Extreme pressure properties of multi-component oil-based nanofluids

Multi-component oil-based nanofluids were prepared by dispersing two different carbons and silver nanoparticles in lubricating oil; then, their tribological properties were investigated using a four-ball tribotester and FZG machine. Each nanofluid demonstrated excellent wear resistance or extreme pressure (EP) properties, but not both properties simultaneously. Therefore, a new concept of a mixed nanofluid was developed to satisfy the wear and EP properties. The multi-component mixed nanofluids containing graphite and Ag nanoparticles not only demonstrated enhanced load carrying and anti-wear properties, but also reduced the electric power consumption by more than 4.8% compared with the base oil in the FZG test.     

金属氧化物纳米流体的导热性能研究

采用瞬态热线法测量了4种不同种类、不同体积份额配比的纳米流体的导热系数,分析了纳米颗粒属性、体积分数、悬浮稳定性及温度等因素对纳米流体导热系数的影响.实验结果表明,在流体中加入纳米颗粒将显著提高流体的导热系数.     

Characterization and stability analysis of oil‐based copper oxide nanofluids for medium temperature solar collectors

Thermal oils are widely used as heat transfer fluids in medium temperature applications. Addition of small amounts of nanoparticles in such fluids can significantly improve their thermophysical properties. This paper presents experimental investigation of an oil‐based nanofluids prepared by dispersing different concentrations (0.25 wt%–1.0 wt%) of copper oxide nanoparticles in Therminol‐55 oil using two‐step method. Shear mixing and ultrasonication were used for uniform distribution and de‐agglomeration of nanoparticles to enhance the stability of the suspensions. The effect of nanoparticles concentrations on thermophysical properties of the nanofluids was analysed by measuring thermal conductivity, dynamic viscosity, effective density and specific heat capacity at different temperatures (25 °C–130 °C). Thermal conductivity exhibited increasing trend with rising temperature and increase in nanoparticles loading. A significant decrease in dynamic viscosity and effective density against increasing temperature makes it suitable for medium temperature applications. Nano‐oils with improved thermal properties are expected to increase the efficiency of concentrating solar thermal collectors.     

Molecular dynamics simulations of the rheological properties of graphene–PAO nanofluids

Graphene is a promising additive for lubricants. The rheological properties of graphene nanofluids have a significant impact on the tribological performance of base oil. In this case, rheological properties including viscosity, density, mean square displacement and diffusion coefficient of graphene–PAO nanofluids were investigated by using the nonequilibrium molecular dynamics simulations in order to understand the effects of graphene on the rheological properties of base oil under extreme conditions. The molecular dynamics model was validated according to the experimental and numerical statistics reported by other researchers. The simulation results reflected that the viscosity of base oil was effectively improved by adding graphene nanoparticles. As the concentration of graphene increased, the viscosity of nanofluids becomes higher. However, the diffusion coefficient reached its highest value (3.73?×?10^?9 m²/s) with nanofluids containing two pieces of graphene in the system. Furthermore, we found that the graphene played a more significant role in enhancing the viscosity of base oil at high temperature and pressure. The viscosity was especially improved by 290.2% at 0.1 MPa, 500 K. The boiling point of the base oil became higher than 800 K after adding graphene. To our best knowledge, this work is the first study of the rheological properties of graphene–PAO nanofluids using molecular dynamic simulations.     

Rheological behavior of carbon nanotube and graphite nanoparticle dispersions

Dispersions containing nanoparticles (nanofluids) are mixtures with unique properties, and their transport properties depend on the three-dimensional network or microstructure of the nanoparticles, which can be affected by various factors including shear stress, particle loading, and temperature. In this research, we studied the rheological behaviors of dispersions containing two different carbon morphologies: multiwalled carbon nanotubes (rodlike nanoparticles with L/D = 30), and graphite particles (disklike nanoparticles with L/D = 0.025). All nanofluids showed shear thinning behavior in steady shear measurements and those containing nanotubes had lower power law indices than graphite dispersions. Shear stress broke down the microstructure network and oriented both rodlike and disklike nanoparticles in the dispersions. The presence of a modest amount of nanotubes in the graphite nanofluid affected the microstructure of the dispersion and caused a remarkable decrease in its power law index. Microstructures of nanofluids strongly depended on the dispersant chemistry used to stabilize the particles, and high temperature may cause dispersant failure. Mechanical methods for dispersing the particles affected the geometry of the nanoparticles and therefore the rheological properties of the nanofluids. In the creep recovery tests, the compliance of graphite nanofluids quickly returned to zero when the stress was removed, while nanotube dispersion with high nanotube loading showed an elastic response during recovery. These results suggest that the microstructure in the dispersions is affected by nanoparticle morphology, dispersant chemistry, and shear stress.     

