Experimental quantification of natural convective heat transfer within annulus space filled with a H2O-Cu nanofluid saturated porous medium. Application to electronics cooling

This experimental study deals with cooling electronics contained in a hemispherical cavity whose cupola is maintained isothermal, being its base inclined at an angle varying from 0° (horizontal disc with the cupola oriented upwards) to 135°. The active component is a dome centered on this base. The space between the differentially heated elements of the assembly is filled with a porous medium of high porosity saturated by a water–copper nanofluid whose volume fraction varies between 0% (pure water) and 7%. The Rayleigh number based on the radius of the cupola reaches high values up to 7.29 × 10¹⁰ given the important surface heat flux generated by the device during operation. The ratio between the thermal conductivity of the solid matrix and that of the base fluid ranges between 0 (interstitial volume without porous medium) and 41.4 corresponding to the intended applications. This experimental study done with an industrial prototype at scale 1 quantifies the natural convective heat transfer via the Nusselt number determined for many configurations obtained by varying the solid-fluid thermal conductivity ratio, the inclination angle, the Rayleigh number, and the volume fraction. The study clearly shows that the cooling performance of the Cu-H₂O nanofluid degrades with its age and the number of times it has been used. Analysis of the results reproducibility also proves the irreversibility of the performance. The measured values were compared with those obtained in a recent numerical study based on the volume control method. The observed deviations taking into account the experimental uncertainty margins validate the mathematical model implemented in the numerical approach.     

Nanofluid MHD natural convection through a porous complex shaped cavity considering thermal radiation

Control volume based finite element method (CVFEM) is applied to simulate H₂O based nanofluid radiative and convective heat transfer inside a porous medium. Non-Darcy model is employed for porous media. Influences of Hartmann number, nanofluid volume fraction, radiation parameter, Darcy number, number of undulations and Rayleigh number on nanofluid behavior were demonstrated. Thermal conductivity of nanofluid is estimated by means of previous experimental correlation. Results show that Nusselt number enhances with augment of permeability of porous media. Effect of Hartmann number on rate of heat transfer is opposite of radiation parameter.     

Natural convection in a square cavity containing a nanofluid and an adiabatic square block at the center

The problem of free convection fluid flow and heat transfer of Cu–water nanofluid inside a square cavity having adiabatic square bodies at its center has been investigated numerically. The governing equations have been discretized using the finite volume method. The SIMPLER algorithm was employed to couple velocity and pressure fields. Using the developed code, a parametric study was conducted and the effects of pertinent parameters such as Rayleigh number, size of the adiabatic square body, and volume fraction of the Cu nanoparticles on the fluid flow and thermal fields and heat transfer inside the cavity were investigated. The obtained results show that for all Rayleigh numbers with the exception of Ra = 10⁴ the average Nusselt number increases with increase in the volume fraction of the nanoparticles. At Ra = 10⁴ the average Nusselt number is a decreasing function of the nanoparticles volume fraction. Moreover at low Rayleigh numbers (10³ and 10⁴) the rate of heat transfer decreases when the size of the adiabatic square body increases while at high Rayleigh numbers (10⁵ and 10⁶) it increases.     

格子Boltzmann方法模拟填充纳米流体三角形腔内的自然对流

采用格子Boltzmann方法研究填充水-氧化铝纳米流体的等腰直角三角形腔体中的自然对流.讨论瑞利数、颗粒体积分数、热源位置等因素对对流换热的影响,以及不同纳米流体模型对模拟结果的影响.结果表明:在低瑞利数下,随着热源在左壁面向上移动,换热效率逐渐增加.而在高瑞利数(Ra=106)时,观察到相反的现象;采用单相纳米流体...     

Effect of thermal stratification on free convection in a square porous cavity filled with a nanofluid using Tiwari and Das' nanofluid model

Natural convection in a square porous cavity filled with a nanofluid in conditions of thermal stratification has been numerically studied. The mathematical model has been formulated in terms of the dimensionless stream function and temperature using the Darcy–Boussinesq approximation and Tiwari and Das' nanofluid model with new more realistic empirical correlations for the physical properties of the nanofluids. Formulated partial differential equations along with the corresponding boundary conditions have been solved by the finite difference method. Particular efforts have been focused on the effects of the Rayleigh number, thermal stratification parameter, porosity of the porous medium, solid volume fraction parameter of nanoparticles, and the solid matrix of the porous medium (glass balls and aluminum foam) on the local and average Nusselt numbers, streamlines and isotherms. It has been observed an essential effect of thermal stratification parameter on heat and fluid flow fields.     

