表明,颗粒的体积分数和导热系数、以及分散剂导热系数的增大,都会引起复合材料导热系数的增大。     

极低体积分数(Au-H2O)纳米流体导热机理研究

纳米流体热物性的研究作为一门新兴交叉学科一直受到关注,然而至今仍没有理论能够准确解释AuH2O这类体积分数极低的纳米流体导热系数极大增强的现象。因此,在前人理论的基础上,提出一种新的算法模型:利用分形理论模拟纳米颗粒分布来解释团聚物对纳米流体导热系数的影响;利用微对流模型以及颗粒扩散修正因子来还原导热系数的动态项。该算法模型充分考虑了团聚、颗粒分布、布朗运动形成的微对流、温度对颗粒和基液分子布朗运动的影响以及颗粒扩散等因素对纳米流体导热系数的影响,能够准确预测出Au-H2O纳米流体导热系数增强的趋势,理论预测值与绝大部分现有实验数据最大偏差不超过1.5%。研究发现,对这类极低浓度纳米流体而言,温度对其影响大于体积分数和粒径的影响,且呈指数形式增长。     

pH对水合Al2O3纳米流体的稳定性和导热性的影响

利用两步法制备了Al2O3水合纳米流体，测试了不同pH值下，不同体积分数的～203纳米流体的zeta电位和导热系数，研究pH值对Al2O3．H20纳米流体稳定性和导热性的影响，实验结果表明：对不同体积分数的Al2O3纳米流体，pH存在一个优化值对应zeta电位的绝对值最大，而电位的绝对值越大，纳米颗粒在流体中分散稳定性越好，因此对Al2O3纳米颗粒来说，要得到稳定分散的纳米流体pH值应在8～9之间为最佳；同样随着pH的变化，也存在一个最优值对应于Al2O3纳米流体的导热系数增加到一个最大的值，并且导热系数随纳米粒子体积分数增加而增大。实验结果表明纳米流体的稳定性和导热性与pH有密切的关系，此结论与文献[6]和[13]中的结论一致。     

SiO_2-EG纳米流体的制备及热物性能

将平均粒径为30nm和50nm的SiO2纳米颗粒添加到EG(乙二醇)的基液中制成SiO2-EG纳米流体,利用粒度分析法、吸光度分析法和Zeta电位分析法分析其悬浮稳定性,当超声时间为3h时,其分散性和稳定性最好。对SiO2-EG纳米流体的热物性能参数如黏度、表面张力、导热系数,在不同体积分数、粒度和温度条件下进行测试分析表明,SiO2-EG纳米流体的黏度高于基液的黏度,表面张力低于基液的表面张力,导热系数高于基液的导热系数。在常温下,体积分数为0.5%时,30nm和50nm两种粒径的SiO2-EG纳米流体的粘度比EG粘度分别提高了15.41%和13.17%,表面张力系数比EG分别降低了3.23%和2.54%,导热系数分别提高38.36%和31.47%。进一步研究发现温度的升高导致SiO2-EG纳米流体的粘度和表面张力系数均有所降低,而导热系数却增加。     

纳米流体强化对流换热的实验研究

建立了纳米流体对流换热系数的实验测试系统,利用实验系统测量YCu-H2O纳米流体的对流换热系数,探讨了纳米颗粒质最分数、Re数和轴向比对Cu-水纳米流体对流换热性能的影响.结果表明:Cu-水纳米流体的对流换热系数随纳米粒子质量分数的增人而增人,但其对流换热系数的增加明显低于导热系数的增加.随着雷诺数的增人,纳米流体的对流换热系数基本呈线性提高.纳米流体在实验管进口段的对流换热系数提岛值明显高于流体在充分发展段的提高值.纳米流体的导热系数、粘度和纳米颗粒迁移是影响纳米流体对流换热系数的主要因素.     

