共查询到19条相似文献,搜索用时 109 毫秒
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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 相似文献
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《工程热物理学报》2017,(2)
将羧基基团引入多壁碳纳米管,改善了碳纳米管在水中的分散性及稳定性。同时研究了不同质量浓度纳米流体的导热系数、加热表面颗粒沉积、接触角变化对核沸腾传热性能的影响。结果表明;羧基化碳纳米流体可强化核沸腾传热。在测试浓度范围内,强化率在低热通时,随着热通量的增加急剧增大,高热通时,趋于稳定;当质量比ω为0.10%,功率为210.6 kW.m~(-2)时,强化率达到最大为138.3%;流体的导热系数随着质量浓度的增大而增大,0.15%浓度导热系数是纯水的1.18倍。分析认为纳米流体表面张力,纳米颗粒沉积,纳米颗粒扰动和导热系数的变化均是影响水基羧基化碳纳米流体沸腾的因素。结论由0.05%的纳米流体沸腾过程高速成像得到验证。 相似文献
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利用同步辐射X射线近边吸收谱(XANES)分别对硝酸氧化处理后的单壁碳纳米管和多壁碳纳米管进行了系统的对比研究。实验结果表明,单壁碳纳米管容易遭到硝酸的破坏,生成大量的碳碎片,氧化所产生的氧化基团大部分形成于碳碎片上,这些吸附在碳纳米管上的碳碎片可以通过氢氧化钠溶液的清洗、过滤去除。相比之下,结构稳定性更高的多壁碳纳米管在硝酸处理过程中则显得比较稳定,大量的氧化基团形成于碳管管壁上。这种不同结构碳纳米管的不同氧化结果可能会对碳纳米管的后续修饰和应用产生重要的影响。 相似文献
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采用F luen软件对封闭腔内Cu-H2O纳米流体强化自然对流换热进行了数值模拟,重点分析Cu纳米粒子添加量和Gr数对换热性能的影响,并解释其换热机理。研究结果表明:在水基液中加入Cu纳米粒子可以显著提高基液的自然对流换热特性。对于一给定的Gr数,随着纳米粒子质量分数的增加,纳米流体的速度组成部分增加,纳米流体质量分数越大,x方向和y方向的速度峰越大,因此加速了流体中能量传输。另一方面,随着Gr数的增加,流线图中旋涡逐渐变大,流线间强度增加,说明换热效果逐渐增强。当Gr数较小时,传热主要是由热壁和冷壁之间的热传导引起的,随着Gr数的增大,换热逐渐变为由对流换热占主导地位。 相似文献
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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。 相似文献
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提出了透射电子显微镜(TEM)纳米云纹法的新技术,首次将该方法用于单根单壁碳纳米管的残余变形测量。纳米云纹由计算机显示器扫描线与碳纳米管束TEM图像干涉而成。该方法具有纳米级空间分辨率,可直接测量碳纳米管的力学性能。对TEM纳米云纹法的原理进行了详细的阐述,并利用不同管径的单壁碳管束产生了云纹。对直径为7.5nm的弯曲碳管束的残余变形进行测量,直接得到了其中一根直径为1.0nm的单壁碳管的残余变形场。实验结果证明了该方法的可行性。该方法为纳米尺度的碳管力学性能测量提供了新途径。 相似文献
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用Pt细丝代替已有3ω方法中的薄膜热线,并设计了基于Labview程序的虚拟测量系统,准确、方便地测量了聚丙烯复合材料的热导率. 测量结果发现,多壁碳纳米管/丁苯橡胶/聚丙烯三元复合材料的热导率随着多壁碳纳米管/丁苯橡胶粉末含量的增加变化不大;多壁碳纳米管/聚丙烯复合材料的热导率随着多壁碳纳米管含量增加而增大;复合材料热导率远小于简单混合规则预测的结果,而与有效介质理论符合很好.