Modeling the onset of thermosolutal convective instability in a non-Newtonian nanofluid-saturated porous medium layer

The onset of double-diffusive (thermosolutal) convection in horizontal porous layer saturated with an incompressible couple stress nanofluid saturated is studied with thermal conductivity and viscosity dependent on the nanoparticle volume fraction. To represent the momentum equation for porous media, a modified Darcy-Maxwell nanofluid model incorporating the effects of Brownian motion and thermophoresis has been used. The thermal energy equation includes regular diffusion and cross diffusion (Soret thermo-diffusion and Dufour diffuso-thermal) terms. A linear stability analysis depends on the normal mode technique and the onset criterion for stationary and oscillatory convection is derived analytically. The nonlinear theory based on the representation of the Fourier series method is applied to capture the behavior of heat and mass transfer. It is found that the couple stress parameter enhances the stability of the system in both the stationary and oscillatory convection modes. The viscosity ratio and conductivity ratio both enhance heat and mass transfer. Transient Nusselt number is found to be oscillatory when time is small. However, when time becomes very large, all the three transient Nusselt number values approach to their steady state values.     

Thermal Lattice Boltzmann Flux Solver for Natural Convection of Nanofluid in a Square Enclosure

In the present study, mathematical modeling was performed to simulate natural convection of a nanofluid in a square enclosure using the thermal lattice Boltzmann flux solver (TLBFS). Firstly, natural convection in a square enclosure, filled with pure fluid (air and water), was investigated to validate the accuracy and performance of the method. Then, influences of the Rayleigh number, of nanoparticle volume fraction on streamlines, isotherms and average Nusselt number were studied. The numerical results illustrated that heat transfer was enhanced with the augmentation of Rayleigh number and nanoparticle volume fraction. There was a linear relationship between the average Nusselt number and solid volume fraction. and there was an exponential relationship between the average Nusselt number and Ra. In view of the Cartesian grid used by the immersed boundary method and lattice model, the immersed boundary method was chosen to treat the no-slip boundary condition of the flow field, and the Dirichlet boundary condition of the temperature field, to facilitate natural convection around a bluff body in a square enclosure. The presented numerical algorithm and code implementation were validated by means of numerical examples of natural convection between a concentric circular cylinder and a square enclosure at different aspect ratios. Numerical simulations were conducted for natural convection around a cylinder and square in an enclosure. The results illustrated that nanoparticles enhance heat transfer in higher Rayleigh number, and the heat transfer of the inner cylinder is stronger than that of the square at the same perimeter.     

Transient electrohydrodynamic convective flow and heat transfer of MWCNT - Dielectric nanofluid in a heated enclosure

A computational research was performed to analyze the electrohydrodynamic (EHD) convective heat transfer in a differentially heated dielectric-MWCNT nanofluid layer. The study was conducted on a square enclosure subjected to a temperature gradient between these two vertical walls as well as a potential difference between these horizontal walls. The enclosure was filled with MWCNT oil-based nanofluid; the MWCNT nanoparticles were dispersed in a perfectly insulating thermal oil with a volume fraction of hardly exceeded 0.4%. The governing equations were derived with the assumption of homogeneous nanofluid and were solved with employing finite volume method. Based on the obtained results, it was found that the increase of Rayleigh number, electric Rayleigh number and nanoparticle concentration enhanced the heat transfer. For high thermal and electric Rayleigh number values, the flow and heat transfer became time dependent and accordingly a frequency study was also performed. It was found that the inclusion of an electric field with the addition of nanoparticles led to a significant heat transfer enhancement of about 43%.     

Lattice Boltzmann method for nanofluid flow in a porous cavity with heat sources and magnetic field

In this article, Lattice Boltzmann method (LBM) has been applied to investigate the influences of magnetic field and heat sources on water based nanofluid natural convection inside a porous cavity with three square heat sources. Koo–Kleinstreuer–Li (KKL) model is applied to study Brownian motion impact on nanofluid flow. Effects of Rayleigh number (Ra), Darcy number (Da), nanofluid volume fraction (ϕ), and Hartmann number (Ha) on heat transfer characteristics are analyzed. From the obtained results we observe a decrease in the temperature gradient with increasing Ha; while quite the opposite effect is true with increasing Da and Ra. In the absence of magnetic field, for higher values of Darcy and Rayleigh numbers, thermal plumes are generated and the temperature gradient is enhanced. Moreover, small eddies are generated near the vertical centerline. However, in the presence of magnetic field, the number of thermal plumes decreases.     

