Scaling analysis for the investigation of slip mechanisms in nanofluids

The primary objective of this study is to investigate the effect of slip mechanisms in nanofluids through scaling analysis. The role of nanoparticle slip mechanisms in both water- and ethylene glycol-based nanofluids is analyzed by considering shape, size, concentration, and temperature of the nanoparticles. From the scaling analysis, it is found that all of the slip mechanisms are dominant in particles of cylindrical shape as compared to that of spherical and sheet particles. The magnitudes of slip mechanisms are found to be higher for particles of size between 10 and 80 nm. The Brownian force is found to dominate in smaller particles below 10 nm and also at smaller volume fraction. However, the drag force is found to dominate in smaller particles below 10 nm and at higher volume fraction. The effect of thermophoresis and Magnus forces is found to increase with the particle size and concentration. In terms of time scales, the Brownian and gravity forces act considerably over a longer duration than the other forces. For copper-water-based nanofluid, the effective contribution of slip mechanisms leads to a heat transfer augmentation which is approximately 36% over that of the base fluid. The drag and gravity forces tend to reduce the Nusselt number of the nanofluid while the other forces tend to enhance it.     

Synthesis of Silica Nanofluid and Application to CO2 Absorption

Abstract

This study focused on the synthesis of stable nanofluids and their direct application to the CO₂ absorption process. A sol-gel process was used as the synthesis method of nanoparticles in nanofluid. The particle size and stability were determined by SEM image and zeta potential of the nanofluid. Three types of nanofluids containing approximately 30 nm, 70 nm, and 120 nm particles were synthesized and all nanofluids had a stable zeta potential of approximately ? 45 mV. Addition of nanoparticles increased the average absorption rate of 76% during the first 1 minute and total absorption amount of 24% in water. The capacity coefficient of CO₂ absorption in the nanofluid is 4 times higher than water without nanoparticles, because the small bubble sizes in the nanofluid have large mass transfer areas and high solubility.     

Numerical simulation of natural convection in a square enclosure filled with nanofluid using the two-phase Lattice Boltzmann method

Considering interaction forces (gravity and buoyancy force, drag force, interaction potential force, and Brownian force) between nanoparticles and a base fluid, a two-phase Lattice Boltzmann model for natural convection of nanofluid is developed in this work. It is applied to investigate the natural convection in a square enclosure (the left wall is kept at a high constant temperature (T_H), and the top wall is kept at a low constant temperature (T_C)) filled with Al₂O₃/H₂O nanofluid. This model is validated by comparing numerical results with published results, and a satisfactory agreement is shown between them. The effects of different nanoparticle fractions and Rayleigh numbers on natural convection heat transfer of nanofluid are investigated. It is found that the average Nusselt number of the enclosure increases with increasing nanoparticle volume fraction and increases more rapidly at a high Rayleigh number. Also, the effects of forces on nanoparticle volume fraction distribution in the square enclosure are studied in this paper. It is found that the driving force of the temperature difference has the biggest effect on nanoparticle volume fraction distribution. In addition, the effects of interaction forces on flow and heat transfer are investigated. It is found that Brownian force, interaction potential force, and gravity-buoyancy force have positive effects on the enhancement of natural convective heat transfer, while drag force has a negative effect.     

Enhancement of thermal conductivity of ethylene glycol based silver nanofluids

Nanofluid is a kind of new engineering material consisting of solid particles with size typically of 1-100 nm suspended in base fluids. Nanofluids offer excellent scope of enhancing thermal conductivity of common heat transfer fluids. In the present study, nanofluids are synthesized using silver nitrate (precursor), ethylene glycol (reducing agent), and poly(acrylamide-co-acrylicacid) (dispersion stabilizer). The different concentrations of silver nanofluid (1000-10,000 ppm) were synthesized. The silver particles present in colloidal phase have been characterized by EDX, XRD, UV-visible spectroscopy, Zeta potential and transmission electron microscopy (TEM). The stability as well as thermal conductivity of these nanofluids was determined with a transient hot-wire apparatus, as a lapse of time after preparation. Typically, 10,000 ppm silver nanofluid exhibited rapid increase in the particle size with the passage of time. Thermal conductivity of silver nanofluids increased to 10, 16, and 18% as the amount of silver particles in nanofluid were 1000, 5000, and 10,000 ppm, respectively. After 30 days of preparation, the thermal conductivity of 1000 and 5000 ppm silver nanofluids decreased slightly from 10% and 15% to 9% and 14%, respectively. In addition, the thermal conductivity of 10,000 ppm nanofluid was decreased from 18% to 14% after 30 days. It is very interesting to report that the silver particles were aggregated in early stage of preparation (up to 15 days), which leads to the increase in the size of silver particles. However, no significant change was observed after 15 days which indicates the stability of silver nanofluids.     

