Higher Nanoparticle Volume Fraction Research Articles

Non-Newtonian nanofluid in a micro planar sudden expansion considering variable properties

The heat transfer and flow pattern of Al2O3-water nanofluid in a micro planar sudden expansion with constant heat flux boundary conditions are numerically studied. The Al2O3-water nanofluid is treated as a single phase non-Newtonian fluid with variable physical properties and the power-law rheology is adopted to describe the characteristics of the flow, in which the flow behavior index depend on the nanoparticle volume fraction. A systematic study of the Al2O3-water nanofluid for a wide range of generalized Reynolds number, 50 ≤ Regen ≤ 500, and nanoparticle volume fraction 0% ≤ φ ≤ 3% is presented. The critical generalized Reynolds number at which flow bifurcation occurs is carefully studied and the heat transfer enhancement due to the non-Newtonian rheology is reported. It is shown that the flow bifurcation is delayed when the nanoparticle volume fraction increases and is advanced as the boundary heat transfer rate increases. The heat transfer deterioration brought by the recirculation area is reduced under a higher nanoparticle volume fraction and the reduction ratio increases with the generalized Reynolds number. The non-Newtonian model results are also compared with the results using a Newtonian model, which indicates that there will be a huge overestimate on the system pressure drop if Newtonian model is taken.

Nanoparticle and poly(methyl methacrylate) co-dispersion in anisole

Aqueous nanoparticle dispersions have been widely studied for their important industrial applications, including photocatalyst, coating, and casting. However, much less work has been conducted on nanoparticle dispersions in organic solvents, especially when co-dispersing with dissolved polymers. In this study, we explore TiO2/ZnO nanoparticle and poly(methyl methacrylate) (PMMA) co-dispersion in organic solvent anisole. To achieve uniformly dispersed suspensions, nanoparticle surface functionalization with nonanoic acid and 1-pentanol is conducted. Nonanoic acid has higher surface coverage on TiO2 and the two dispersants show similar surface coverage on ZnO. Heat treatment enhances the dispersant adsorption on the nanoparticle surfaces. For both TiO2/PMMA and ZnO/PMMA co-dispersions, higher solids loading leads to higher suspension viscosity. Higher volume fraction of nanoparticles in the nanoparticle plus PMMA solid phase results in lower viscosity, mainly by reducing the entanglement of the dissolved PMMA chains.

Numerical investigation on the melting of nanoparticle-enhanced phase change materials (NEPCM) in a bottom-heated rectangular cavity using lattice Boltzmann method

Convection melting of water (ice)–copper nanoparticles as a nanoparticle-enhanced phase change material (NEPCM) in a bottom-heated rectangular cavity is investigated numerically in this paper. A novel lattice Boltzmann (LB) method is developed to solve the solid–liquid phase change coupled with natural convection, in which two sets of evolution functions (density evolution function and temperature evolution function) are constructed to simulate fluid flow and heat transfer. The latent heat source term in temperature evolution function is treated with an implicit scheme, avoiding iteration steps as compared with previous methodologies. The moving melting interface is treated with an immersed moving boundary scheme, which makes it simple for the implementation of non-slip velocity condition on the solid–liquid interface. The predictions of the current model are compared with previous numerical and experimental results, and a reasonable agreement is obtained. And then the developed model is employed to study the impacts of the volume fraction of nanoparticles and the Grashof number on the flow structure and heat transfer characteristics. The computed results show that the NEPCM exhibits high heat transfer efficiency in comparison to the pure phase change material (PCM). As the volume fraction of nanoparticles increases, the temperature field and melting interface evolves faster, and the melt fraction and energy stored increases. In addition, for high volume fraction of nanoparticles at a Grashof number Gr = 2.5 × 10 5 , an asymmetric melting interface forms due to the asymmetric distribution of the convection cells.