含不同形状内热源的圆管内纳米流体自然对流及换热数值研究

采用格子玻尔兹曼方法对有三种恒温热源(圆形、三角形、方形)参与的圆管内纳米流体(铜-水)自然对流进行数值研究.主要研究瑞利(Ra)数,纳米颗粒体积分数以及热源几何形状等控制参数对纳米流体的流动与传热的影响.结果发现纳米颗粒体积分数的增加有利于强化传热,且在Ra数较小时,平均努塞尔(Nu)数增加的幅度要优于Ra数较大的情...     

Lattice Boltzmann simulation for hydrothermal analysis of free convection within dumbbell-shaped heat exchanger

The lattice Boltzmann simulation of nanofluid flow and heat transfer during natural convection within a dumbbell-shaped heat exchanger is carried out. The heat exchanger is filled with CuO–water. The KKL model is employed to predict the thermo-physical properties of nanofluid. In order to perform a comprehensive hydrothermal investigation, different post-processing approaches such as heatline visualization, total entropy generation, local entropy generation based on local fluid friction irreversibility and heat transfer irreversibility, average and local Nusselt variation are employed. In the present investigation, it is tried to present the impact of different influential parameters like Rayleigh number, solid volume fraction of nanofluid and thermal arrangement of internal fins-bodies on the fluid flow, heat transfer rate and entropy generation.     

Natural Convection of Fe_3O_4-Ethylene Glycol Nanofluid under the Impact of Electric Field in a Porous Enclosure

Free convection of Fe_3O_4-Ethylene glycol nanofluid in existence of Coulomb forces is studied. Effect of thermal radiation is taken into account. Properties of nanofluid are varied with supplied voltage and shape of nanoparticles. The bottom wall is considered as positive electrode. Control Volume based Finite Element Method is used to obtain the results, which are the roles of Darcy number(Da), radiation parameter(Rd), Rayleigh number(Ra), nanofluid volume fraction(φ), and supplied voltage(?φ). Results indicate that Nusselt number is an enhancing function of supplied voltage and Darcy number. Maximum values for temperature gradient are occurred for platelet shape nanoparticles.     

Heat Transfer and Pressure Drop Characteristics of Graphene Oxide/Water Nanofluid in a Circular Tube Fitted with Wire Coil Insert

In this work, heat transfer and pressure drop characteristics of graphene oxide/water nanofluid flow through a circular tube having a wire coil insert were studied. The required graphene oxide was synthesized via the Hummer method and characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (SRD), and scanning electron microscope (SEM) methods. Dispersing graphene oxide in the water, nanofluids with 0.02, 0.07, and 0.12% volume fraction were prepared. An experimental set-up was designed and made to investigate the heat transfer performance and pressure loss of nanofluids. All experiments were carried out in the constant heat flux at tube wall conditions. The volumetric flow rates of the nanofluid were adjusted at 6, 8, and 10 L/min. Thermal conductivity, specific heat, density, and viscosity as thermophysical properties of the nanofluid were calculated using graphene oxide and water properties at the average temperature via appropriate relations. These properties were applied to calculate the convective heat transfer coefficient, Nusselt number, and friction factors for each experiment. Finally, the constant and exponents of Duangthongsuk and Wongwises's correlations for Nusselt number and friction factor were corrected by experimental results. The achieved experimental data have shown good agreement with those predicted. The results have shown that 0.12 vol% of graphene oxide in the water can enhance convective heat transfer coefficient by about 77%. As a result, it can be concluded that the graphene oxide/water can be used in the heat transfer devices to achieve more efficiency.     

Convective Heat Transfer of a Cu/Water Nanofluid Flowing Through a Circular Tube

Abstract

Fluids in which nanometer-sized solid particles are suspended are called nanofluids. These fluids can be employed to increase the heat transfer rate in various applications. In this study, the convective heat transfer for Cu/water nanofluid through a circular tube was experimentally investigated. The flow was laminar, and constant wall temperature was used as thermal boundary condition. The Nusselt number of nanofluids for different nanoparticle concentrations, as well as various Peclet numbers, was obtained. Also, the rheological properties of the nanofluid for different volume fractions of nanoparticles were measured and compared with theoretical models. The results show that the heat transfer coefficient is enhanced by increasing the nanoparticle concentrations as well as the Peclet number.     