Brownian diffusion effect on nanometer aerosol classification in electrical mobility spectrometer

A multi-channel differential mobility analyzer (MCDMA) or aerosol spectrometer is widely used for classifying and measuring nanometer aerosol particles in the size range from 1 nm to 1 μm because of its better time response than a typical differential mobility analyzer. In the present study, the effect of Brownian diffusion on electrical mobility classification and trajectory of nanometer aerosol particles in an electrical mobility spectrometer developed at Chiang Mai University has been analytically investigated. Th Brownian diffusion of particles inside the spectrometer increased with decreasing particle size and flow rates of aerosol and clean sheath air, and with increasing inner electrode voltage, and also decreased with decreasing operating pressure. The particle trajectories considering Brownian diffusion motion inside the spectrometer were found to be broader than those under no Brownian diffusion. Smaller particles were found to have higher degree of broadening of trajectory than the larger particles. Brownian diffusion effect was found to be significant for particles smaller than 10 nm.     

Natural convection heat transfer of nanofluids along a vertical plate embedded in porous medium

The unsteady natural convection heat transfer of nanofluid along a vertical plate embedded in porous medium is investigated. The Darcy-Forchheimer model is used to formulate the problem. Thermal conductivity and viscosity models based on a wide range of experimental data of nanofluids and incorporating the velocity-slip effect of the nanoparticle with respect to the base fluid, i.e., Brownian diffusion is used. The effective thermal conductivity of nanofluid in porous media is calculated using copper powder as porous media. The nonlinear governing equations are solved using an unconditionally stable implicit finite difference scheme. In this study, six different types of nanofluids have been compared with respect to the heat transfer enhancement, and the effects of particle concentration, particle size, temperature of the plate, and porosity of the medium on the heat transfer enhancement and skin friction coefficient have been studied in detail. It is found that heat transfer rate increases with the increase in particle concentration up to an optimal level, but on the further increase in particle concentration, the heat transfer rate decreases. For a particular value of particle concentration, small-sized particles enhance the heat transfer rates. On the other hand, skin friction coefficients always increase with the increase in particle concentration and decrease in nanoparticle size.     

Convective heat transfer of alumina nanofluids in laminar flows through a pipe at the thermal entrance regime

The convective heat transfer characteristics of aqueous alumina nanofluids were investigated experimentally under forced laminar tube flows. The particles had different shapes of cylinders, bricks and blades, and particle loading was between 0?C5 volume%. The nanofluids were characterized rheologically, and the heat transfer system was validated by using water without particles. In calculating Nusselt and Peclet numbers to assess heat transfer enhancement of nanofluids, physical properties of water were used so as not to exaggerate the amount of heat transfer. It was found that heat transfer coefficients of nanofluids are almost the same or a little smaller than that of water. The heat transfer coefficient can be reduced by the lowering the thermal conductivity of the nanofluid under shearing conditions and particle depletion by the cluster migration from the wall to the tube center. The reduction in thermophysical properties also contributes to the reduction in heat transfer coefficient. It has been concluded that nanofluids from metal particles with appropriate stabilizing agents can satisfy the requirements to be a practically usable nanofluid.     

