Discussion on the thermal conductivity enhancement of nanofluids

Increasing interests have been paid to nanofluids because of the intriguing heat transfer enhancement performances presented by this kind of promising heat transfer media. We produced a series of nanofluids and measured their thermal conductivities. In this article, we discussed the measurements and the enhancements of the thermal conductivity of a variety of nanofluids. The base fluids used included those that are most employed heat transfer fluids, such as deionized water (DW), ethylene glycol (EG), glycerol, silicone oil, and the binary mixture of DW and EG. Various nanoparticles (NPs) involving Al₂O₃ NPs with different sizes, SiC NPs with different shapes, MgO NPs, ZnO NPs, SiO₂ NPs, Fe₃O₄ NPs, TiO₂ NPs, diamond NPs, and carbon nanotubes with different pretreatments were used as additives. Our findings demonstrated that the thermal conductivity enhancements of nanofluids could be influenced by multi-faceted factors including the volume fraction of the dispersed NPs, the tested temperature, the thermal conductivity of the base fluid, the size of the dispersed NPs, the pretreatment process, and the additives of the fluids. The thermal transport mechanisms in nanofluids were further discussed, and the promising approaches for optimizing the thermal conductivity of nanofluids have been proposed.     

液态金属微液滴脉动热管的传热性能

利用室温液态金属和表面活性剂溶液混合工质的振荡运动,在脉动热管中形成液态金属微纳液滴分散的高热导率混合流体并提高其传热性能。本文将液态金属表面活性剂混合工质引入六弯管板式脉动热管中,在不同液态金属填充量和加热功率下开展可视化和传热性能实验。实验结果表明,液态金属在表面活性剂混合工质中通过振荡自分散成球形液滴且相互之间不易发生合并,并在表面活性剂工质中留下粒径在410~520nm的纳米颗粒。传热性能方面,液态金属填充量在20%~25%时,液态金属球形液滴的黏度高、质量大,会阻碍混合工质的振荡运动从而降低脉动热管的传热性能,填充量在5%~10%时,混合工质耦合了液态金属的高热导率特性,有效提高传热性能,热阻最多降低11.21%。     

A generalized corresponding states method for the prediction of the thermal conductivity of liquids and liquid mixtures

We have recently developed an extension of the three parameter corresponding states principle based on the properties of two non-spherical reference fluids for the viscosity of liquids and liquid mixtures. We extend the method here to the thermal conductivity. We have tested the method for a large number of binary liquid mixtures using the two pure components in each case as our reference fluids. Good agreement between predicted and experimental thermal conductivities was obtained using only the data for the pure components. This agreement becomes excellent if a single binary interaction consta independent of temperature and composition, is used in the mixture calculations. If the pure component reference properties are not available, then the may he obtained from the properties of two similar fluids.     

Thermal conductivity and viscosity of self-assembled alcohol/polyalphaolefin nanoemulsion fluids

Very large thermal conductivity enhancement had been reported earlier in colloidal suspensions of solid nanoparticles (i.e., nanofluids) and more recently also in oil-in-water emulsions. In this study, nanoemulsions of alcohol and polyalphaolefin (PAO) are spontaneously generated by self-assembly, and their thermal conductivity and viscosity are investigated experimentally. Alcohol and PAO have similar thermal conductivity values, so that the abnormal effects, such as particle Brownian motion, on thermal transport could be deducted in these alcohol/PAO nanoemulsion fluids. Small angle neutron-scattering measurement shows that the alcohol droplets are spheres of 0.8-nm radius in these nanoemulsion fluids. Both thermal conductivity and dynamic viscosity of the fluids are found to increase with alcohol droplet loading, as expected from classical theories. However, the measured conductivity increase is very moderate, e.g., a 2.3% increase for 9 vol%, in these fluids. This suggests that no anomalous enhancement of thermal conductivity is observed in the alcohol/PAO nanoemulsion fluids tested in this study.     

Thermal conductivity and viscosity of normal C2–C6 aliphatic carboxylic Acids

Equipment for measurements of thermal conductivity and viscosity of liquids in the temperature range 273–350 K and at atmospheric pressure is described. The thermal conductivity and viscosity of acetic, propionic, butyric, valeric and hexanoic acids in the liquid state have been measured. The values obtained are compared with published data and the ratio of thermal conductivity, λ, to viscosity, η, is examined in the form of the dimensionless quantities, Mλ/Rη and cpη/λ (Prandtl number).     