关键词:
ω法')" href="#">3ω法
多壁碳纳米管
聚丙烯复合材料
热导率 相似文献
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This article reports the thermal conductivity modeling of nanofluids containing decorated multi-walled carbon nanotubes with TiO2 nanoparticles. TiO2 nanoparticles and decorated multi-walled carbon nanotubes are synthesized with different amounts of TiO2 nanoparticles. The experimental results show that the measured thermal conductivities of TiO2 nanofluids and multi-walled carbon nanotube nanofluids are higher than the predicted values by theoretical models. The comparison results of multi-walled carbon nanotube nanofluids and multi-walled carbon nanotube–TiO2 nanofluids reveal that the predicted values by the Xue model are closer to the measured values. In addition, the results show that the thermal conductivity of nanofluids containing multi-walled carbon nanotube–TiO2 increases with respect to TiO2 content of hybrid. 相似文献
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Mechanism of thermal conductivity of nanofluids is analysed and calculated, including Brownian motion effects, particle agglomeration and viscosity, together influenced by temperature. The results show that only Brown- Jan motion as reported is not enough to describe the temperature dependence of the thermal conductivity of nanofluids. The change of particle agglomeration and viscosity with temperature are also important factors. As temperature increases, the reduction of the particle surface energy would decrease the agglomeration of nanopartieles, and the reduction of viscosity would improve the Brownish motion. The results egree well with the experimental data reported. 相似文献
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纳米流体热导率和粘度的分子动力学模拟计算 总被引:5,自引:0,他引:5
本文采用分子动力学(MD)模拟来计算纳米流体比较重要的热物性:热导率和粘度,与已有实验结果比较符合 较好,为进一步研究纳米流体传热效率提供了依据。 相似文献
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T. Ishigaki S. Suzuki H. Kataura W. Krätschmer Y. Achiba 《Applied Physics A: Materials Science & Processing》2000,70(2):121-124
The microstructure and field emission properties of films prepared from commercial powders of single-wall and multi-wall carbon nanotube films were investigated. In addition to nanotubes, these materials contained amorphous and crystalline carbon phases, and the single-wall powder also contained nano-particles of the Fe-Co catalysts. The turn-on fields were measured to be 2.3 V 7m-1 for films prepared with the material containing single-wall nanotubes and 2.6 V 7m-1 for those with multi-wall nanotubes. Long-term stability of emission current for these films was also investigated and correlated to their microstructure. Single-wall material showed a decrease in emission current that extended over several days and that was related to permanent degradation of emission sites. Emission current from the multi-wall material stabilized after an initial increase. 相似文献
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Nanofluids present a new type of dispersed fluids consisting of a carrier fluid and solid nanoparticles. Unusual properties of nanofluids, particularly high thermal conductivity, make them eminently suitable for many thermophysical applications, e.g., for cooling of equipment, designing of new heat energy transportation and production systems and so on. This requires a systematic study of heat exchange properties of nanofluids. The present paper contains the measurement results for the heat transfer coefficient of the laminar and turbulent flow of nanofluids on the basis of distilled water with silica, alumina and copper oxide particles in a minichannel with circular cross section. The maximum volume concentration of particles did not exceed 2%. The dependence of the heat transfer coefficient on the concentration and size of nanoparticles was studied. It is shown that the use of nanofluids allows a significant increase in the heat transfer coefficient as compared to that for water. However, the obtained result strongly depends on the regime of flow. The excess of the heat transfer coefficient in the laminar flow is only due to an increase in the thermal conductivity coefficient of nanofluid, while in the turbulent flow the obtained effect is due to the ratio between the viscosity and thermal conductivity of nanofluid. The viscosity and thermal conductivity of nanofluids depend on the volume concentration of nanoparticles as well as on their size and material and are not described by classical theories. That is why the literature data are diverse and contradictory; they do not actually take into account the influence of the mentioned factors (size and material of nanoparticles). It has been shown experimentally and by a molecular dynamics method that the nanofluid viscosity increases while the thermal conductivity decreases with the decreasing dispersed particle size. It is found experimentally for the first time that the nanofluid viscosity coefficient depends on the particle material. The higher is the density of particles, the higher is the thermal conductivity coefficient of nanofluid. 相似文献