Effect of Using CuO–Oil Nanofluid on Surface Roughness and Thermal Performance during Superfinishing Process

This research aims to investigate the effect of adding copper oxide nanoparticles to the oil Gr-6004 base fluid and its concentration changes from 0.1% to 0.4% on the surface roughness of gudgeon pin and the thermal conductivity of the nanofluids during the superfinishing process. The main novelty of this investigation is analyzing the impact of utilizing CuO/oil Gr-6004 nanofluid on thermal conductivity of oil and surface quality of gudgeon pin during superfinishing process. Based on the results, adding nanoparticles to the oil Gr-6004 has significantly reduced the surface roughness. In addition, by increasing the concentration of nanoparticles to 0.4%, the surface roughness has decreased by 57% compared to oil Gr-6004. Also, by adding nanoparticles, the thermal conductivity of the nanofluids has increased to 19.5%. In addition, dispersing CuO nanoparticles into base fluid reduces oil temperature by 17.44%.     

Investigation on tribological performance of CuO vegetable-oil based nanofluids for grinding operations

With ball-bearing and tribofilm lubrication effects, CuO vegetable oil-based nanofluids have exhibited excellent anti-wear and friction reduction properties. In this study, CuO nanofluids were synthesized by a one-step electro discharge process in distilled water containing polysorbate-20 and vegetable oil as a nanoparticle stabilizer and source of fatty-acid molecules in the base fluid, respectively. Pin-on-disk tribotests were conducted to evaluate the lubrication performance of synthesized CuO nanofluids between brass/steel contact pairs under various loadings. Surface grinding experiments under minimum lubrication conditions were also performed to evaluate the effectiveness of the synthesized nanofluids in improving the machining characteristics and surface quality of machined parts. The results of pin-on-disk tests revealed that adding nanofluids containing 0.5% and 1% (mass fraction) CuO nanoparticles to the base fluid reduced the wear rate by 66.7% and 71.2%, respectively, compared with pure lubricant. The lubricating action of 1% (mass fraction) CuO nanofluid reduced the ground surface roughness by up to 30% compared with grinding using lubricant without nano-additives. These effects were attributed to the formation of a lubrication film between the contact pairs, providing the rolling and healing functions of CuO nanoparticles to the sliding surfaces. The micrography of ground surfaces using a scanning electron microscope confirmed the tribological observations.The full text can be downloaded at https://link.springer.com/article/10.1007/s40436-020-00314-1     

A study of ZnO nanoparticles and ZnO-EG nanofluid

This study deals with the synthesis and characterisation of ZnO nanoparticles and suspension of ZnO-ethylene glycol (EG) nanofluid. Crystalline ZnO nanoparticles are synthesised using the chemical method. The nanofluids were synthesised by the dispersion of ZnO nanoparticles in EG solution using an ultrasonicator. The prepared ZnO nanoparticles are characterised by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and UV-Vis absorption spectrum. The particle size distribution and ultrasonic parameters of the synthesised nanofluid are measured with the help of acoustic particle sizer (APS-100) and ultrasonic interferometer. The observed features of ZnO nanoparticles and ZnO-EG nanofluids are discussed in correlation with known properties of other nanofluids.     

Experimental evaluation of thermophysical properties of oil-based titania nanofluids for medium temperature solar collectors

Thermal oils are widely used as working fluids in the medium temperature heat transfer applications including concentrating solar thermal collectors. However, the weak thermal characteristics of these oils are major drawbacks in their successful application in the medium-high temperature solar collectors. Fortunately, the emergence of nanotechnology has provided the opportunity to alter thermo-physical properties of base fluids by adding small amount of sub-micron size solid particles possessing better properties. This paper presents an experimental investigation of thermophysical properties of an oil-based nanofluid to be used in the medium temperature solar collector for enhanced thermal energy transport. The colloidal suspensions were prepared by dispersing different weight fractions (0.25 wt.%, 0.5 wt.%, 0.75 wt.% and 1.0 wt.%) of Titania nanoparticles in Therminol-55 oil using two-step method. Shear mixing and high energy ultrasonication was employed to achieve uniform mixing and de-agglomeration of the nanoparticles in order to enhance the stability of the colloidal suspensions. Thermophysical properties of the nanofluids were determined as a function of nanoparticles concentrations in the temperature range of 25 °C–130 °C. The experimental results demonstrated substantial improvement in thermal conductivity of the nano-oils with an increase in nanoparticles loading which further enhanced at higher temperatures. Dynamic viscosity and effective density displayed a decreasing trend against rising temperature which indicate the effectiveness of these nanofluids for medium temperature heat supply. Nano-oils with superior thermal properties can improve the performance of medium temperature solar thermal collectors.     

