Experimental investigation toward obtaining a new correlation for viscosity of WO3 and Al2O3 nanoparticles-loaded nanofluid within aqueous and non-aqueous basefluids

In this study, the effect of temperature and mass fraction of Al₂O₃ and WO₃ nanoparticles dispersed in deionized water and liquid paraffin was investigated on dynamic viscosity of nanofluid. The results of the TEM tests showed that the size of Al₂O₃ and WO₃ nanoparticles was ranged from 10 to 60 nm, and the results showed that nanoparticles were semi-spherical. Also the results of DLS and zeta potential tests, respectively, exhibited the uniform size and high stability of the nanoparticles in the basefluid environment. The findings showed that adding a certain amount of nanoparticles to water and liquid paraffin increases dynamic viscosity, and in the case of various shear rates, the viscosity is constant for the water-based nanofluids, which indicates the Newtonian behavior of the nanofluid. In addition, for those prepared by liquid paraffin as a basefluid, the viscosity does not remain constant at different shear rates and at low amount of shear rate the viscosity achieves higher value, indicating non-Newtonian behavior of liquid paraffin-based nanofluids. The results showed that by increasing the temperature in liquid paraffin-based nanofluid the uniformity and linearity of the viscosity curve at various shear rates could be observed, which represents an approach for Newtonian behavior of nanofluid at higher temperatures. These results also showed that with increasing the mass fraction of nanoparticles in water and liquid paraffin, the viscosity increases at different shear rates. Finally, the correlation presented in this study shows that for nanofluid viscosity as a function of nanoparticles load and temperature, the deviation of correlated data from experimental values is less than 10%.

     

A new semi-analytical model for effective thermal conductivity of nanofluids

A new theoretical model for thermal conductivity of nanofluids is developed incorporating effective medium theory, interfacial layer, particle aggregation and Brownian motion-induced convection from multiple nanoparticles/aggregates. The predicated result using aggregate size, which represents the particle size in the actual condition of nanofluids, fits well with the experimental data for water-, R113- and ethylene glycol (EG)-based nanofluids. The present model also gives much better predictions compared to the existing models. A parametric analysis, particularly particle aggregation, is conducted to investigate the dependence of effective thermal conductivity of nanofluids on the properties of nanoparticles and fluid. Aggregation is the main factor responsible for thermal conductivity enhancement. The dynamic contribution of Brownian motion on thermal conductivity enhancement is surpassed by that of static mechanisms, particularly at high volume fraction. Predication also indicated that the viscosity increases faster than the thermal conductivity, causing the highly aggregated nanofluids to become unfavourable, especially for d_f = 1.8.     

Experimental study on the rheological behavior of nanolubricant-containing MCM-41 nanoparticles with viscosity measurement

In this study, the rheological behavior and viscosity of a stable nanofluid, which is prepared with the suspension of MCM-41 nanoparticles in SAE40 engine oil as base fluid, would be presented. Two-step method has been used to stabilize the nanoparticles in engine oil. To obtain structural and morphological properties of the synthesized nanoparticles, small-angle X-ray scattering, N₂ adsorption/desorption analysis and scanning electron microscopy have been done. Then, viscosity of nanofluids has been measured in temperature range of 25–55 °C, shear rates up to 13,000 s^?1 and different concentrations (0 mass%, 0.5 mass%, 1 mass%, 3 mass% and 5 mass% of MCM-41 nanoparticles). For all the samples, the shear stress versus shear rate diagrams showed that SAE40 oil has Newtonian behavior, in which adding mesoporous silica nanoparticles causes non-Newtonian or pseudoplastic behavior. The results declared that viscosity decreases with increasing temperature and increases with an enhancement in concentration. Furthermore, based on experimental results, an accurate correlation has been proposed to predict the viscosity of SAE40/MCM-41 nanolubricants.

     

Synthesis of metal-based nanofluids and their thermo-hydraulic performance in compact heat exchanger with multi-louvered fins working under laminar conditions

Present experimental investigation incorporates characterization of Al nanopowder, synthesis of Al/water nanofluids, and effect of these nanofluids on thermal performance of compact heat exchanger. Al nanoparticles are characterized using TEM and XRD. Al/water nanofluid is prepared by dispersing metal basis aluminium nanoparticles of average 100 nm size into double distilled water at two different particle volume concentrations of 0.1 and 0.2%. The nanofluids are prepared by two-step method and cetyl trimethyl ammonium bromide surfactant is used to stabilize the nanofluid. Thermo-physical properties of nanofluids at two different concentrations and their variation with fluid temperature are measured experimentally. It is examined that thermal conductivity, viscosity, and density of the nanofluid increased with the increase of volume concentrations. Furthermore, by increasing the fluid temperature, thermal conductivity is intensified, while the viscosity and density are decreased. Heat transfer parameters are strong functions of these thermo-physical properties. Therefore, comprehensive findings on heat transfer coefficient, Nusselt number, colburn factor, friction factor, and effectiveness are determined experimentally for prepared nanofluids passing under laminar conditions through single-pass cross-flow compact heat exchanger attached with multi-louvered fins.