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Aiming at the dispersion stability of nanopartieles regarded as the guide of heat transfer enhancement, we investigate the viscosity and the thermal conductivity of Cu and Al2O3 nanoparticles in water under different pH values. The results show that there exists an optimal pH value for the lowest viscosity and the highest thermal conductivity, and that at the optimal pH value the nanofluids containing a small amount of nanoparticles have noticeably higher thermal conductivity than that of the base fluid without nanoparticles. For the two nanofluids the enhancements of thermal conductivity are observed up to 13% (Al2O3-water) or 15% (Cu-water) at 0.4 wt%, respectively. Therefore, adjusting the pH values is suggested to improve the stability and the thermal conductivity for practical applications of nanofluid. 相似文献
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Vegetable oils (Ground nut) are being investigated to serve as a possible substitute for non-biodegradable mineral oils, which are currently being used as metal-cutting fluids in machining processes. In this study, thermophysical properties of hybrid nanofluids (vegetable oil) to be used as metalworking cutting fluids are investigated. In-situ synthesis of copper (Cu) and zinc (Zn) combined hybrid particles is performed by mechanical alloying with compositions of 50:50, 75:25, and 25:75 by weight. Characterizations of the synthesized powder were carried out using X-ray diffraction, a particle size analyzer, FE-SEM, and TEM. Hybrid nanofluids with all the three combinations of hybrid nanoparticles were prepared by dispersing them into a base fluid (vegetable oil). The thermophysical properties, such as thermal conductivity and viscosity, were studied for various volume concentrations and at a range of temperatures. Experimental results have shown enhancement in thermal conductivity in all cases and also an increase in viscosity. The enhancement in viscosity is similar in all three combinations, as the particle size and shape are almost identical. The enhancement in thermal conductivity is higher in Cu–Zn (50:50), resulting in better enhancement in thermal conductivity due to the Brownian motion of the particles and higher thermal conductivity of the nanoparticles incorporated. 相似文献
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Homogeneous and stable nanofluids have been produced by suspending well dispersible multi-walled carbon nanotubes (CNTs) into ethylene glycol base fluid. CNT nanofluids have enhanced thermal conductivity and the enhancement ratios increase with the nanotube loading and the temperature. Thermal conductivity enhancement was adjusted by ball milling and cutting the treated CNTs suspended in the nanofluids to relatively straight CNTs with an appropriate length distribution. Our findings indicate that the straightness ratio, aspect ratio, and aggregation have collective influence on the thermal conductivity of CNT nanofluids. 相似文献
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Yijun Yang Alparslan Oztekin Sudhakar Neti Satish Mohapatra 《Journal of nanoparticle research》2012,14(5):852
The present study demonstrates the importance of actual agglomerated particle size in the nanofluid and its effect on the
fluid properties. The current work deals with 5 to 100 nm nanoparticles dispersed in fluids that resulted in 200 to 800 nm
agglomerates. Particle size distributions for a range of nanofluids are measured by dynamic light scattering (DLS). Wet scanning
electron microscopy method is used to visualize agglomerated particles in the dispersed state and to confirm particle size
measurements by DLS. Our results show that a combination of base fluid chemistry and nanoparticle type is very important to
create stable nanofluids. Several nanofluids resulted in stable state without any stabilizers, but in the long term had agglomerations
of 250 % over a 2 month period. The effects of agglomeration on the thermal and rheological properties are presented for several
types of nanoparticle and base fluid chemistries. Despite using nanodiamond particles with high thermal conductivity and a
very sensitive laser flash thermal conductivity measurement technique, no anomalous increases of thermal conductivity was
measured. The thermal conductivity increases of nanofluid with the particle concentration are as those predicted by Maxwell
and Bruggeman models. The level of agglomeration of nanoparticles hardly influenced the thermal conductivity of the nanofluid.
The viscosity of nanofluids increased strongly as the concentration of particle is increased; it displays shear thinning and
is a strong function of the level of agglomeration. The viscosity increase is significantly above of that predicted by the
Einstein model even for very small concentration of nanoparticles. 相似文献