Electrokinetic mixing of two fluids with equivalent conductivity

Electrokinetic (EK) micromixers have been widely studied in the past decade for biochemical applications, biological and chemical analysis, etc. Unfortunately, almost all EK mixers require different electrical conductivity between the two fluids to be mixed, which has greatly limited their wide applications, in cases where the two streams to be mixed have equivalent electrical conductivity. Here we show that mixing enhancement between two fluids with identical conductivity can be achieved in an EK micromixer with conductive sidewalls, where the electric field is in transverse direction of the flow. The results revealed that the mixing became stronger with increased conductivity value. This mixing method provides a novel and convenient strategy for mixing two liquids with the same or similar electrical conductivity in microfluidic systems, and could potentially serves as a powerful tool for sample preparation in applications such as liquid biopsy, and environmental monitoring, etc.     

Thermal transport properties of diamond-based nanofluids and nanocomposites

The focus of this work is to examine the effects nanoparticles, in particular nanodiamond, have on the heat transfer of fluids and polymer solids. Sample preparation techniques that provide suitable nanoparticle dispersion in both liquid and solid samples are discussed. Liquid suspensions are characterized by measuring particle size distributions and liquid viscosities; heat transfer properties are qualitatively compared via an ad-hoc thermal transport test setup. Polymer samples are visually characterized to ensure nanoparticle dispersion and thermal conductivity is measured using a flash lamp technique.     

Preparation and properties of copper-oil-based nanofluids

In this study, the lipophilic Cu nanoparticles were synthesized by surface modification method to improve their dispersion stability in hydrophobic organic media. The oil-based nanofluids were prepared with the lipophilic Cu nanoparticles. The transport properties, viscosity, and thermal conductivity of the nanofluids have been measured. The viscosities and thermal conductivities of the nanofluids with the surface-modified nanoparticles have higher values than the base fluids do. The composition has more significant effects on the thermal conductivity than on the viscosity. It is valuable to prepare an appropriate oil-based nanofluid for enhancing the heat-transfer capacity of a hydrophobic system. The effects of adding Cu nanoparticles on the thermal oxidation stability of the fluids were investigated by measuring the hydroperoxide concentration in the Cu/kerosene nanofluids. The hydroperoxide concentrations are observed to be clearly lower in the Cu nanofluids than in their base fluids. Appropriate amounts of metal nanoparticles added in a hydrocarbon fuel can enhance the thermal oxidation stability.     

液体导热系数的自由体积模型

本文用统一的自由体积模型描述温度，压力和组成对稠密流体，尤其是液体的导热系数的影响。首先，在低压下纯液体的导热系数提出了简单的三参数关联式，对４３种纯物质，４４３个数据点的总平均误差仅为０．２６％，在此基础上，提出了液体混合的模型，其参数可视与温度无关，对４３个二元体系，包括含水体系，计８６９个点的计算误差仅为２．３１％，优于被认为成功的Ｔｅｊａ－Ｒｉｃｅ模型，最后，提出了适用于高压下，包括气液两     

The use of drying experiments in the study of the effective thermal conductivity in a solid containing a multicomponent liquid mixture

The effective thermal conductivity of a porous solid containing multicomponent liquid mixtures has been studied. To achieve this, the liquid composition, liquid content and temperature distributions have been measured in a cylindrical sample dried by convection from the open upper side and heated by contact with a hot source at the bottom side. A quasi-steady state reached at high source temperatures permits to calculate the total heat flux from temperatures measured on the surface and the gas stream. The simulations performed and compared with experimental data made it possible to estimate the adjusting geometric parameter of Krischer's model for the effective thermal conductivity. The effective thermal conductivity has been widely studied for two-phase systems, mostly with regard to thermal insulation elements. The calculation of this transport parameter includes the contribution to heat transfer of the evaporation–diffusion–condensation mechanism undergone by the multicomponent mixture. The influence of liquid composition and temperature on the thermal conductivity due to the evaporation–diffusion–condensation mechanism and the effective thermal conductivity is described. The results reveal that in this case the resistance to heat transfer seems to correspond to a parallel arrangement between the phases.     