     

Microwave synthesis of silver nanofluids with polyvinylpyrrolidone (PVP) and their transport properties

Microwave synthesis has been applied to prepare stable silver nanofluids in ethanol by reduction of AgNO₃ with polyvinylpyrrolidone (PVP), used as stabilizing agent, having Ag concentrations of 1% by volume. The nanofluids were characterized by UV-vis spectroscopy, Fourier transform infrared, energy-dispersive X-ray spectroscopy, and transmission electron microscopy and systematically investigated for refractive index, electrical and thermal conductivity, and viscosity for different polymer concentrations. The size of nanoparticles was found to be in the range of 30–60 nm for two different salt-to-PVP ratios. For higher concentration of polymer in nanofluid, nanoparticles were 30 nm in size showing increase in thermal conductivity but a decrease in viscosity and refractive index, which is due to the polymer structure around nanoparticles. Thermal conductivity measurements of nanofluids show substantial increment in the thermal conductivity of nanofluid relative to the base fluid and nonlinear enhancement over the 283–323 K temperature range. Rheology of nanofluids was studied at room temperature showing effect of polymer on viscosity and confirming the Newtonian behavior of nanofluid.     

Experimental determination of thermal conductivity and viscosity of different nanofluids and its effect on a hybrid solar collector

In this research, three different volume concentrations (??=?0.05, 0.1 and 0.2%) of Al₂O₃/water, CuO/water and Al₂O₃–CuO/water (50:50) nanofluids are prepared by adopting a two-step nanofluid preparation method. Al₂O₃ and CuO nanoparticles with the average diameter of 50 nm and 27 nm were dispersed in distilled water. The thermal conductivity and viscosity of prepared nanofluids are measured for different temperatures by using KD2 Pro thermal property analyzed and Brookfield viscometer, respectively. The effects of nanofluids on the thermal, electrical and overall efficiency of photovoltaic thermal (PVT) solar collector are also studied. The experimental results revealed that the thermal conductivity and viscosity increase with the increase in percentage volume concentration and viscosity decreases with the increase in temperature. Furthermore, the obtained maximum thermal and electrical efficiencies of a PVT solar collector for 0.2% volume concentration of hybrid nanofluids are 82% and 15%, respectively, at peak solar radiation. The highest overall efficiency of a PVT collector with .2% volume concentration of hybrid nanofluid was 97% at peak solar radiation. Results recommend that nanofluids can be used as a heat transfer in PVT solar collector.

     

Synthesis and thermo-physical properties of water-based novel Ag/ZnO hybrid nanofluids

The comparative study on the thermo-physical properties of water-based ZnO nanofluids and Ag/ZnO hybrid nanofluids is reported in the present study. The outer surface of ZnO nanoparticles was modified with a thin coating of Ag nanoparticles by a wet chemical method for improved stability and heat transfer properties. The ZnO and Ag/ZnO nanofluids were prepared with varying volume concentration (??=?0.02–0.1%). The synthesized nanoparticles and nanofluids were characterized with different characterization methods viz., scanning electron microscopy, X-ray diffraction, dynamic light scattering, thermal conductivity measurement, and viscosity measurement. Results show that thermal conductivity of Ag/ZnO hybrid nanofluids is found to be significantly higher compared to ZnO nanofluids. The maximum thermal conductivity an enhancement for Ag/ZnO nanofluid (??=?0.1%) is found to 20% and 28% when it compared with ZnO nanofluid (??=?0.1%) and water, respectively.     