Acoustics and rheological studies of aqueous ethylene glycol blend copper oxide nanofluids

Experimental investigations have been performed to synthesize copper oxide nanoparticles by conventional chemical precipitation method and nanofluids were prepared by two-step method using CuO nanoparticles in different proportions of ethylene glycol–water mixtures (EG–water). Powder X-ray diffraction (PXRD), energy dispersive X-ray (EDX), scanning electron microscope (SEM), particle size, and zeta potential analysis have been studied to characterize both solid and fluid samples for their sizes, shapes, stability, and arrangement. Besides, acoustics and rheological properties such as ultrasonic velocity, density, and viscosity have been measured for all fluid samples at three different temperatures. Interpretations of all these parameters have been made on the basis of particle stability and dispersion capacity of nanoparticles in different proportions of base fluids. The variation of dynamic viscosity with shear rate shows the nanofluids to be behaved like non-Newtonian fluids at very less shear rate but shows Newtonian behavior as the shear rate increases.     

Determination of effective specific heat of nanofluids

The effective specific heat of several types of nanofluids are measured by transient double hot-wire technique. Sample nanofluids are prepared by suspending 1–5 volume percentages of titanium dioxide (TiO₂), aluminium oxide (A1₂O₃) and aluminium (Al) nanoparticles in various base fluids, such as deionised water, ethylene glycol and engine oil. The effective specific heats of these nanofluids were found to decrease substantially with increased volume fraction of nanoparticles. Besides particle volume fraction, particle materials and base fluids also have influence on the effective specific heat of nanofluids. Except Al/engine oil-based nanofluid, predictions of the effective specific heat of nanofluids by the volume fraction mixture rule-based model showed reasonably good agreement with the experimental results. Based on the calibration results obtained for the base fluids, the measurement error is estimated to be within 2.77%.     

Gold nanofluids synthesised using antioxidant catechin,its characterisation and studies on nucleation

Nanofluids are a new class of fluids where nanosized solid particles are dispersed in a liquid medium. This study aims to experimentally investigate the production of gold nanofluids using catechin (CAT), a polyphenolic compound. The characterisation of the green-synthesised nanoparticles using UV–visible spectrophotometer, differential scanning calorimeter, dielectric spectroscopy, and transmission electron microscopy, attempts to generate information to understand the properties of green-synthesised gold nanofluids. Four nanofluids were prepared by varying their concentrations between CAT and tannic acid to form gold nanoparticles (AuNP) in water. The gold nanofluids exhibited interesting thermal, surface plasmon resonance, and dielectric properties. The formation of the AuNP was studied using two different methods using transmission electron microscope and they exhibited interesting growth patterns within 1 minute. These results are significant as they shed light on the importance of the onset of the AuNP formation.     

Investigation on Stability and Optical Properties of Titanium Dioxide and Aluminum Oxide Water-Based Nanofluids

Water is regarded as a poor absorber of solar energy. This affects the efficiency of solar thermal systems. The addition of nanoparticles to heat transfer fluids used in solar thermal systems can enhance their optical properties. These new-generation heat transfer fluids are known as nanofluids. The present study investigates the stability and optical properties of three nanofluids, including aluminum oxide (13 nm and <50 nm) and titanium dioxide (21 nm) nanofluids. The stability of the nanofluids was observed through a photo-capturing method and zeta potential measurements. Ultraviolet–visible (UV–Vis) spectrophotometer was used to measure the absorbance and transmittance of the prepared nanofluids. The effect of factors such as type of particle, type of surfactant, and pH of the solution on the optical properties of the nanofluids was also investigated. We found that the titanium dioxide nanofluid had better optical properties but lower stability compared to aluminum oxide nanofluids.     

Radiative properties of dense nanofluids

The radiative properties of dense nanofluids are investigated. For nanofluids, scattering and absorbing of electromagnetic waves by nanoparticles, as well as light absorption by the matrix/fluid in which the nanoparticles are suspended, should be considered. We compare five models for predicting apparent radiative properties of nanoparticulate media and evaluate their applicability. Using spectral absorption and scattering coefficients predicted by different models, we compute the apparent transmittance of a nanofluid layer, including multiple reflecting interfaces bounding the layer, and compare the model predictions with experimental results from the literature. Finally, we propose a new method to calculate the spectral radiative properties of dense nanofluids that shows quantitatively good agreement with the experimental results.     