Al2O3纳米粉体悬浮液热物性实验研究

分别采用瞬态热线法、比较量热法和旋转粘度计测试了不同温度、粒子浓度和粒径下的Al2O3-DW（蒸馏水）纳米流体的导热系数、比热容、粘度等热物性参数。试验结果表明,粒子浓度、粒径和温度都是影响Al2O3-DW纳米流体热物性参数的重要因素。与水相比,纳米流体导热系数和粘度增加,常温4％体积份额下增幅分别为21．5％和52．3％;纳米流体比热容随着粒子体积份额增加而降低,并推导出了常温下低浓度纳米流体比热容的预测公式。     

Cu-水纳米流体的分散行为及导热性能研究

通过测定Cu-水纳米悬浮液的Zeta电位和吸光度,采用Hotdisk热物性分析仪测量了其导热系数,探讨了不同pH值和分散剂浓度对Cu-水纳米悬浮液分散稳定性和导热性能的影响.结果表明,pH值和分散剂加入量是影响Cu-水纳米悬浮液分散稳定和导热系数的重要因素.最优化的pH值和分散剂加入量能显著提高水溶液中Cu表面Zeta电位绝对值,增大了颗粒间静电排斥力,悬浮液分散稳定性较好,导热系数较高.从分散稳定和导热系数提高两个方面来考虑,pH=9.5左右被选为最优化值,在0.1%Cu-H2O纳米流体中,0.07%SDBS被选为最优化浓度.另外,Cu-水纳米流体的导热系数随纳米粒子质量分数的增大而增大,呈非线性关系,且比现有理论(Hamilton-Crosser模型)预测值大.     

二元混合表面活性剂对纳米流体热物性影响

采用两步法制备了添加二元混合表面活性剂的氧化锌纳米流体，纳米颗粒的体积浓度为0.398%～2.292%。XRD、TEM对氧化锌纳米粒子进行表征，吸光度法和沉降法分析了纳米流体稳定性，之后研究15～55℃时，制备流体的导热与黏度，并与添加单一表面活性剂的纳米流体进行对比。实验结果显示，添加SDS/CTAB纳米流体稳定性更优。纳米流体导热系数随温度及纳米粒子体积浓度的增加而增大，55℃,2.292%的纳米流体热导率增强最大，提高了38%,且热导率增强作用明显优于单一表面活性剂的添加。纳米流体黏度随温度及纳米粒子体积浓度的增加而逐渐降低，在55℃,2.292%时拥有最小黏度0.645 mPa·s。与单一表面活性剂相比，SDS/CTAB的添加能有效降低纳米流体黏度，对减小纳米流体黏度有积极作用。     

影响纳米流体导热系数实验测试的因素

导热系数是衡量纳米流体强化换热的最重要的参数,但在不同学者的研究中,对于同一种纳米流体所测得的导热系数却有很大差别。本文针对影响纳米流体导热系数实验测量的因素进行研究,在相同的实验条件下,分别运用Hotdisk导热仪和闪光法导热仪对水基二氧化钛纳米流体的导热系数进行了测量。实验结果表明,用Hotdisk法的测量结果比用闪光法测得的高21%～34%。通过计算分析发现:自然对流是引起纳米流体导热系数测量结果多变的重要原因之一。     

Ag-TiO_2纳米悬浮液的制备及其导热性与黏性

为了提高TiO_2纳米流体的导热性能,将Ag纳米颗粒与TiO_2纳米颗粒混合,利用两步法制备出不同银钛物质的量比的Ag-TiO_2纳米悬浮液,并研究其导热性能与黏性。采用X射线衍射仪和扫描电子显微镜表征Ag-TiO_2纳米颗粒晶型和形貌;通过沉降法、激光粒度分析和吸光度测量法探究Ag-TiO_2纳米悬浮液的稳定性。结果表明:Ag纳米颗粒含量低有利于Ag-TiO_2纳米悬浮液稳定;质量分数为0. 3%的银钛物质的量的比为1∶6、1∶4、1∶2的Ag-TiO_2悬浮液,导热系数在常温下比TiO_2悬浮液导热系数分别增加了0. 83%、1. 66%、2. 33%,而黏度与基液水相比分别增加了37. 5%、27. 5%、22. 5%;银钛物质的量比为1∶2的Ag-TiO_2纳米悬浮液导热系数高于银钛物质的量比为1∶6和1∶4悬浮液的,而其黏度却低于银钛物质的量比为1∶6和1∶4悬浮液的。     

An effective thermal conductivity model of nanofluids with a cubical arrangement of spherical particles

The theoretical investigation of the effective thermal conductivities of nanofluids, a new class of solid-liquid suspensions, is important in both predicting and designing nanofluids with effective thermal conductivities. We have developed a new thermal conductivity model for nanofluids that is based on the assumption that monosized spherical particles are uniformly dispersed in the liquid and are located at the vertexes of a simple cubic lattice, with each particle surrounded by a liquid layer having a thermal conductivity that differs from that of the bulk liquid. This model nanofluid with a cubical arrangement of nanoparticles gives a more practical upper limit of thermal conduction than a model nanofluid with a parallel arrangement of nanoparticles. The new model unexpectedly shows a nonlinear relationship of thermal conductivity with particle concentration, whereas the conductivity-concentration curve changes from convex upward to concave upward with increasing volume concentration. The effects of particle and layer parameters on the effective thermal conductivities are also analyzed. A comparison of predicted thermal conductivity values and experimental data shows that the predicted values are much higher than the experimental data, a finding that indicates that there is a potential to further improve the effective thermal conductivities of nanofluids with more uniformly dispersed particles.     