Thermophysical properties of CTBN and HTPB liquid rubber modified epoxy blends

Thermal conductivity and diffusivity of carboxyl‐terminated copolymer of polybutadiene and acrylonitrile (CTBN) and hydroxyl‐terminated polybutadiene (HTPB) liquid rubber‐ modified epoxy blends were investigated. A good agreement was observed between the calculated values of the specific heat estimated from thermal conductivity, diffusivity, and density measurements and the DSC results. Measurements of the thermal conductivity values of HTPB/Epoxy blends were in good agreement with three simple theoretical models, which have been used thereafter for the estimation of the unknown value of the thermal conductivity of CTBN (k_CTBN = 0.24 Wm^?1K^?1). The morphology of the rubber‐modified epoxy blends has been quantified and indicate a tendency towards co‐continuous phase upon the inclusion of higher weight percentage of rubber (≥30 wt %). Moreover, we notice a significant enhancement of the thermal conductivity during this morphological shift. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Equilibrium,interfacial and transport properties of n-alkanes: Towards the simplest coarse grained molecular model

In this work, using molecular dynamics simulations, we have investigated how simple could be a coarse grained molecular model yielding simultaneously equilibrium densities, surface tensions and transport properties of some n-alkanes (from methane to n-decane) along the vapour–liquid equilibrium curve. For that purpose, as an initial model, the fully flexible Lennard–Jones Chain (3 “molecular” parameters) model has been chosen. Using this simple molecular model, good results have been obtained on equilibrium properties of all tested n-alkanes despite a systematic slight overestimation of critical temperatures, critical pressures and surface tensions. In addition, concerning transport properties, good results have been obtained for methane and n-butane except for thermal conductivity in the gas state. For n-heptane and n-decane it has been found that thermal conductivity is systematically underestimated while viscosity is well estimated except at low temperatures. Concerning thermal conductivity, this misevaluation can be corrected if the zero-density thermal conductivity is known. Concerning shear viscosity, it is found that an additional “rigidity” fourth parameter is required to improve the results when dealing with the longest chains at the lowest temperatures.     

Thermal properties of silicate esters

Fourteen dimethyldialkoxysilanes are prepared by reacting dimethyldichlorosilane with corresponding alcohols in the liquid phase. Their physicochemical properties are determined by standard methods and their tentative empirical formulae are elucidated. The thermal conductivity values of dimethyldialkoxysilanes are determined using a two slab guarded hot plate apparatus over a temperature range of 30–190°C. The thermal conductivity values of dimethyldialkoxysilanes calculated from empirical equations are found to vary from -14% to +19%. Because thermal conductivity is an important parameter in heat transfer calculations, and to minimise the deviation of the values, new equations involving easily measurable ultrasonic parameters have been formulated. The proposed equations have been tested for the series of dimethyldialkoxysilanes, tetraalkoxysilanes and polydimethylsiloxanes. The thermal conductivity values computed from the proposed equations are comparable with experimental thermal conductivity values. Further, an equation involving the calculation of thermal conductivity values at varying temperatures is also proposed.     

Ultrasonication effects on thermal and rheological properties of carbon nanotube suspensions

The preparation of nanofluids is very important to their thermophysical properties. Nanofluids with the same nanoparticles and base fluids can behave differently due to different nanofluid preparation methods. The agglomerate sizes in nanofluids can significantly impact the thermal conductivity and viscosity of nanofluids and lead to a different heat transfer performance. Ultrasonication is a common way to break up agglomerates and promote dispersion of nanoparticles into base fluids. However, research reports of sonication effects on nanofluid properties are limited in the open literature. In this work, sonication effects on thermal conductivity and viscosity of carbon nanotubes (0.5 wt%) in an ethylene glycol-based nanofluid are investigated. The corresponding effects on the agglomerate sizes and the carbon nanotube lengths are observed. It is found that with an increased sonication time/energy, the thermal conductivity of the nanofluids increases nonlinearly, with the maximum enhancement of 23% at sonication time of 1,355 min. However, the viscosity of nanofluids increases to the maximum at sonication time of 40 min, then decreases, finally approaching the viscosity of the pure base fluid at a sonication time of 1,355 min. It is also observed that the sonication process not only reduces the agglomerate sizes but also decreases the length of carbon nanotubes. Over the current experimental range, the reduction in agglomerate size is more significant than the reduction of the carbon nanotube length. Hence, the maximum thermal conductivity enhancement and minimum viscosity increase are obtained using a lengthy sonication, which may have implications on application.     

