Natural Convection Of Water-based Nanofluids Research Articles

Natural convection of water-based nanofluids in three-dimensional enclosures

The single-phase approach, in which nanofluids are treated as pure fluids assuming that the solid and liquid phases are in local thermal and hydrodynamic equilibrium, has been used with good results to simulate forced convection flows. Conversely, its extension to natural convection has revealed to be more problematic, as the obtained numerical results are substantially different from the experimental data, mainly due to the effects of the slip motion occurring between the suspended nanoparticles and the base liquid, whose consequent nonuniform distribution of the solid phase concentration can significantly affect both heat and momentum transfer. In the present article, a two-phase model based on a double-diffusive approach is proposed to study the natural convection flows of water-based metal oxide nanofluids inside cylindrical and rectangular enclosures heated and cooled at their opposite sides with the scope to reproduce a number of experimental data-sets available in the literature. Given the assumption of local thermal equilibrium between nanoparticles and host liquid, and considering the Brownian and thermophoretic diffusion as responsible for causing significant relative velocity between the solid and liquid phases, the governing equations are solved by a control-volume numerical method implemented using the open-source platform OpenFOAM (Open Field Operation and Manipulation). It has been found that the nanoparticle diffusion gives the major contribution to the decrease of the heat transfer rate red usually observed experimentally in natural convection flows of nanofluids inside differentially-heated enclosures. This means that a two-phase model, in conjunction with proper correlations for the evaluation of the nanofluid effective physical properties, must be necessarily enforced to obtain reliable results for many engineering applications.

Natural convection of water-based nanofluids in three-dimensional enclosures

The single-phase approach, in which nanofluids are treated as pure fluids assuming that the solid and liquid phases are in local thermal and hydrodynamic equilibrium, has been used with good results to simulate forced convection flows. Conversely, its extension to natural convection has revealed to be more problematic, as the obtained numerical results are substantially different from the experimental data, mainly due to the effects of the slip motion occurring between the suspended nanoparticles and the base liquid, whose consequent nonuniform distribution of the solid phase concentration can significantly affect both heat and momentum transfer. In the present article, a two-phase model based on a double-diffusive approach is proposed to study the natural convection flows of water-based metal oxide nanofluids inside cylindrical and rectangular enclosures heated and cooled at their opposite sides with the scope to reproduce a number of experimental data-sets available in the literature. Given the assumption of local thermal equilibrium between nanoparticles and host liquid, and considering the Brownian and thermophoretic diffusion as responsible for causing significant relative velocity between the solid and liquid phases, the governing equations are solved by a control-volume numerical method implemented using the open-source platform OpenFOAM (Open Field Operation and Manipulation). It has been found that the nanoparticle diffusion gives the major contribution to the decrease of the heat transfer rate red usually observed experimentally in natural convection flows of nanofluids inside differentially-heated enclosures. This means that a two-phase model, in conjunction with proper correlations for the evaluation of the nanofluid effective physical properties, must be necessarily enforced to obtain reliable results for many engineering applications.

Natural convection of water-based nanofluid in a chamber with a solid body of periodic volumetric heat generation

Natural convection of water-based nanofluid in a chamber with a solid body of periodic volumetric heat generation

Natural convection of water-based nanofluids near the density maximum in an annulus

This paper presents a series of numerical simulations on natural convection of a water-based nanofluid near its density maximum in an annulus. The volume fraction of the nanoparticles ranged from 0 to 0.08, the nanoparticle diameter from 10 nm to 100 nm, the radius ratio of the annulus from 0.1 to 0.8, and the density inversion parameter from 0 to 1. The results indicate that Brownian motion should be considered in natural convection of water-based nanofluid with the density inversion phenomenon. Both the solid volume fraction and the radius ratio affect greatly the flow and thermal fields, while the nanoparticle diameter has a little impact. Adding nanoparticles can weaken or even overcome the negative effect of the density inversion phenomenon on the heat transfer performance, possibly making it higher than that of the common fluid. Additionally, the heat transfer performance of the nanofluid with metal or metal oxide nanoparticles is better than that of the nanofluid with non-metallic compound nanoparticles.