Experimental research on microscale grinding temperature under different nanoparticle jet minimum quantity cooling

To solve the limited cooling capacity of drip cooling technique in current clinical grinding orthopedic surgery, this paper carried out a nanoparticle jet minimum quantity cooling grinding experiment by using hydroxyapatite, SiO₂, Fe₂O₃, carbon nanotubes nanofluids. The mass fraction of each nanofluids was 2%, 4%, 6%, 8%, 10%, respectively. Grinding temperature during the grinding process was measured, and results show that the temperature peak of bone micro-grinding isn’t always proportional to the mass fraction of nanofluids. The measured temperature peak shows an inversely proportional relationship with the mass fraction of nanofluids within a certain mass fraction range, but a proportional relationship beyond this mass fraction range. Such a critical mass fraction varies when using different types of nanoparticles. Combined with heat transfer enhancement of nanofluids, the influence law of nanofluids type and concentration on grinding temperature was disclosed from the rheological properties of nanofluids and the micromechanisms of nanoparticles in the grinding zone. The bigger the mass fraction, the more easily it displays rheological properties. As for hydroxyapatite, whose length-diameter ratio is high and for SiO₂ and CNTs, whose hardness values are high, they more easily display shear thinning and shear thickening, and the effect of rheological properties on temperature is obvious.     

Free convection of hybrid Al2O3-Cu water nanofluid in a differentially heated porous cavity

Hybrid nanofluids are a new type of enhanced working fluids, engineered with enhanced thermo-physical properties. The hybrid nanofluids profit from the thermo-physical properties of more than one type of nanoparticles. The present study aims to address the free convective heat transfer of the Al₂O₃-Cu water hybrid nanofluid in a cavity filled with a porous medium. Two types of important porous media, glass ball and aluminum metal foam, are considered for the porous matrix. The experimental data show dramatic enhancement in the thermal conductivity and dynamic viscosity of the synthesized hybrid nanofluids, and hence, these thermophysical properties could not be modeled using available models of nanofluids. Thus, the actual available experimental data for the thermal conductivity and the dynamic viscosity of hybrid nanofluids are directly utilized in the present theoretical study. Various comparison with results published previously in the literature are performed and the results are found to be in excellent agreement. In most cases, the average Nusselt number Nu_l is decreasing function of the volume fraction of nanoparticles. The results show the reduction of heat transfer using nanoparticles in porous media. The observed reduction of the heat transfer rate is much higher for hybrid nanofluid compared to the single nanofluid.     

石墨烯纳米流体研究进展

润滑与冷却是当前工业领域两个重要的议题。前者与机械装置、零部件的使用可靠性和寿命直接相关,对减少摩擦产生的能耗具有重大意义,而后者对于高功率密度器件的热管理至关重要。二者的结合在航空航天、汽车机械等领域广泛存在,而纳米流体是一种很好的承载二者的工作介质。本文针对石墨烯纳米流体这一热点,综述了石墨烯纳米流体的分散理论基础与方法,对影响石墨烯纳米流体悬浮稳定性因素进行了调研,归纳总结了纳米流体的导热机理、影响因素以及石墨烯纳米流体进展,分析了纳米流体未实现大面积应用的主要原因,同时对石墨烯作为添加剂应用于润滑领域的进展进行了评述。最终提出石墨烯纳米流体协同增强换热与减磨润滑的应用设计。在航天器等应用领域中,由于对石墨烯纳米流体的力热耦合综合性能缺乏广泛研究,以及航天器稳定性和长期运行可靠性等问题,未来的研究应以航天传热工质为基础,进行纳米粒子针对性设计,通过系统开展基于空间环境动态流动换热性能与回路寿命的研究,为未来实现纳米流体的航天器应用奠定理论基础和提供技术支撑。     

多壁碳纳米管水基纳米流体的对流换热特性

实验研究了纳米粉体浓度、雷诺数Re和热流密度对多壁碳纳米管水基纳米流体(MWNTs/H2O)对流换热性能的影响。纳米粉体浓度分别为0.05 g/L、0.1 g/L、0.2 g/L和0.4 g/L,雷诺数Re为500~900,热流密度为10~20 k W/m2。结果表明:1)纳米流体对流换热系数随着纳米粉体浓度、Re、热流密度的增加而增加。如在Re为631且纳米粉体浓度为0.4 g/L时,纳米流体对流换热系数比基液增大了17.6%;2)纳米流体对流换热系数的提高率明显大于对应的导热系数提高率,当纳米粉体浓度为0.05g/L时,其对流换热系数和导热系数的提高率分别为7.4%和0.15%;3)在Eubank-Proctor方程的基础上,建立了适合于低Re条件下的混和对流换热的实验关联式。     

Nanofluids for energetics: effect of stabilization on the critical heat flux at boiling

Investigation of aqueous nanofluids synthesized for the first time based on the natural alumosilicate attapulgite showed their high efficiency as heat-transfer agents and the ability to provide a more than twofold increase of the critical heat flux (CHF) as compared to that of the base liquid (pure water). It is shown that insignificant additives of sodium pyrophosphate, which produce disaggregation of nanoparticles and stabilization of attapulgite-containing nanofluids, significantly improve their thermal properties and heatexchange parameters. Several mechanisms are proposed for interpretation of the observed CHF growth. It is established that the use of attapulgite nanofluids not only allows higher CHF to be reached as compared to that of pure water, but also eliminates sudden onset of the boiling crisis.     

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.