Experimental Study on the Effective Thermal Conductivity and Thermal Diffusivity of Nanofluids

This paper reports measurements of the effective thermal conductivity and thermal diffusivity of various nanofluids using the transient short-hot-wire technique. To remove the influences of the static charge and electrical conductance of the nanoparticles on measurement accuracy, the short-hot-wire probes are carefully coated with a pure Al₂O₃ thin film. Using distilled water and toluene as standard liquids of known thermal conductivity and thermal diffusivity, the length and radius of the hot wire and the thickness of the Al₂O₃ film are calibrated before and after application of the coating. The electrical leakage of the short-hot-wire probes is frequently checked, and only those probes that are coated well are used for measurements. In the present study, the effective thermal conductivities and thermal diffusivities of Al₂O₃/water, ZrO₂/water, TiO₂/water, and CuO/water nanofluids are measured and the effects of the volume fractions and thermal conductivities of nanoparticles and temperature are clarified. The average diameters of Al₂O₃, ZrO₂, TiO₂, and CuO particles are 20, 20, 40, and 33 nm, respectively. The uncertainty of the present measurements is estimated to be within 1% for the thermal conductivity and 5% for the thermal diffusivity. The measured results demonstrate that the effective thermal conductivities of the nanofluids show no anomalous enhancement and can be predicted accurately by the model equation of Hamilton and Crosser, when the spherical nanoparticles are dispersed into fluids.     

Mechanisms and models of effective thermal conductivities of nanofluids

The physical mechanisms and mathematical models of the effective thermal conductivities of nanofluids have long been of interest to the nanofluid research community because the effective thermal conductivities of nanofluids cannot generally be fully explained and predicted by classical effective medium theories. This review article summarizes considerable progress made on this topic. Specifically, the physical mechanisms and mathematical models of the effective thermal conductivities of nanofluids are reviewed, the potential contributions of those physical mechanisms are evaluated, and the comparisons of the theoretical predictions and experimental data are presented along with opportunities for future research.     

Evidence for enhanced thermal conduction through percolating structures in nanofluids

The unusually large enhancement of thermal conductivity (k/k(f)～4.0, where k and k(f) are the thermal conductivities of the nanofluid and the base fluid, respectively) observed in a nanofluid containing linear chain-like aggregates provides direct evidence for efficient transport of heat through percolating paths. The nanofluid used was a stable colloidal suspension of magnetite (Fe(3)O(4)) nanoparticles of average diameter 6.7?nm, coated with oleic acid and dispersed in kerosene. The maximum enhancement under magnetic field was about 48φ (where φ is the volume fraction). The maximum enhancement is observed when chain-like aggregates are uniformly dispersed without clumping. These results also suggest that nanofluids containing well-dispersed nanoparticles (without aggregates) do not exhibit significant enhancement of thermal conductivity. Our findings offer promising applications for developing a new generation of nanofluids with tunable thermal conductivity.     

Particle shape effect on the viscosity and thermal conductivity of ZnO nanofluids

The viscosity and thermal conductivity of ZnO nanofluids with nanoparticle shapes of nearly rectangular and of sphere, were experimentally investigated under various volume concentrations of the nanoparticles, ranging from 0.05 to 5.0 vol.%. The viscosity of the nanofluids increased with increases in the volume concentration by up to 69%. In addition, the enhancement of the viscosity of the nearly rectangular shape nanoparticles was found to be greater by 7.7%, than that of the spherical nanoparticles. The thermal conductivity of the ZnO nanofluids increased by up to 12% and 18% at 5.0 vol.% for the spherical and the nearly rectangular shape nanoparticles, respectively, compared to that of the base fluid (water). The shape of the particles is found to have a significant effect on the viscosity and thermal conductivity enhancements.     