Transient natural convection in a partially open trapezoidal cavity filled with a water-based nanofluid under the effects of Brownian diffusion and thermophoresis

PurposeThe purpose of this paper is to study about the natural convection of water-based nanofluid in a partially open trapezoidal cavity under the influence of Brownian diffusion and thermophoresis.Design/methodology/approachGoverning equations formulated in dimensionless stream function – vorticity variables – have been solved by finite difference method with a homemade code C++. Effects of Rayleigh number (Ra = 50-1,000), Lewis number (Le = 10-1,000), buoyancy-ratio parameter (Nr = 0.1-5.0), Brownian motion parameter (Nb = 0.1, 1.0) and thermophoresis parameter (Nt = 0.1, 1.0) on nanofluid flow and heat transfer have been studied.FindingsIt is found that high values of Rayleigh and Lewis numbers lead to the homogenization of nanoparticles distributions. For high values of Nt and Nb, heating is more essential and the cavity average temperature rises.Originality/valueThe originality of this work is to analyze natural convection in an open-sided trapezoidal cavity with Brownian diffusion and thermophoresis.

A Demonstrative Study on the Two-phase vs. Single-phase Modeling of Buoyancy-driven Flows of Enclosed Nanofluids

ABSTRACTA demonstrative numerical study on natural convection of water-based nanofluids in square enclosures with different boundary conditions imposed at the walls, and different orientations with respect to the gravity vector, is performed using both the single-phase and the two-phase approaches, with the main scope to evaluate in what measure the single-phase approach fails in describing the basic heat and fluid flow features, as well as in determining the thermal performance of nanofluids. The system of the mass, momentum and energy transfer governing equations is solved by way of a computational code based on the SIMPLE-C algorithm. Empirical correlations are used for the calculation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient. The following configurations are investigated: a tilted cavity differentially-heated at two opposite walls; a vertical cavity partially-heated at the bottom wall and cooled at both sides; and a vertical cavity differentially-heated at the vertical and horizontal walls. It is found that the non-uniform distribution of the suspended solid phase throughout the enclosure gives rise to a solutal buoyancy force, whose competition with the thermal buoyancy force results in a periodic flow detectable only if the two-phase approach is applied. Moreover, the impact of the dispersion of the nanoparticles into the base liquid, which turns out to be notably higher at higher average temperatures, is found to be systematically underestimated by the single-phase approach.

Buoyancy-driven convection of nanofluids in inclined enclosures

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in square cavities differentially heated at two opposite walls, and inclined with respect to gravity so that the heated wall faces upwards. It is assumed that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum and energy for the nanofluid, and continuity for the nanoparticles, is solved through an in-house developed computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on literature experimental data. The pressure-velocity coupling is handled by way of the SIMPLE-C algorithm. Numerical simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, as well as the tilting angle of the enclosure, the enclosure width, the average temperature of the nanofluid, and the temperature difference imposed across the cavity, as independent variables, whose effects are thoroughly analyzed and discussed. The existence of an optimal tilting angle of the enclosure and an optimal particle loading for maximum heat transfer is detected. In addition, a thermophoresis-induced periodic flow at tilting angles larger than 50° is found to occur for low to moderate values of both the nanofluid average temperature and the volume fraction of the suspended solid phase.

Thermophoresis-induced oscillatory natural convection flows of water-based nanofluids in tilted cavities

ABSTRACTA two-phase model based on the double-diffusive approach is used to perform a numerical study on the natural convection of water-based nanofluids in differentially heated square cavities, inclined with respect to gravity so that the heated wall is positioned below the cooled wall, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum, and energy for the nanofluid, and continuity for the nanoparticles, is solved through a computational code that incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on several sets of literature experimental data. Pressure–velocity coupling is handled by the way of the SIMPLE-C algorithm. Numerical simulations are executed for tilting angles in the range 50−70 deg, such that the nonuniform distribution of the suspended solid phase gives rise to a nonnegligible solutal buoyancy force, whose effects are investigated using the nanoparticle diameter and average volume fraction, the cavity width, the nanofluid average temperature, and the temperature difference imposed across the cavity, as independent variables. It is found that the competition between the solutal and thermal buoyancy forces results in an oscillatory flow, with an oscillation amplitude that increases on increasing the cavity size and the imposed temperature difference. Moreover, the impact of the nanoparticle dispersion into the base liquid is found to be higher at higher average temperatures, whereas, by contrast, the other variables have moderate or negligible effects.

