Effective Viscosity Of Nanofluid Research Articles

On the Controversy of Nanofluid Rheological Behavior

Different findings about suitable correlations to describe nanofluid viscosity can be explained based on the research presented in this paper. The effective viscosity of nanofluids is crucial when nanofluids are considered as heat carrier fluids. Despite many publications, however, no consensus about suitable correlations could be found in past years. Particularly, the impact of the shear rate on the viscosity is being discussed controversially. It is shown in this paper that these different findings can be explained considering the theory for the rheology of suspensions. Any measurement of viscosity over shear rate only shows a section of the entire rheological behavior. Thus, experimental results of shear thinning, Newtonian behavior and shear thickening of nanofluids can all be a part of this overall range of possible shear rates. This hypothesis is validated based on viscosity data from the literature and viscosity measurements over a wide range of shear rates for different nanofluids showing all three types of behavior.

Effective viscosity of nanofluids — A modified Krieger–Dougherty model based on particle size distribution (PSD) analysis

Nanofluids (colloidal suspensions of nano-sized metallic and non-metallic particles in conventional cooling liquids) are well known for their potential to enhance the thermal transport. Excessive attempts have been made to utilize these nanofluids in heat transfer applications and energy conversion systems, etc. Effective viscosity of nanofluids is a pivotal parameter in determining the flow and heat transfer performance of nanofluids. The prime aim of this work is to develop a new model for effective viscosity of nanofluids. The influences of aggregation and interfacial layer formation have been incorporated into the Krieger–Dougherty (K–D) equation to predict the effective viscosity of nanofluids. This is accomplished by characterizing the clusters based on particle size distribution (PSD) analysis. Furthermore, attention has been paid to showcase the effects of cluster volume fraction, particle diameter and surfactants on effective viscosity of nanofluids. The predicted results are in good agreement with a wide variety of experimental data from literature consisting of different combinations of nanoparticles and basefluid. The accuracy and ease of application of the newly proposed model make it more interesting and useful for practical engineers in design and development of heat transfer systems using nanofluids.

A new model for density of nanofluids including nanolayer

Nanofluids, which are suspension of nanoparticles in conventional heat transfer fluids, attracted researchers while they show higher thermal conductivity and specific heat capacity. The important parameters that have influence on thermal fluid properties of nanofluids include the volume fraction of the nanoparticles, temperature, density of fluid base and nanoparticles, nanoparticles size, nanolayer, thermal conductivity of base fluid and particles, and pH. Nanolayer is an approved interfacial layer between particles and base fluid involved in some of modeling for effective thermal conductivity and effective viscosity of nanofluids. Therefore, this layer must effect on other properties of nanofluids such as density. In this study, investigation into the density of nanofluids has done experimentally. The nanofluids investigated for density measurements consist of SiO2–water, MgO–glycerol, CuO–glycerol, and SiOx-ethylene glycol/water for a range of 1%–6% volume fraction as well as a temperature range of 10°C to 40°C. The results show that mixture model for density of nanofluids (density of nanofluid=density of base fluid multiply by volume fraction of base fluid + density of nanoparticles multiply by volume fraction of nanoparticles), which is generally cited in literature, has higher value than experimental data. For higher volume fraction of nanoparticles, the gap between the experimental results and the mixture model gets more. This is due to the nanolayer that also shows nanolayer density can be between void and the base fluid density. Therefore, based on the experimental data, a new model for density of nanofluids developed, which includes nanolayer. It was also found that the amount of the void in the nanolayer is more sensitive to nanoparticle size and not to base fluids or nanoparticles material.

Numerical study of mixed convection heat transfer in lid-driven cavity utilizing nanofluid: Effect of type and model of nanofluid

Numerical investigation of the laminar mixed convection in two-dimensional lid driven cavity filled with water-Al2O3, water-Cu or water-TiO2 nanofluids is done in this work. In the present study, the top and bottom horizontal walls are thermally insulated while the vertical walls are kept at constant but different temperatures. The governing equations are given in term of the stream function-vorticity formulation in the non-dimensionalized form and then solved numerically by second-order central difference scheme. The thermal conductivity and effective viscosity of nanofluid have been calculated by Maxwell-Garnett and Brinkman models, respectively. An excellent agreement between the current work and previously published data on the basis of special cases are found. The governing parameters are Rayleigh number 103 ? Ra ? 106 and solid concentration 0 ? ? ?0.2 at constant Reynolds and Prandtl numbers. An increase in mean Nusselt number is found as the volume fraction of nanoparticles increases for the whole range of Rayleigh numbers. In addition, it is found that significant heat transfer enhancement can be obtained by increasing thermal conductivity coefficient of additive particles. At Ra=1.75?105, the Nusselt number increases by about 21% for TiO2-Water, and almost 25% for Al2O3-Water, and finally around 40% for Cu-Water nanofluid. Therefore, the highest values are obtained when using Cu nanoparticles. The result obtained using variable thermal conductivity and variable viscosity models are also compared to the results acquired by the Maxwell-Garnett and the Brinkman model.

Lattice Boltzmann simulation of magnetohydrodynamic natural convection heat transfer of Al 2 O 3 –water nanofluid in a horizontal cylindrical enclosure with an inner triangular cylinder

Abstract In this study the lattice Boltzmann method is applied to investigate the effect of magnetic field on natural convection heat transfer of Al 2 O 3 –water nanofluid in a two-dimensional horizontal annulus. In this model, the effect of Brownian motion on the effective thermal conductivity is also considered. The effective thermal conductivity and the effective viscosity of nanofluid are calculated by KKL (Koo–Kleinstreuer–Li) correlation. The effect of nanoparticle volume fraction for the enhancement of heat transfer was examined for several sets of values of Rayleigh and Hartmann numbers. Also, a correlation of the Nusselt number with physical parameters is presented. The obtained results indicate that the value of the maximum stream function decreases with increasing Hartmann number. Furthermore, we notice that the Nusselt number has a direct relationship with the Rayleigh number; but quite the opposite is true with the Hartmann number. The obtained results indicate that the Lattice Boltzmann method with double-population is a powerful approach for the simulation of natural convection heat transfer in nanofluids in regions with curved boundaries.