Thermal diffusivity of nanofluids containing Au/Pd bimetallic nanoparticles of different compositions

Colloidal suspensions of bimetallic Au/Pd nanoparticles were prepared by simultaneous reduction of the metal ions from their corresponding chloride salts with polymer (PVP) stabilizer. Thermal properties of water containing bimetallic nanoparticles with different nominal compositions (Au/Pd = 12/1, 5/1, 1/1, 1/5) were measured using the mode mismatched dual-beam thermal lens technique to determine the effect of particle composition on the thermal diffusivity of the nanofluids. The characteristic time constant of the transient thermal lens was estimated by fitting the experimental data to the theoretical expression for transient thermal lens. The thermal diffusivity of the nanofluids (water, containing Au/Pd bimetallic nanoparticles) is seen to be strongly dependent on the composition of the particles. The maximum diffusivity was achieved for the nanoparticles with highest Au/Pd molar ratio. A possible mechanism for such high thermal diffusivity of the nanofluids with bimetallic particles is given. UV-Vis spectroscopy, TEM and high-resolution electron microscopy (HREM) techniques were used to characterize the Au/Pd bimetallic nanoparticles.     

Elaboration of thermophysical performance enhancement mechanism of functionalized boron nitride/graphite hybrid nanofluids

This study mainly investigated the physicochemical characteristics of ethylene glycol/ water (EG/W) based hydroxyl-functionalized boron nitride (BN-OH) and graphite (G) hybrid nanofluids. A novel simple and efficient annealing method was proposed to have hexagonal boron nitride (h-BN) nanoparticles functionalized to improve the synergistic role between hybrid G/BN-OH nanoparticles. Meanwhile, the dispersion stability, thermal stability, and rheological behavior of diverse nanofluids (h-BN, BN-OH, G, G/BN and G/BH-OH) were comprehensively evaluated. The results showed that the G/BN-OH hybrid nanofluids demonstrate both better dispersion stability and thermal stability, as well as a lower increase in viscosity. In addition, the thermal conductivity of G/BN-OH hybrid nanofluids was increased by up to 18.05% with a concentration of 0.2 wt% when compared to the base fluid. Ultimately, the complicated theoretical mechanism of thermophysical performance augment for G/BH-OH hybrid nanofluids was reliably presented. The enhanced thermal conductivity of nanofluids may be attributed to the formation of adsorption layers and the synergistic effect of the thermal conductivity network.     

Electrical conductivity measurements of nanofluids and development of new correlations

In this study the electrical conductivity of aluminum oxide (Al2O3), silicon dioxide (SiO2) and zinc oxide (ZnO) nanoparticles dispersed in propylene glycol and water mixture were measured in the temperature range of 0 degrees C to 90 degrees C. The volumetric concentration of nanoparticles in these fluids ranged from 0 to 10% for different nanofluids. The particle sizes considered were from 20 nm to 70 nm. The electrical conductivity measuring apparatus and the measurement procedure were validated by measuring the electrical conductivity of a calibration fluid, whose properties are known accurately. The measured electrical conductivity values agreed within +/- 1% with the published data reported by the manufacturer. Following the validation, the electrical conductivities of different nanofluids were measured. The measurements showed that electrical conductivity of nanofluids increased with an increase in temperature and also with an increase in particle volumetric concentration. For the same nanofluid at a fixed volumetric concentration, the electrical conductivity was found to be higher for smaller particle sizes. From the experimental data, empirical models were developed for three nanofluids to express the electrical conductivity as functions of temperature, volumetric concentration and the size of the nanoparticles.     

Investigation of Al2O3-MWCNTs hybrid dispersion in water and their thermal characterization

Synthesis of water based Al2O3-MWCNTs hybrid nanofluids have been investigated and characterized. Al2O3-MWCNTs nanoparticles in weight proportion of 97.5:2.5 to 90:10 have been studied over 1% to 6% weight concentration. Dispersion quality of nanofluids is assured by additional synthesis process like acids treatment and grinding of MWCNTs by planetary ball mill. The effects of ground and non-ground MWCNTs over dispersion quality and thermal conductivity have been investigated. Sedimentation effect of hybrid nanofluids with time length has been studied by sample visualization and TEM micrographs. The augmentative absorbance and thermal conductivity of hybrid nanofluids have been compared with pure Al2O3/water nanofluids. The overall result shows that the enhancement in normalized thermal conductivity of hybrid nanofluids is still not so sharp though the absorbance and other qualities show much better comparing mono type nanofluids. Hybrid nanofluids with spherical particles show a smaller increase in thermal conductivity comparing cylindrical shape particles.