Natural Convection of Water-based Nanofluids in a Square Cavity with Partially Heated of the Bottom Wall

Numerical study of natural convection of water-based nanofluids in a square cavity with partially heated at the bottom wall and filled with nanofluids is carried out using different types of nanoparticles. The remaining wall of this enclosure is kept at cold temperature. The numerical procedure used in this work is based on the Galerkin weighted residual method of finite element formulation.Calculation are performed for Rayleigh numbers in the range 103 -106 and different solid volume fraction of nanoparticles 0 ≤ φ ≤ 0.2. An enhancement in heat transfer rate is observed with the increase of nanoparticles volume fraction for the whole range of Rayleigh numbers. It is observed that the heat transfer enhancement strongly depends on the type of nanofluids.

Buoyancy-induced convection of water-based nanofluids in differentially-heated horizontal semi-annuli

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in differentially-heated horizontal semi-annuli, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. The system of the governing equations of continuity, momentum, and energy for the nanofluid, and continuity for the nanoparticles, is solved by the way of a computational code which incorporates three empirical correlations for the evaluation of the effective thermal conductivity, the effective dynamic viscosity, and the thermophoretic diffusion coefficient, all based on a wide number of literature experimental data. The pressure-velocity coupling is handled through the SIMPLE-C algorithm. Numerical simulations are executed for three different nanofluids, using the diameter and the average volume fraction of the suspended nanoparticles, the cavity size, the average temperature, and the temperature difference imposed across the cavity, as independent variables. It is found that the impact of the nanoparticle dispersion into the base liquid increases remarkably with increasing the average temperature, whereas, by contrast, the other controlling parameters have moderate effects. Moreover, at temperatures of the order of room temperature or just higher, the heat transfer performance of the nanofluid is significantly affected by the choice of the solid phase material.

Use of nanofluids as coolants in buoyancy-driven thermal management of embedded heating components of small-scale devices

A two-phase model based on the double-diffusive approach is used to perform a numerical study on natural convection of water-based nanofluids in square cavities partially heated at the bottom wall and cooled at both sides, assuming that Brownian diffusion and thermophoresis are the only slip mechanisms by which the solid phase can develop a significant relative velocity with respect to the liquid phase. Numerical simulations are basically executed for Al2O3 + H2O, using the diameter and the average volume fraction of the suspended nanoparticles, the cavity width, the heated fraction of the bottom wall, the average temperature and the temperature difference imposed across the cavity, as independent variables. Additional simulations are also performed using CuO or TiO2 nanoparticles. It is found that the cooperation between the solutal and thermal buoyancy forces results in a significant enhancement of the heat transfer performance of the nanofluid compared with the pure base liquid.

Magnetic field effect on the unsteady natural convection in a right-angle trapezoidal cavity filled with a nanofluid