A numerical study of laminar natural convective heat transfer inside a closed cavity with different aspect ratio

Two-dimensional steady-state laminar natural convection was studied numerically for differentially heated air-filled closed cavity with adiabatic top and bottom walls. The temperature of the left heated wall and cooled right wall was assumed to be constant. The governing equations were iteratively solved using the control volume approach. In this paper, the effects of the Rayleigh number and the aspect ratio were examined. Flow and thermal fields were exhibited by means of streamlines and isotherms, respectively.Variations of the maximum stream function and the average heat transfer coefficient were also shown. The average Nusselt number and was correlated to the Rayleigh number based on curve fitting for each aspect ratio. The investigation covered the range 10⁴ ≤ RA ≤ 10⁷ and is done at Prandtl number equal to 0.693. The result shows the average Nusselt number is the increasing function of Rayleigh number. As the aspect ratio increases, Nusselt number decreases along the hot wall of the cavity. As Rayleigh number increases, Nusselt number increases. Result indicates that at constant aspect ratio, with increase in Rayleigh number the heat transfer rate increases.     

Non-Darcy free convection of Fe3O4-water nanoliquid in a complex shaped enclosure under impact of uniform Lorentz force

Effect of Lorentz forces on natural convection in a complex shaped cavity filled with nanoliquid immersed in porous medium is investigated by means of Control volume based finite element method (CVFEM). Non Darcy model is taken into account for porous media. The working fluid is Fe₃O₄ –water and its viscosity considered as function of magnetic field. Figures are illustrated for different values of Darcy number (Da), Fe₃O₄ -water volume fraction (?), Rayleigh (Ra) and Hartmann (Ha) numbers. Results depict that enhancing in Lorentz forces results in reduce in nanofluid motion and increase the thickness of thermal boundary. Convective heat transfer enhances with rise of Darcy number.     

Natural convective heat transfer of Ag-water nanofluid flow inside enclosure with center heater and bottom heat source

A numerical study on natural convective heat transfer inside an enclosure with center heater using nanofluid has been carried out. The effect of different length of center heater on the flow and temperature fields is analysed for different Rayleigh numbers. Results are displayed in terms of streamlines, isotherms, mid height velocity profile and average Nusselt number. The numerical results reveal heat transfer increases with increasing heater length at both vertical and horizontal positions for increasing values of Rayleigh numbers. In particular, a higher increase in heat transfer is obtained with heater situated with vertical position of maximum length. Also it is obtained that enhancement of heat transfer is high for Ag - water nanofluid than CuO -water and Al₂O₃ -water nanofluids.     

Numerical investigation of natural convection heat transfer of nanofluids in a Γ shaped cavity

This paper analyzes the heat transfer and fluid flow of natural convection in a Γ shaped enclosure filled with Al₂O₃/Water nanofluid that operates under differentially heated walls. The Navier–Stokes and energy equations are solved numerically. Heat transfer and fluid flow are examined for parameters of non-uniform nanoparticle size, mean nanoparticle diameter, nanoparticle volume fraction, Grashof number and different geometry of enclosure. Finite volume method is used for discretizating positional expressions, and the forth order Rung-Kuta is used for discretizating time expressions. Also an artificial compressibility technique was applied to couple continuity to momentum equations. Results indicate that using nanofluid causes an increase in the heat transfer and the Nusselt number so that for R = 0.001 in Gr = 10³, the Nusselt number 25%, in Gr = 10⁴ 26%, and in Gr = 10⁵ 28% increases. Furthermore; by decreasing the mean diameters of nanoparticles, Nusselt number increases. By increasing R parameter (d_p,min/d_p,max) and nano particle volume fraction, Nusselt number increases.     

小通道扁管内纳米流体流动与传热特性

建立了测量小通道扁管内纳米流体流动与对流换热性能的实验系统,测量了不同粒子体积份额的水-Cu纳米 流体的管内对流换热系数和摩擦阻力系数,实验结果表明,在相同雷诺数条件下,小通道扁管内纳米流体的对流换热系数 大于纯液体,且随粒子的体积份额的增加而增大,而纳米流体的阻力系数并未明显增大。     

Experimental Study of Two Phase Closed Thermosyphon Using Cuo/Water Nanofluid in the Presence Of Electric Field

In this article, the effect of applying an electric field on the performance of a two-phase closed thermosyphon is investigated experimentally. A CuO/water nanofluid is used as the working fluid in the present investigation; 40% of the evaporator volume is filled with the working fluid during the tests. An electric field in various voltages ranging from 5 to 20 kV is applied to the system. Also, the input power supplied to the evaporator varies between 60 to 120 W. The thermal efficiency and the thermal resistance of the two-phase closed thermosyphon are evaluated in various strengths of electric field and different volume fractions. It is found that using the nanofluid and applying an electric field could increase the thermal efficiency by up to 30% as compared with the case in which the working media is pure water and no electric field is applied. To illustrate the effect of the electric field on the heat transfer enhancement, the augmentation Nusselt number, defined as the ratio of the Nusselt numbers after and before applying the electric field, is discussed. The results show that utilizing an electric field is more advantageous when the input power applied to the system is lower.