Computational Analysis of Nanofluid Cooling of High Concentration Photovoltaic Cells

High concentration photovoltaic devices require effective heat rejection to keep the solar cells within a suitable temperature range and to achieve acceptable system efficiencies. Various techniques have been developed to achieve these goals. For example, nanofluids as coolants have remarkable heat transfer characteristics with broad applications; but, little is known of its performance for concentration photovoltaic cooling. Generally, a cooling system should be designed to keep the system within a tolerable temperature range, to minimize energy waste, and to maximize system efficiency. In this paper, the thermal performance of an Al2O3-water cooling system for densely packed photovoltaic cells under high concentration has been computationally investigated. The model features a representative 2D cooling channel with photovoltaic cells, subject to heat conduction and turbulent nanofluid convection. Considering a semi-empirical nanofluid model for the thermal conductivity, the influence of different system design and operational parameters, including required pumping power, on cooling performance and improved system efficiency has been evaluated. Specifically, the varied system parameters include the nanoparticle volume fraction, the inlet Reynolds number, the inlet nanofluid temperature, and different channel heights. Optimal parameter values were found based on minimizing the system's entropy generation. Considering a typical 200-sun concentration, the best performance can be achieved with a channel of 10 mm height and an inlet Reynolds number of around 30,000, yielding a modest system efficiency of 20%. However, higher nanoparticle volume fractions and lower nanofluid inlet temperatures further improve the cell efficiency. For a more complete solar energy use, a combined concentration photovoltaic and thermal heating system are suggested.

Numerical analysis of melting of nano-enhanced phase change material in latent heat thermal energy storage system

The heat transfer enhancement in the latent heat thermal energy storage system through dispersion of nanoparticle is reported. The resulting nanoparticle-enhanced phase change materials exhibit enhanced thermal conductivity in comparison to the base material. Calculation is performed for nanoparticle volume fraction from 0 to 0.08. In this study rectangular and cylindrical containers are modeled numerically and the effect of containers dimensions and nano particle volume fraction are studied. It has been found that the rectangular container requires half of the melting time as for the cylindrical container of the same volume and the same heat transfer area and also, higher nano particle volume fraction result in a larger solid fraction. The increase of the heat release rate of the nanoparticle-enhanced phase change materials shows its great potential for diverse thermal energy storage application.

Experimental Investigation of Thermal Conductivity and Effusivity of Ferrite Based Nanofluids under Magnetic Field

We investigate the effect of magnetic field (H) on the thermal conductivity (λ) and effusivity (ε) of cobalt ferrite based nanofluids having different concentrations (ϕ). Cobalt ferrite nanoparticles (NPs) have been synthesized using the microwave assisted method. At high volume fraction of cobalt ferrite nanoparticles in water (ϕ > 0), both thermal parameters have been found to be suppressed relative to ϕ and λ of water in the absence of H. However, it is seen that percentage values of the effusivity in perpendicular field direction show negative to positive variation, and thermal conductivity in parallel field direction shows negative to zero variation.

Combined Effect of Magnetic Field and Nanofluid Variable Properties on Heat Transfer Enhancement in Natural Convection

The current work investigated, numerically, enhancement of heat transfer in natural convection using CuO-water nanofluid in the presence of a magnetic field. The governing equations were discretized using the control volume method and solved numerically via the SIMPLE algorithm. For the case of absence of a magnetic field and for low Rayleigh number, the heat transfer was almost insensitive to the presence of nanoparticles. For moderate and high Rayleigh numbers, the presence of nanoparticles had an adverse effect on heat transfer at high volume fraction of nanoparticles. The highest reduction in heat transfer was registered for the case of Ra = 105. Contour maps are generated for the normalized Nusselt number (Nu*) to determine the optimum selection of volume fraction of nanoparticles and magnetic field that gives maximum heat transfer enhancement. The results demonstrated the effectiveness and practicality of using high values of magnetic field in enhancing heat transfer using nanofluids.

Crystallization kinetics and phase transformation of poly(vinylidene fluoride) films incorporated with functionalized baTiO3 nanoparticles

AbstractThe transformation of α to β‐phase in poly(vinylidene fluoride) (PVDF) induced by the addition of tetradecylphosphonic acid (TDPA)‐BaTiO3 nanoparticles and subsequently the isothermal crystallization kinetics of pristine PVDF and its nanocomposites have been investigated. The result of infrared spectra showed that the relative crystalline fraction of β‐phase was enhanced greatly after the introduction of TDPA–BaTiO3 nanoparticles, and reached the peak of 93% when the concentration of nanofillers was 20%. The interaction between TDPA–BaTiO3 nanoparticles and PVDF macromolecular chains induced the change of conformation from trans‐gauche to all‐trans crystal structure in PVDF segment. The isothermal crystallization of TDPA–BaTiO3/PVDF nanocomposites was carried out by the differential scanning calorimetry (DSC). The influence of TDPA–BaTiO3 nanoparticles concentration on crystallization rate, activate energy, melting enthalpy, and peak temperature were studied. The nanocomposite film loaded 20% TDPA–BaTiO3 nanoparticles exhibited the highest crystallization rate and activate energy, which decreased after loading more nanofillers in the host because of high volume fraction of nanoparticles leading to steric hindrance and further weakening the mobility of PVDF chains during the crystallization. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Composition, nanostructure, and optical properties of silver and silver-copper lusters