Laserspektroskopische Bestimmung thermophysikalischer Eigenschaften transparenter Fluide

Determination of thermophysical properties of transparent fluids by laser spectroscopy. This review deals with the determination of thermophysical properties of transparent fluids by laser-spectrometric methods. On the basis of the phenomena of laser-Rayleigh and laser-Brillouin scattering the paper demonstrates that the transfer coefficients thermal conductivity, diffusion coefficient, and viscosity, as well as the velocity of sound, sound attenuation, specific heats, and compressibility are fundamentally derivable at thermodynamic equilibrium over a wide range of temperature and pressure. Typical measurements show that the methods used for measuring some of the quantities have already reached standards maturity while others are still in their initial stages.     

Radial heat transfer to fixed beds of particles

Radial temperature distributions have been measured when air flows through a fixed bed of large particles heated at the wall. The measurements have been interpreted by two models. In the first, the axial and radial thermal conductivities are assumed constant throughout the bed, and there is a thermal resistance at the wall. In the second, thermal properties have been assumed constant in the bulk, but allowed to vary in a narrow boundary region adjacent to the wall without an additional resistance so that both temperature and temperature gradient are continuous throughout both regions. For small particles and particles of high thermal conductivity, both models interpreted the measurements equally well. For large particles of low thermal conductivity the two-region model was much better.     

基于尿素/氯化胆碱低共熔溶剂体系纳米流体制备及其热物性研究

随着电子工业的快速发展,传统换热工质由于其较低的热导率已无法满足越来越高的换热需求。另一方面,传统的换热工质受限其相对较窄的液程范围而无法使用于复杂的温况或特殊的工作条件。低共熔溶剂（DESs）具有与离子液体相似的低饱和蒸气压、高沸点及强稳定性等优势,在传热领域具有巨大的潜力。制备了以尿素/氯化胆碱低共熔溶剂体系为基液,石墨烯、Al₂O₃、TiO₂三种纳米粒子填充的纳米流体,研究了黏度、热导率等热物性与纳米粒子和基液组成之间的关系,并系统地研究了纳米粒子结构对其稳定性的影响。实验结果表明,纳米粒子的填充会在一定程度上增加基液的黏度,其中石墨烯填充的纳米流体的黏度增加最大。此外,石墨烯能显著提高DESs的导热性能,其中6%（质量）石墨烯纳米流体热导率相比基液可增加29.0%。     

Rate of strain effect on the thermal conductivity of a polymer liquid

The effective thermal conductivity of a polymer liquid was measured as a function of strain rate. The polymer liquids studied were three commercial grades of polydimethylsiloxane. The experimental results show a dependence of effective thermal conductivity on strain rate. This dependence is believed due to either the rotation of large clusters of entangled macromolecules, or the orientation of the individual macromolecules, or possibly the combination of both mechanisms simultaneously in shear flow. The effective thermal conductivity, for all three samples studied, increased with increasing strain rate. The rate of increase was found to decrease with increasing temperature. The thermal conductivity measurements were made in an annular axial flow test cell that utilizes a hot wire probe technique. Convection problems were eliminated by proper placement of the temperature measuring section of the wire in the cell. A tentative model was used through which the size of the cluster can be estimated. Once the size of the cluster is obtained, the rotational contribution to the energy transport can be estimated. Initial predictions agree well with the observed phenomena.     

HFE7100和HFE7500的热物理性质

为了获得氢氟醚HFE7100、HFE7500的热物理性质参数,补充现有数据不足,为其作为电子元器件的冷却介质、工业清洗剂等工程应用提供技术支持,利用瞬态热线法测量了常压下HFE7100和HFE7500的热导率,并用表面光散射法测量了HFE7100和HFE7500的液相黏度和表面张力。HFE7100和HFE7500的热导率和液相黏度均关联成温度的多项式函数,HFE7100热导率和黏度的实验值和关联式的平均绝对偏差分别为0.37%和1.19%,HFE7500热导率和黏度的实验值和关联式的平均绝对偏差分别0.08%和1.10%。利用改进的van der Waals关联表面张力和温度的关系,HFE7100和HFE7500的表面张力的实验值和关联式的平均绝对偏差分别为0.03 mN·m^-1和0.02 mN·m^-1。获得的HFE7100和HFE7500的热导率、黏度和表面张力实验数据及方程,可为其工程应用提出数据支持。