Pore‐network modeling of particle retention in porous media

Transport and filtration of micron and submicron particles in porous media is important in applications such as water purification, contaminants dispersion, and drilling mud invasion. Existing macroscopic models often fail to be predictive without empirical adjustments and a more fundamental approach may be required. We develop a physically‐representative, 3D pore network model based on a particle tracking method to simulate particle retention and permeability impairment in polydisperse particle systems. The model includes the effect of hydraulic drag, gravity, electrostatic and van der Waals forces, as well as Brownian motion. A converging‐diverging pore throat geometry is used to capture the mechanism of interception. With the analytical solution of fluid velocity within a pore throat, the trajectory of each particle is calculated explicitly. We also incorporate surface roughness and particle–surface interaction to determine particle attachment and detachment. Pore throat structure and conductivity are updated dynamically to account for the effect of deposited particles. Predictions of effluent concentration and macroscopic filtration coefficient are in good agreement with published experimental data. We find that the filtration coefficient is dependent on the relative angle between fluid flow and gravity. Particle deposition by interception is significant for large particle/grain size ratios. Brownian diffusion is the primary cause of retention at low Peclet numbers, especially for small gravity numbers. Particle size distribution is found to be a cause of hyperexponential deposition often observed in experiments. Permeability reduction was small for strong repulsive forces because particles only deposited in paths of slow velocity. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3118–3131, 2017     

The Effect of Size and Concentration of Nanoparticles on the Mass Transfer Coefficients in Irregular Packed Liquid–Liquid Extraction Columns

This investigation explored the effects of nanofluids on mass transfer enhancement using an irregularly packed liquid–liquid extraction column and the chemical systems of water–acetic acid–toluene. SiO₂ nanoparticles with sizes of 10, 30, or 80 nm are dispersed in toluene–acetic acid to produce nanofluids with different volume fractions of 0, 0.01, 0.05, and 0.1 vol.%. The effects of nanoparticle size and concentration on dispersed phase mass transfer coefficient were discussed based on the experimental data. This is for the first time that the effect of nanoparticle size is studied in liquid–liquid extraction systems. It was found that the mass transfer enhancement was more significant in nanofluids with smaller particles. It was also observed that mass transfer coefficient is larger in nanofluids compared to that in dispersed phase without nanoparticles, with a peak enhancement at a nanoparticle volume fraction of 0.05 vol.% for 10-nm particles and 0.01 vol.% for 30- and 80-nm particles. The maximum mass transfer coefficient enhancement was approximately 42% at 0.05% concentration of nanoparticles using smaller particles (10 nm). Finally, a novel correlation for prediction of effective diffusivity in the presence of nanoparticles has been proposed, which is a function of nanoparticle size and its concentration. The main advantage of this approach is that the principal effect of these two parameters is considered in correlation without which the experimental data could not be fitted with an acceptable accuracy.     

Toward nanofluids of ultra-high thermal conductivity

The assessment of proposed origins for thermal conductivity enhancement in nanofluids signifies the importance of particle morphology and coupled transport in determining nanofluid heat conduction and thermal conductivity. The success of developing nanofluids of superior conductivity depends thus very much on our understanding and manipulation of the morphology and the coupled transport. Nanofluids with conductivity of upper Hashin-Shtrikman (H-S) bound can be obtained by manipulating particles into an interconnected configuration that disperses the base fluid and thus significantly enhancing the particle-fluid interfacial energy transport. Nanofluids with conductivity higher than the upper H-S bound could also be developed by manipulating the coupled transport among various transport processes, and thus the nature of heat conduction in nanofluids. While the direct contributions of ordered liquid layer and particle Brownian motion to the nanofluid conductivity are negligible, their indirect effects can be significant via their influence on the particle morphology and/or the coupled transport.     

Particle transport in a small square enclosure in laminar natural convection

The transport of particles with diameters in the range of 50 nm to 1 μm in laminar free convection of air in square enclosures was numerically investigated by an Eulerian–Lagrangian method. Two-dimensional square enclosures with widths from 2.5 mm to 5 cm, with two adiabatic surfaces and 100 and 200 °C temperature difference between the other two surfaces, were considered. The Rayleigh numbers varied from 100 to 8×10⁵. The air flow was simulated in Eulerian frame using a commercial CFD software, whose predictions were compared with published benchmark results. Lagrangian particle transport calculations were carried out by tracking 1000 particles that were initially randomly distributed in the flow field, and assuming one-way coupling between the particles and the carrier gas. Particle motion mechanisms considered included gravity, drag, lift force, thermophoresis and Brownian dispersion.The results showed that at Rayleigh numbers lower than about 10 000 the entire flow field was dominated by a single recirculation pattern. For these low Rayleigh number cases most of the particles disperse towards the walls, while a fraction of particles were trapped in a quasi-steady recirculation zone. Inside this recirculation zone the particles were at quasi-equilibrium with respect to the hydrodynamic and dispersive forces that acted on them, and left the zone due to Brownian dispersion only at a very low rate. This quasi-equilibrium zone was not observed at the higher Rayleigh numbers where a single recirculation pattern no longer governed the entire flow field. The results also confirmed the important role of thermophoresis and Brownian dispersion, in particular for submicron size particles.     