Lusters are composite thin layers of coinage metal nanoparticles in glass displaying peculiar optical properties and obtained by a process involving ionic exchange, diffusion, and crystallization. In particular, the origin of the high reflectance (golden-shine) shown by those layers has been subject of some discussion. It has been attributed to either the presence of larger particles, thinner multiple layers or higher volume fraction of nanoparticles. The object of this paper is to clarify this for which a set of laboratory designed lusters are analysed by Rutherford backscattering spectroscopy, transmission electron microscopy, x-ray diffraction, and ultraviolet-visible spectroscopy. Model calculations and numerical simulations using the finite difference time domain method were also performed to evaluate the optical properties. Finally, the correlation between synthesis conditions, nanostructure, and optical properties is obtained for these materials.

Morphology Evolution of PS-b-P2VP Diblock Copolymers via Supramolecular Assembly of Hydroxylated Gold Nanoparticles

We report on the strong segregation of core–shell Au nanoparticles, with a shell layer consisting of a random copolymer brush of styrene and vinylphenol (PS-r-PVPh-SH), in poly(styrene-b-2-vinylpyridine) (PS-b-P2VP) diblock copolymer. Because of the formation of multiple hydrogen bonds between the hydroxyl groups within the shell of the nanoparticles and the pyridine group in PS-b-P2VP, the Au nanoparticles were strongly localized into P2VP domains with a very high volume fraction of nanoparticles (ϕp ∼ 0.53). The spatial distribution of Au nanoparticles, observed by transmission electron microscopy (TEM), is compared with results of previous experiments where homopolymers were blended with block copolymers. If the diameter d of the nanoparticles is much less than the width D of the P2VP lamellar domains, these nanoparticles are more uniformly distributed across the P2VP domain than if d is comparable to D, in which case the nanoparticles are pushed toward the center of the P2VP domains. This behavior is ...

Investigation of the electrical conductivity of propylene glycol-based ZnO nanofluids

Electrical conductivity is an important property for technological applications of nanofluids that has not been widely studied. Conventional descriptions such as the Maxwell model do not account for surface charge effects that play an important role in electrical conductivity, particularly at higher nanoparticle volume fractions. Here, we perform electrical characterizations of propylene glycol-based ZnO nanofluids with volume fractions as high as 7%, measuring up to a 100-fold increase in electrical conductivity over the base fluid. We observe a large increase in electrical conductivity with increasing volume fraction and decreasing particle size as well as a leveling off of the increase at high volume fractions. These experimental trends are shown to be consistent with an electrical conductivity model previously developed for colloidal suspensions in salt-free media. In particular, the leveling off of electrical conductivity at high volume fractions, which we attribute to counter-ion condensation, represents a significant departure from the "linear fit" models previously used to describe the electrical conductivity of nanofluids.

Microwave Absorption Properties of Polymer Modified Ni-Zn Ferrite Nanoparticles

Polymer composites of magnetic particles are widely used as microwave absorbers. An effective method for obtaining thinner microwave absorbers for device design is increasing the volume fraction of magnetic nanoparticles by enhancing the permeability of composites. In this study, composites were prepared using Ni-Zn ferrite nanoparticles surface-modified with 4-META (4-methacryloylioxyethyl trimellitate anhydride) and cross-linked with PEG-4SH (pentaerythritol tetra-polyethylene glycol ether with four thiol-modified terminals). These composites have a high volume fraction of nanoparticles (up to 72 vol%) and permeability (μr max″=5.9). In addition, the prepared composites showed good microwave absorption properties (R.L.<−20 dB) with a smaller matching thickness than conventional microwave absorber using spinel-type ferrite.