Effect of temperature on volumetric and transport properties of nanofluids containing ZnO nanoparticles poly(ethylene glycol) and water

The nanoparticles of ZnO have been dispersed in base fluids of poly(ethylene glycol), PEG, and its aqueous solutions. Stability of these nanofluids has been verified with UV-Vis spectroscopy. Dynamic light scattering and size analyze laser methods were used to obtain particle size of the nanofluids investigated. The density, speed of sound and viscosity values for these nanofluids have been measured at T = (293.15, 298.15, 308.15 and 318.15) K. From these experimental data, the excess molar volume, $V_m^E$ and isentropic compressibility, κ_s, have been determined. The behaviour of these values with temperature and concentration has been interpreted for clarifying the dispersion of ZnO nanoparticles in PEG and aqueous solution of PEG. The effects of ZnO nanoparticles and temperature have also been investigated on volumetric and transport properties of aqueous solutions of PEG. The $V_m^E$, was adequately fitted to the Redlich–Kister, Ott et al. and Singh et al. equations. The isentropic compressibility values were correlated with the polynomial equation. The Eyring-NRTL and Eyring-mNRF models have successfully been used for correlating the viscosity values of the nanofluids investigated with temperature dependency considered. The performance of the Einstein, Brinkman, Lundgren and Batchelor models in the prediction of viscosity values of the (ZnO + PEG) nanofluid has also been tested.     

Effect of EM propagation medium on electrorheological characteristics of dielectric nanofluids

The effect of dielectric loss on the electrorheological (ER) characteristic of dielectric nanofluids under shear was studied. When nanofluids are activated by an applied electric field, it behaves like a non-Newtonian fluid under ER effect by creating the chains of nanoparticles. ER characteristics of ZnO and Al₂O₃ nanofluids with various nanoparticles concentration (0.1, 0.05, 0.01 wt%) were measured. For this purpose, a solenoid-based electromagnetic (EM) transmitter was used under different propagation media including air, tap water, and salt water. The result shows that all the nanofluids exhibit pseudo-plastic behavior, while the electric field causes a significant increase in viscosity in the presence of tap water, followed by salt water. Additionally, the viscosity of nanofluid shows a high dependence on particle loading. A possible mechanism was also proposed to describe the effect of dielectric properties on the ER behavior of dielectric nanofluids.     

Assessing the flow characteristics of nanofluids during turbulent natural convection

High-performance cooling is of vital importance for the cutting-edge technology of today, from nanoelectronic mechanical systems to nuclear reactors. Advances in nanotechnology have allowed the development of a new category of coolants, termed nanofluids that have the potential to enhance the thermal performance of conventional heat transfer fluids. At the present time, nanofluids are a controversial research theme, since there is yet no conclusive answer to explain the underlying physical mechanisms of heat transfer. The current study investigates experimentally the heat and mass transfer behaviour of dilute Al₂O₃–H₂O nanofluids under turbulent natural convection—Rayleigh number of the order of 10⁹—in a cubic Rayleigh–Bénard cell with optical access. Traditional heat transfer measurements were combined with a velocimetry method to obtain a deeper understanding of the impact of nanoparticles on the heat transfer performance of the base fluid. Particle image velocimetry was employed to quantify the resulting mean velocity field and flow structures in dilute nanofluids under natural convection, at three parallel planes inside the cubic cell. All the results were compared with that for the base fluid, i.e. deionised water. It was observed that the presence of a minute amount of Al₂O₃ nanoparticles in deionised water, φ_v =?0.00026 vol.%, considerably modifies the mass transfer behaviour of the fluid in the bulk region of turbulent Rayleigh–Bénard convection. Simultaneously, the general heat transport, as expressed by the Nusselt number, remained unaffected within the experimental uncertainty.

     

Viscosity modification of lubricating oil based on high-concentration silica nanoparticle colloidal system

In this study, colloidal systems with SiO₂ nanoparticle as viscosity modifier additive were synthesized in the lubricating oil via an in situ Stober sol-gel method. The fluid characters of lubricating oil and viscosity variation were carefully investigated via rheological methods. The results showed that the lubricating oil transformed from Newtonian fluid to non-Newtonian fluid with increasing the concentration of nanoparticles, and smaller particles displayed better thickening effect toward lubricating oil. For the system with highly concentrated nanoparticle (20?wt%), the rheological behavior mainly depends on the size of nano-SiO₂. The lubricating oil with smaller nano-SiO₂ particles displayed higher structural strength and response rate, resulting in good recoverability after high-speed shear. The viscosity changed with temperature and also displayed a thermo-responsive behavior, which significantly alleviated the effect of shear thinning on the viscosity under high temperature. This study presented a new strategy for effectively tuning the fluid characters and modifying the viscosity of lubricating oils by adding highly concentrated inorganic nanoparticles.     