Measurement of the Aerodynamic Drag Force on Single Aerosol Particles

The “picobalance” (quadrupole) was used to measure the aerodynamic drag force on individual solid particles and droplets by suspending the object in a laminar jet of gas introduced through the bottom electrode. Particles ranging in diameters from about 1 to 150 μm can be studied in this manner. The DC voltage required to maintain the particle position against the opposing forces of aerodynamic drag and gravity was measured to determine the drag force. The flow velocity at which the aerodynamic drag force balances the gravitational force yields information on the aerodynamic size, and the DC voltage required to suspend the particle against gravity with no flow provides a measure of the particle mass. Particle mobilities for spherical and irregularly shaped solids are presented. Light-scattering measurements for spherical particles provide an independent determination of size; the results are generally in good agreement with the aerodynamic size. It is shown that the electrodynamic balance can be used to measure drag forces much larger than the particle weight.     

Effect of Sub-Grid Scales on Large Eddy Simulation of Particle Deposition in a Turbulent Channel Flow

Large-eddy simulations (LES) of particle transport and deposition in turbulent channel flow were presented. Particular attention was given to the effect of subgrid scales on particle dispersion and deposition processes. A computational scheme for simulating the effect of subgrid scales (SGS) turbulence fluctuation on particle motion was developed and tested. Large-eddy simulation of Navier-Stokes equations using a finite volume method was used for finding instantaneous filtered fluid velocity fields of the continuous phase in the channel. Selective structure function model was used to account for the subgrid-scale Reynolds stresses. It was shown that the LES was capable of capturing the turbulence near wall coherent eddy structures.

The Lagrangian particle tracking approach was used and the transport and deposition of particles in the channel were analyzed. The drag, lift, Brownian, and gravity forces were included in the particle equation of motion. The Brownian force was simulated using a white noise stochastic process model. Effects of SGS of turbulence fluctuations on deposition rate of different size particles were studied. It was shown that the inclusion of the SGS turbulence fluctuations improves the model predictions for particle deposition rate especially for small particles. Effect of gravity on particle deposition was also investigated and it was shown that the gravity force in the stream wise direction increases the deposition rate of large particles.     

Hydrodynamic forces on monodisperse assemblies of axisymmetric elongated particles: Orientation and voidage effects

We investigate the average drag, lift, and torque on static assemblies of capsule-like particles of aspect ratio 4. The performed simulations are from Stokes flow to high Reynolds numbers (0.1 ≤ Re ≤ 1,000) at different solids volume fraction (0.1 ≤ ɛ_s ≤ 0.5). Individual particle forces as a function of the incident angle ϕ with respect to the average flow are scattered. However, the average particle force as a function of ϕ is found to be independent of mutual particle orientations for all but the highest volume fractions. On average, a sine-squared scaling of drag and sine-cosine scaling of lift holds for static multiparticle systems of elongated particles. For a packed bed, our findings can be utilized to compute the pressure drop with knowledge of the particle-orientation distribution, and the average particle drag at ϕ = 0° and 90°. We propose closures for average forces to be used in Euler–Lagrange simulations of particles of aspect ratio 4.     

Particle-particle interactions during normal flow filtration: Model simulations

Although particle trajectory calculations have been used previously to analyze the behavior of membrane systems, these studies have ignored the effects of particle-particle interactions. Particle motion was evaluated by numerical integration of the Langevin equation accounting for the combined effects of electrostatic repulsion, enhanced hydrodynamic drag, Brownian diffusion, and interparticle forces. In the absence of Brownian forces, particles are unable to enter the pore unless the drag force associated with the filtration velocity can overcome the electrostatic repulsion. The presence of a second particle alters the particle trajectories, forcing the particles to attain equilibrium positions located symmetrically about the pore centerline. Interparticle forces can effectively push the particle over the energy barrier, significantly reducing the magnitude of the critical filtration velocity required for particle transmission. Brownian forces also allow particles to enter the pore, with the particle transmission increasing with increasing filtration velocity.     