Magnetic Properties of Nanoparticle&amp;ndash;Polymer Composites Prepared Using Surface Modification and Cross-Linking Reaction

Polymer composites of magnetic particles are widely used as microwave absorbers. An effective method for obtaining thinner microwave absorbers for device mounting is to increase the volume fraction of magnetic nanoparticles within the composites such that the permeability is enhanced. In this study, composites were prepared using magnetite nanoparticles surface-modified with 4-META (4-methacryloylioxyethyl trimellitate anhydride) and cross-linked with PEG-4SH (pentaerythritol tetra-polyethylene glycol ether with four thiol-modified terminals). These composites have a higher volume fraction of nanoparticles (up to 62 vol%) and higher permeability than conventional epoxy resin composites. In addition, the prepared composites showed good microwave absorption properties (R:L :< � 20 dB) with a smaller matching thickness (d ¼ 8:0 mm) than the epoxy resin composites (d ¼ 9:0 mm). [doi:10.2320/matertrans.MAW201020]

Low-Temperature Thermoelectric Power Factor Enhancement by Controlling Nanoparticle Size Distribution

Coherent potential approximation is used to study the effect of adding doped spherical nanoparticles inside a host matrix on the thermoelectric properties. This takes into account electron multiple scatterings that are important in samples with relatively high volume fraction of nanoparticles (>1%). We show that with large fraction of uniform small size nanoparticles (∼1 nm), the power factor can be enhanced significantly. The improvement could be large (up to 450% for GaAs) especially at low temperatures when the mobility is limited by impurity or nanoparticle scattering. The advantage of doping via embedded nanoparticles compared to the conventional shallow impurities is quantified. At the optimum thermoelectric power factor, the electrical conductivity of the nanoparticle-doped material is larger than that of impurity-doped one at the studied temperature range (50-500 K) whereas the Seebeck coefficient of the nanoparticle doped material is enhanced only at low temperatures (∼50 K).

Nanofluid convective heat transfer in a parallel-disk system

Inherently low thermal conductivities of basic fluids form a primary limitation in high-performance cooling which is an essential requirement for numerous thermal systems and micro-devices. Nanofluids, i.e., dilute suspensions of, say, metal-oxide nanoparticles in a liquid, are a new type of coolants with better heat transfer performances than their pure base fluids alone. Using a new, experimentally validated model for the thermal conductivity of nanofluids, numerical simulations have been executed for alumina-water nanofluid flow with heat transfer between parallel disks. The results indicate that, indeed, nanofluids are promising new coolants when compared to pure water. Specifically, smoother mixture flow fields and temperature distributions can be achieved. More importantly, given a realistic thermal load, the Nusselt number increases with higher nanoparticle volume fraction, smaller nanoparticle diameter, reduced disk-spacing, and, of course, larger inlet Reynolds number, expressed in a novel form as Nu = Nu(Re and Br). Fully-developed flow can be assumed after a critical radial distance, expressed in a correlation R crit = fct(Re), has been reached and hence analytic solutions provide good approximations. Nanofluids reduce the system’s total entropy generation rate while hardly increasing the required pumping power for any given Re in. Specifically, minimization of total entropy generation allows for operational and geometric system-optimization in terms of S gen = fct (Re and δ).

Effects of Variable Viscosity and Thermal Conductivity of CuO-Water Nanofluid on Heat Transfer Enhancement in Natural Convection: Mathematical Model and Simulation

Heat transfer enhancement in horizontal annuli using variable thermal conductivity and variable viscosity of CuO-water nanofluid is investigated numerically. The base case of simulation used thermal conductivity and viscosity data that consider temperature property dependence and nanoparticle size. It was observed that for Ra≥104, the average Nusselt number was deteriorated by increasing the volume fraction of nanoparticles. However, for Ra=103, the average Nusselt number enhancement depends on aspect ratio of the annulus as well as volume fraction of nanoparticles. Also, for Ra=103, the average Nusselt number was less sensitive to volume fraction of nanoparticles at high aspect ratio and the average Nusselt number increased by increasing the volume fraction of nanoaprticles for aspect ratios ≤0.4. For Ra≥104, the Nusselt number was deteriorated everywhere around the cylinder surface especially at high aspect ratio. However, this reduction is only restricted to certain regions around the cylinder surface for Ra=103. For Ra≥104, the Maxwell–Garnett and the Chon et al. conductivity models demonstrated similar results. But, there was a deviation in the prediction at Ra=103 and this deviation becomes more significant at high volume fraction of nanoparticles. The Nguyen et al. data and the Brinkman model give completely different predictions for Ra≥104, where the difference in prediction of the Nusselt number reached 50%. However, this difference was less than 10% at Ra=103.