Purpose – Unsteady natural convection of water-based nanofluid within a right-angle trapezoidal cavity under the influence of a uniform inclined magnetic field using the mathematical nanofluid model proposed by Buongiorno is presented. The paper aims to discuss these issues. Design/methodology/approach – The left vertical and right inclined walls of the enclosure are kept at constant but different temperatures whereas the top and bottom horizontal walls are adiabatic. All boundaries are assumed to be impermeable to the base fluid and to nanoparticles. In order to study the behavior of the nanofluid, a non-homogeneous Buongiorno’s mathematical model is taken into account. The physical problems are represented mathematically by a set of partial differential equations along with the corresponding boundary conditions. By using an implicit finite difference scheme the dimensionless governing equations are numerically solved. Findings – The governing parameters are the Rayleigh, Hartmann and Lewis numbers along with the inclination angle of the magnetic field relative to the gravity vector, the aspect ratio and the dimensionless time. The effects of these parameters on the average Nusselt number along the hot wall, as well as on the developments of streamlines, isotherms and isoconcentrations are analyzed. The results show that key parameters have substantial effects on the flow, heat and mass transfer characteristics. Originality/value – The present results are new and original for the heat transfer and fluid flow in a right-angle trapezoidal cavity under the influence of a uniform inclined magnetic field using the mathematical nanofluid model proposed by Buongiorno. The results would benefit scientists and engineers to become familiar with the flow behavior of such nanofluids, and the way to predict the properties of this flow for possibility of using nanofluids in advanced nuclear systems, in industrial sectors including transportation, power generation, chemical sectors, ventilation, air-conditioning, etc.

Conjugate Natural Convection of Nanofluids in an Enclosure with a Volumetric Heat Source

This article presents a numerical investigation of conjugate natural convection of water-based nanofluids in a square cavity. Two different types of nanofluids, namely Al2O3–water and CuO–water are considered as the working fluids. A square volumetric heat generating source is located within the cavity, resembling a heat generating electronic device. All the cavity walls are considered to be adiabatic except the right-hand side wall which is at the cold temperature. Transport equations for Newtonian fluid have been solved numerically, using finite volume method. The effects of relevant parameters such as Rayleigh number (103 ≤ Ra ≤ 106), type of nanofluid, solid volume fraction of the nanoparticles, and the location of the heat source on the cooling performance of the cavity have been studied. The results show that in natural convection flows, nanofluids are more effective in heat transfer enhancement for moderate values of Rayleigh number and low values of solid particles volume fractions. In addition, to maximize the heat transfer performance, the best placement of heat source has been determined. Also it is shown that CuO–water nanofluid exhibits elevated thermal performance in comparison with Al2O3–water nanofluid.

Natural Convection of Water-Based Nanofluids in a Square Enclosure with Non-Uniform Heating of the Bottom Wall

In this paper, a numerical study of natural convection in a square enclosure with non-uniform temperature distribution maintained at the bottom wall and filled with nanofluids is carried out using different types of nanoparticles. The remaining walls of the enclosure are kept at a lower temperature. Calculations are performed for Rayleigh numbers in the range 5 × 103 ≤ Ra ≤ 106 and different solid volume fraction of nanoparticles 0 ≤ χ ≤ 0.2. An enhancement in heat transfer rate is observed with the increase of nanoparticles volume fraction for the whole range of Rayleigh numbers. It is also observed that the heat transfer enhancement strongly depends on the type of nanofluids. For Ra = 106, the pure water flow becomes unsteady. It is observed that the increase of the volume fraction of nanoparticles makes the flow return to steady state.

Natural convection of water-based nanofluids in an inclined enclosure with a heat source

This study investigates natural convection heat transfer of water-based nanofluids in an inclined square enclosure where the left vertical side is heated with a constant heat flux, the right side is cooled, and the other sides are kept adiabatic. The governing equations are solved using polynomial differential quadrature (PDQ) method. Calculations were performed for inclination angles from 0° to 90°, solid volume fractions ranging from 0% to 20%, constant heat flux heaters of lengths 0.25, 0.50 and 1.0, and a Rayleigh number varying from 10 4 to 10 6. The ratio of the nanolayer thickness to the original particle radius is kept at a constant value of 0.1. The heat source is placed at the center of the left wall. Five types of nanoparticles are taken into consideration: Cu, Ag, CuO, Al 2O 3, and TiO 2. The results show that the average heat transfer rate increases significantly as particle volume fraction and Rayleigh number increase. The results also show that the length of the heater is also an important parameter affecting the flow and temperature fields. The average heat transfer decreases with an increase in the length of the heater. As the heater length is increased, the average heat transfer rate starts to decrease for a smaller inclination angle (it starts to decrease with inclination at 90° for ɛ = 0.25, 60° for ɛ = 0.50, 45° for ɛ = 1.0, respectively).