Effect of viscous dissipation on MHD water-Cu and EG-Cu nanofluids flowing through a porous medium

This paper provides a comparative analysis of two different types of nanofluids for Stokes second problem. Additional effects of MHD, porosity and viscous dissipation are also considered. Two types of Newtonian liquids (water and ethylene glycol) are considered as base fluids with suspended nanosized Cu particles. A homogenous model of Newtonian nanofluids over a flat plate is used to describe this phenomenon with Stokes boundary conditions such that the ambient fluid is static and with uniform temperature. The problem is first written in terms of nonlinear partial differential equations with physical conditions; then after non-dimensional analysis, the Laplace transform method is used for its closed-form solution. Exact expressions are determined for the dimensionless temperature, velocity field, Nusselt number and skin friction coefficient and arranged in terms of exponential and complementary error functions satisfying the governing equations and boundary conditions. They are also reduced to the known solutions of Stokes second problem for Cu-water nanofluids. Results are computed using Maple software. The results showed that both skin friction and rate of heat transfer increase with increasing solid volume fraction of nanoparticles. MHD and porosity had an opposite effect on velocity for both types of nanofluids. The dimensionless temperature increases by increasing the Eckert and Hartmann numbers.

     

Prediction of rheological behavior of MWCNTs–SiO2/EG–water non-Newtonian hybrid nanofluid by designing new correlations and optimal artificial neural networks

In this paper, at the first, new correlations were proposed to predict the rheological behavior of MWCNTs–SiO₂/EG–water non-Newtonian hybrid nanofluid using different sets of experimental data for the viscosity, consistency and power law indices. Then, based on minimum prediction errors, two optimal artificial neural network models (ANNs) were considered to forecast the rheological behavior of the non-Newtonian hybrid nanofluid. One hundred and ninety-eight experimental data were employed for predicting viscosity (Model I). Two sets of forty-two experimental data also were considered to predict the consistency and power law indices (Model II). The data sets were divided to training and test sets which contained respectively 80 and 20% of data points. Comparisons between the correlations and ANN models showed that ANN models were much more accurate than proposed correlations. Moreover, it was found that the neural network is a powerful instrument in establishing the relationship between a large numbers of experimental data. Thus, this paper confirmed that the neural network is a reliable method for predicting the rheological behavior of non-Newtonian nanofluids in different models.

     

A review on molten-salt-based and ionic-liquid-based nanofluids for medium-to-high temperature heat transfer

Molten-salt-based nanofluids and ionic-liquid-based nanofluids are developed for thermal storage and heat transfer at relatively high temperatures, in the past few years. Preparation and stabilization techniques are briefly introduced firstly, and then, thermal properties, e.g., specific heat, thermal conductivity and viscosity, are summarized and discussed in detail. The properties are not only affected by the characteristics of nanomaterials and base fluids, but also affected by the synthesis method, such as the sonication intensity and duration. Some of the thermophysical property data are still incomplete, especially the thermal conductivity of molten-salt-based nanofluids, and properties of ionic-liquid-based nanofluids at high temperatures. While several literature works show that the Krieger–Dougherty model can well predict the viscosity, no general models for thermal conductivity and specific heat have been developed yet for both types of nanofluids.

     

Pressure Drop and Performance Characteristics of Water-Based Al2O3 and CuO Nanofluids in a Triangular Duct

An experimental study is performed to determine the pressure drop and performance characteristics of Al₂O₃/water and CuO/water nanofluids in a triangular duct under constant heat flux where the flow is laminar. The effects of adding nanoparticles to the base fluid on the pressure drop and friction factor are investigated at different Reynolds numbers. The results show that at a specified Reynolds number, using the nanofluids can lead to an increase in the pressure drop by 35%. It is also found that with increases in the Reynolds number, the rate of increase in the friction factor with the volume fraction of nanoparticles is reduced. Finally, the performance characteristics of the two nanofluids are investigated using the data of pressure drop and convective heat transfer coefficient. The results show that the use of Al₂O₃/water nanofluid with volume fractions of 1.5% and 2% is not helpful in the triangular duct. It is also concluded that at the same volume fraction of nanoparticles, using Al₂O₃ nanoparticles is more beneficial than CuO nanoparticles based on the performance index.     

Viscosity of nanofluids: particle shape and fractal aggregates

Colloidal dispersions of nanoparticles in thermal base fluids are known to alter their spherical shapes thereby affecting their surface properties. This aspect has been investigated with respect to the effective viscosity of nanofluids presuming the particles to acquire the shape of prolate spheroid. Also, the contributions of the interfacial layer formed around these particles and their possible agglomeration has been taken in to account. The analysis has been carried out by modifying the Krieger and Dougherty model. The relative viscosity of these nanofluids has been computed as a function of volume fraction, particle size and the eccentricity of the particle. The model also incorporates the concept of fractal dimensions. The results thus obtained compare significantly well with the available experimental data and reaffirms an improvement over earlier models.     