Heat- and mass-transport in aqueous silica nanofluids

Using the transient hot wire and pulsed field gradient nuclear magnetic resonance methods we determined the thermal conductivity and the solvent self-diffusion coefficient (SDC) in aqueous suspensions of quasi-monodisperse spherical silica nanoparticles. The thermal conductivity was found to increase at higher volume fraction of nanoparticles in accordance with the effective medium theory albeit with a smaller slope. On the other hand, the SDC was found to decrease with nanoparticle volume fraction faster than predicted by the effective medium theory. These deviations can be explained by the presence of an interfacial heat-transfer resistance and water retention by the nanoparticles, respectively. We found no evidence for anomalous enhancement in the transport properties of nanofluids reported earlier by other groups.

Effects of variable viscosity and thermal conductivity of Al 2O 3–water nanofluid on heat transfer enhancement in natural convection

Heat transfer enhancement in horizontal annuli using variable properties of Al 2O 3–water nanofluid is investigated. Different viscosity and thermal conductivity models are used to evaluate heat transfer enhancement in the annulus. The base case uses the Chon et al. expression for conductivity and the Nguyen et al. experimental data for viscosity which take into account the dependence of these properties on temperature and nanoparticle volume fraction. It was observed that for Ra ⩾ 10 4, the average Nusselt number was reduced by increasing the volume fraction of nanoparticles. However, for Ra = 10 3, the average Nusselt number increased by increasing the volume fraction of nanoparticles. For Ra ⩾ 10 4, the Nusselt number was deteriorated every where around the cylinder surface especially at high expansion ratio. However, this reduction is only restricted to certain regions around the cylinder surface at Ra = 10 3. For Ra ⩾ 10 4, the difference in Nusselt number between the Maxwell Garnett and Chon et al. model prediction is small. But, there was a deviation in prediction at Ra = 10 3 and this deviation becomes more significant at high volume fraction of nanoparticles. The Nguyen et al. data and Brinkman model gives completely different predictions for Ra ⩾ 10 4 where the difference in prediction of Nusselt number reached 30%. However, this difference was less than 10% at Ra = 10 3.

Retrieval of the effective complex refractive index of Bruggeman nanoliquids from surface plasmon resonance reflectance using the maximum entropy method

We show that surface plasmon resonance reflectance measurement can be used to extract the effective complex refractive index of a Bruggeman nanoliquid, i.e. a solution consisting of spherical nanoparticles embedded into a host liquid such as water, using the maximum entropy method. This retrieval can be immediately performed once the optical properties of the host liquid are known. In this paper we investigate the upper limit of the volume fraction of spherical nanoparticles in a solution that still yields a reliable result in the retrieval based on the maximum entropy method. Finally, we show that by squeezing the spectrum we can significantly reduce the errors in the analysis and retrieve the effective complex refractive index for a high volume fraction of nanoparticles. ¶Permanent address: Department of Physics, University of Joensuu, PO Box 111, FI-80101 Joensuu, Finland. ∥Emails: erik.vartiainen@lut.fi (EMV); kai.peiponen@joensuu.fi (K-EP)

Strengthening and Toughness of AZ61 Mg with Nano SiO&lt;SUB&gt;2&lt;/SUB&gt; Particles

The strengthening mechanisms and bending toughness of the AZ61 Mg based composites reinforced by nano SiO 2 particles are examined. The composites were prepared either by spray forming, ingot metallurgy, or powder metallurgy, followed by severe hot extrusion. The spray formed composites exhibit the best nano particle distribution and toughness, but the volume fraction of the nano particles that can be inserted is limited. The nano composites fabricated through the powder metallurgy method possess the highest strength due to the extra strengthening effect from the MgO phase. Strengthening analysis based on the Orowan strengthening mechanism can predict well the composite strength provided that the nano particles are in reasonably uniform dispersion. For composites containing higher nano particle volume fractions greater than 3%, the experimental strength data fall well below the theoretical predictions, suggesting poor dispersion of the reinforcement.