Synthesis of the Silver Nanofluid by ASNSS Process

Arc Spray Nanoparticle Synthesis System (ASNSS) has been used to prepare the silver nanofluids in this study. The metal electrodes under the electrical discharge will melt and evaporate rapidly and condense to form the nanoparticles in the dielectric fluid at lower temperature and produce the suspended nanoparticle fluid. Thus, the mechanism of the ASNSS process is superheating the electrodes by plasma to form metallic nuclei and supercooling these nuclei by dielectric liquid to produce nanofluid. This study considers the different controlling parameters such as discharge current,discharge voltage, pulse-duration time, electrode diameter, and the temperature of dielectric liquid. The optimally operated parameters can be obtained to produce the finer particle size in nanofluid. The results indicate the silver electrodes in alcohol fluid will produce the spherical nanosilver particles. The mean particle size of silver in different dielectric liquid temperatures of-40, -20, 0, and 10℃ is about13.4, 15.8, 17.5, and 21.6 nm, respectively. This indicates that the well suspended fluid can be obtained by controlling the lower dielectric fluid temperature.     

Numerical simulation of microscopic motion and deposition of nanoparticles via electrodynamic focusing

A computational model was developed to simulate microscopic motion and deposition of charged aerosols during the nanoparticle patterning process utilizing electrodynamic focusing concept (Kim et al., 2006). Our computational model includes Brownian random force, Coulomb and image forces, fluid drag and van der Waals force for determining Lagrangian particle trajectories after solving electrostatic fields in the deposition chamber. Our results are in agreement with the previous experimental findings. The effects of operation parameters such as surface charge density, applied voltage and particle charges were investigated. It was found that the electric field-induced motion of particles dominated over Brownian random motion of 10 nm nanoparticles near the surface and the inertial motion of charged nanoparticles under high electric field would be important to determine the precise deposition pattern within submicrometer scale structures.     

低压条件下纳米流体的沸腾换热特性

在不同低压压力和不同纳米流体浓度下对光滑传热面上的水基纳米流体的池内沸腾特性进行了试验研究.纳米流体由平均直径50 nm的氧化铜粒子加入去离子水中组成,没有加入任何添加剂.研究主要针对7.2 kPa到100kPa的压力区间和0.1%到2%的质量浓度区间内压力和颗粒浓度对光滑表面沸腾换热特性的影响,研究结果表明:压力对纳米流体的沸腾换热特性有强烈影响,沸腾换热系数和临界热流密度(CHF)强化率随着压力的降低而大幅度增加.纳米流体浓度对沸腾换热系数和临界热流密度(CHF)有重要影响,并且在质量浓度约1%附近存在一个最佳颗粒浓度.研究结果显示由与去离子水相比,质量分数为1%,压力为7.2 kPa的纳米流体在光滑表面上的沸腾换热系数和临界热流密度都得到了显著提高.     

Production and dispersion stability of nanoparticles in nanofluids

This paper presents an experimental study on the homogeneous dispersion of nanoparticles in nanofluids. In this study, various physical treatment techniques based on two-step method, including stirrer, ultrasonic bath, ultrasonic disruptor, and high-pressure homogenizer were systematically tested to verify their versatility for preparing stable nanofluids. Initially carbon black and silver nanoparticles dispersed in base fluids with the presence of surfactant were found to be highly agglomerated with the hydrodynamic diameter of 330 nm to 585 nm, respectively. After both CB and Ag nanofluids were treated by various two-step methods, stirrer, ultrasonic bath, and ultrasonic disrupter was found to do a poor performance in deagglomeration process for the initial particle clusters. However, the high-pressure homogenizer produced the average diameter of the CB and Ag particles of 45 nm and 35 nm, respectively, indicating that among various physical treatment techniques employed in this study, the high-pressure homogenizer was the most effective method to break down the agglomerated nanoparticles suspended in base fluids. In order to prepare another nanofluid with much smaller primary nanoparticles, we also employed a modified magnetron sputtering system, in which the sputtered nanoparticles were designed to directly mix with the running surfactant-added silicon oil thin film formed on a rolling drum (i.e. one-step method). We observed that Ag nanoparticles produced by the modified magnetron sputtering system were homogeneously dispersed and long-term stable in the silicon oil-based fluid, and the average diameter of Ag nanoparticles was found to be ~ 3 nm, indicating that the modified magnetron sputtering system is also an effective one-step method to prepare stable nanofluids.