Effect of temperature on AC conductivity of poly(butyl methacrylate)/cerium dioxide nanocomposites and applicability of different conductivity modeling studies

In this research, the effect of cerium dioxide (CeO₂) nanoparticles on electrical properties of poly(butyl methacrylate) (PBMA) has been investigated. Polymer nanocomposites reinforced with variable contents of CeO₂ nanoparticles (3, 5, 7 and 10 wt%) were fabricated by an in situ polymerization method. The formation of nanocomposites was analyzed by FTIR, XRD, SEM and TEM analysis. Also, the AC and DC conductivities of CeO₂ nanoparticles-reinforced PBMA were systematically studied with respect to different loadings of CeO₂ fillers. The FTIR, XRD and morphological studies revealed that the nanoparticles were well inserted and uniformly dispersed into the macromolecular chain of PBMA. The AC conductivity of PBMA/CeO₂ composite increases not only with the loading of nanoparticles but also with the temperature of the system. The activation energy determined from AC electrical conductivity was found to decrease with the frequency and temperature. DC conductivity of the nanocomposites was increased with the insertion of nanoparticles into PBMA. The DC conductivity of all the composites was greater than pure PBMA. The applicability of different theoretical models such as Scarisbrick, Bueche and McCullough equations was compared with the experimentally determined DC conductivity of PBMA/CeO₂ nanocomposites. These models fail to explain the conductivity of polymer composite in the entire loading of fillers. Hence, a new theoretical model is proposed in this study and it shows good agreement with the experimentally observed conductivity values.

     

Evaporation-induced formation of fractal-like structures from nanofluids

After the nanofluids are fully dried, the self-assembled nanoparticles can form various structures on the substrate. The fractal-like patterns are among them. The two-dimensional Kinetic Monte Carlo model is developed to predict the drying patterns of the nanofluids in an open domain, where the dewetting front shrinks from the edge toward the center. The simulation reveals that the initially dispersed particles can be deposited into an isotropic branched structure which remains frozen after full evaporation of the base fluid. The well-developed fractal-like particle aggregates are different from the fractal cavities obtained in the previous closed domain simulation. The present prediction of the fractal particle aggregation is verified by the experiments with the water-based nanofluids. The images taken using a scanning electron microscope prove that the evaporation-induced branched microstructures are formed by the nanoparticles as the water is totally dried.     

Effect of pH on deagglomeration and rheology/morphology of aqueous suspensions of goethite nanopowder

The kinetics of deagglomeration in diluted suspensions of goethite nanopowder, as well as the rheology and morphology of the resulting suspensions, strongly depends on pH. At pH 3, nanopowder can be dispersed as separate nanoparticles, and the resulting suspension is Newtonian, with the viscosity only marginally higher than the viscosity of water. At pH between 5 and 12, nanoparticles tend to reaggregate and form weak aggregates/flocs. Morphology changes from a Newtonian suspension of primary nanoparticles to a non-Newtonian, shear-thinning suspension of large, porous, interconnected flocs with the yield stress reaching a maximum at an isoelectric point. The effect of pH on morphology and rheology is reversible, and as pH is reduced to 3, the suspension becomes Newtonian, with viscosity marginally higher than the viscosity of water. The rheological models based on DLVO theory do not allow prediction of the effect of pH on viscosity and yield stress, but the flow curves of goethite suspensions can be described by a fractal model with five adjustable parameters.     

Thermal conductivity of Al2O3/water nanofluids

A considerable number of studies can be found on the thermal conductivity of nanofluids in which Al₂O₃ nanoparticles are used as additives. In the present study, the aim is to measure the thermal conductivity of very narrow Al₂O₃ nanoparticles with the size of 5 nm suspended in water. The thermal conductivity of nanofluids with concentrations up to 5 % is measured in a temperature range between 26 and 55 °C. Using the experimental data, a correlation is presented as a function of the temperature and volume fraction of nanoparticles. Finally, a sensitivity analysis is performed to assess the sensitivity of thermal conductivity of nanofluids to increase the particle loading at different temperatures. The sensitivity analysis reveals that at a given concentration, the sensitivity of thermal conductivity to particle loading increases when the temperature increases.