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.

Heat transfer and flow characteristics of AL2O3–water nanofluid in a double tube heat exchanger

An experimental study was carried out in order to find out the effects of Al2O3 nanofluid with a mean diameter of 20nm on heat transfer, pressure drop and thermal performance of a double tubes heat exchanger. The effective viscosity of nanofluid was measured in various temperatures ranging from 27°C to 55°C. Experiments were carried out at different Reynolds numbers ranging from 5000 to 20,000, approximately, and in various nanoparticles concentration up to 1% by volume. Results indicate that there is a good potential in promoting the thermal performance of heat exchanger by adding nanoparticles in the investigated ranges where there is not a severe pressure drop penalty. The empirical correlation was created for Nusselt number variation based on the Reynolds number and nanoparticles concentration.

Effective viscosity of magnetic nanofluids through capillaries

The simultaneous effect of magnetic field and temperature on the capillary viscosity of magnetic nanofluid is an important parameter for a new class of applications such as nanoduct flow, nanomotors, micro- and nanofluidic devices, for transformer cooling, magnetic targeted drug delivery, etc. The effective viscosity of a nanofluid is explained based on the rotation of the particles and the effect of torque on it due to an externally applied magnetic field. Two types of fluids are used here, temperature-sensitive and non-temperature-sensitive magnetic nanofluids. In both types of fluids, decrease in effective viscosity with temperature is observed, but in both cases the mechanism for the decrement is quite different. One is due to temperature dependence of the magnetic moment and the other is due to removal of the secondary surfactant. For temperature-sensitive magnetic nanofluids, a Curie temperature of ~80 °C is extracted from this study. For non-temperature-sensitive magnetic nanofluids ~65% of the secondary surfactant is removed for a change in temperature, ΔT = 40 °C. This is analogous with removal of a drug from magnetic particles for targeted drug delivery. Further, a linear dependence of effective viscosity with different capillary size and ξ (angle between magnetic field and flow direction, ξε[0,π/2]) is also observed. This linear dependence can also be a good approximation for the study of magnetic drug targeting, as in the human body the capillaries are of different sizes, and the externally applied magnetic field is not always parallel or perpendicular to the drug flow direction.

Numerical investigation of fluid flow and heat transfer around a solid circular cylinder utilizing nanofluid

In this study, flow-field and heat transfer through a copper–water nanofluid around circular cylinder has been numerically investigated. Governing equations containing continuity, N–S equation and energy equation have been developed in polar coordinate system. The equations have been numerically solved using a finite volume method over a staggered grid system. SIMPLE algorithm has been applied for solving the pressure linked equations. Reynolds and Peclet numbers (based on the cylinder diameter and the velocity of free stream) are within the range of 1 to 40. Furthermore, volume fraction of nanoparticles (φ) varies within the range of 0 to 0.05. Effective thermal conductivity and effective viscosity of nanofluid have been estimated by Hamilton–Crosser and Brinkman models, respectively. The effect of volume fraction of nanoparticles on the fluid flow and heat transfer characteristics are investigated. It is found that the vorticity, pressure coefficient, recirculation length are increased by the addition of nanoparticles into clear fluid. Moreover, the local and mean Nusselt numbers are enhanced due to adding nanoparticles into base fluid.

Numerical study of mixed convective cooling in a square cavity ventilated and partially heated from the below utilizing nanofluid

Abstract A numerical investigation of mixed convection flows through a copper–water nanofluid in a square cavity with inlet and outlet ports has been executed. The natural convection effect is attained by heating from the constant flux heat source which is symmetrical located at the bottom wall and cooling from the injected flow. The governing equations have been solved using the finite volume approach, using SIMPLE algorithm on the collocated arrangement. The study has been carried out for the Reynolds number in the range 50 ≤ Re ≤ 1000, with Richardson numbers 0 ≤ Ri ≤ 10 and for solid volume fraction 0 ≤ ϕ ≤ 0.05. The thermal conductivity and effective viscosity of nanofluid have been calculated by Patel and Brinkman models, respectively. Results are presented in the form of streamlines, isotherms, average Nusselt number and average bulk temperature. In addition, the effects of solid volume fraction of nanofluids on the hydrodynamic and thermal characteristics have been investigated and discussed. The results indicate that increase in solid concentration leads to increase in the average Nusselt number at the heat source surface and decrease in the average bulk temperature.

Numerical study of mixed convection flows in a square lid-driven cavity utilizing nanofluid

A numerical investigation of laminar mixed convection flows through a copper–water nanofluid in a square lid-driven cavity has been executed. In the present study, the top and bottom horizontal walls are insulated while the vertical walls are maintained at constant but different temperatures. The study has been carried out for the Rayleigh number 10 4 to 10 6, Reynolds number 1 to 100 and the solid volume fraction 0 to 0.05. The thermal conductivity and effective viscosity of nanofluid have been calculated by Patel and Brinkman models, respectively. The effects of solid volume fraction of nanofluids on hydrodynamic and thermal characteristics have been investigated and discussed. It is found that at the fixed Reynolds number, the solid concentration affects on the flow pattern and thermal behavior particularly for a higher Rayleigh number. In addition it is observed that the effect of solid concentration decreases by the increase of Reynolds number.