Showing papers on "Natural convection published in 2020"

Conjugate natural convection flow of Ag–MgO/water hybrid nanofluid in a square cavity

Abstract: The conjugate natural convection of a new type of hybrid nanofluid (Ag–MgO/water hybrid nanofluid) inside a square cavity is addressed. A thick layer of conductive solid is considered over the hot wall. The governing partial differential equations (PDEs) representing the physical model of the natural convection of the hybrid nanofluid along with the boundary conditions are reported. The thermophysical properties of the nanofluid are directly calculated using experimental data. The governing PDEs are transformed into a dimensionless form and solved by the finite element method. The effect of the variation of key parameters, such as the volume fraction of nanoparticles, Rayleigh number, and the ratio between the thermal conductivity of the wall and the thermal conductivity of the hybrid nanofluid (Rk), is studied. Furthermore, the effects of the key parameters are investigated on the temperature distribution, local Nusselt number, and average Nusselt number. The results of this study show that the heat transfer rate increases by adding hybrid nanoparticles for a conduction-dominant regime (low Rayleigh number). The heat transfer rate is an increasing function of both the Rayleigh number and the thermal conductivity ratio (Rk). In the case of a convective-dominant flow (high Rayleigh number flow) and an excellent thermally conductive wall, the local Nusselt number at the surface of the conjugate wall decreases substantially by moving from the bottom of the cavity toward the top.

Numerical study on mixed convection of a non-Newtonian nanofluid with porous media in a two lid-driven square cavity

Abstract: In the present numerical study, mixed flow of the non-Newtonian water/Al2O3 nanofluid with 0–4% nanoparticles volume fractions (φ) inside a two-dimensional square cavity with hot and cold lid-driven motion and porous media is simulated at Richardson numbers (Ri) of 0.01, 10 and 100 and Darcy numbers (Da) of 10−4 ≤ Da ≤ 10−2 using Fortran computer code. The obtained results for temperature domain, velocity, Nusselt number and streamlines indicate that by increasing Richardson number and decreasing axial velocity parameter of walls and similarity of flow behavior to natural flow mechanism, variations of velocity are reduced, which is due to the reduction in fluid momentum. By increasing Darcy number, penetrability of fluid motion enhances and fluid lightly moves along the cavity. Figuration of streamlines at lower Richardson numbers highly depends on the Darcy number changes. In case (2), due to the counterflow motion and buoyancy force, distinction of flow domain profiles is more obvious. On the other hand, this issue causes more velocity gradients and vortexes in special sections of cavity (central regions of cavity). In case (2), the behavior of streamlines is affected by some parameters such as variations of Darcy number, nanoparticles volume fraction and Richardson number more than case (1). By increasing Darcy number, flow lightly passes among hot and cold sources and leads to improve the heat transfer. Moreover, reduction in flow penetrability in cavity results in the reduction in fluid flow in its direction, sectional distribution and regions with higher temperature. Consequently, in these regions the growth of thermal boundary layer is more significant. In case (2), at lower Richardson numbers compared to higher ones, the affectability of lid-driven motion contrary to buoyancy force caused by density variations is less because of higher fluid momentum. At Ri = 0.01, because of the strength of lid-driven motion, flow direction is compatible with lid-driven motion. Also, temperature distribution is not uniform, and in these regions, fluid has the minimum velocity which leads to the enhancement of dimensionless temperature. In both studied cases, the increment of nanoparticles volume fraction as well as Darcy number and reduction in Richardson number result in the improvement of temperature distribution and decrease in dimensionless temperature.

Natural convection analysis in a square enclosure with a wavy circular heater under magnetic field and nanoparticles

Abstract: The aim of the current study is to investigate the natural convection heat transfer in a square enclosure with a wavy circular heater under magnetic field and nanoparticles. The governing equations that are expressed in dimensionless form are solved by means of control volume finite element method employing a validated FORTRAN code. The dimensionless controlling parameters of the present investigation are shape factor of nanoparticles (m), Hartmann number (Ha), Rayleigh number (Ra), and nanoparticle volume fraction (ϕ). In this study, the amplitude A and number of undulations N are fixed at a constant value of 0.2 and 8, respectively. The obtained results portray that in the presence of waves on the inner wall of the annulus, the heat transfer rate is an ascending function of Rayleigh number, nanoparticle volume fraction and less obvious function of their shape factor, while it is a descending function of the Hartmann number.

Numerical simulation of hydrothermal features of Cu–H2O nanofluid natural convection within a porous annulus considering diverse configurations of heater

Abstract: The purpose of the current study is to numerically investigate the effects of shape factors of nanoparticles on natural convection in a fluid-saturated porous annulus developed between the elliptical cylinder and square enclosure. A numerical method called the control volume-based finite element method is implemented for solving the governing equations. The modified flow and thermal structures and corresponding heat transfer features are investigated. Numerical outcomes reveal very good grid independency and excellent agreement with the existing studies. The obtained results convey that at a certain aspect ratio, an increment in Rayleigh and Darcy numbers significantly augments the heat transfer and average Nusselt number. Further, enhancement of Rayleigh number increases the velocity of nanofluid, while that of aspect ratio of the elliptical cylinder shows the opposite trend.

Effects of magnetic field on micro cross jet injection of dispersed nanoparticles in a microchannel

Abstract: Purpose Water/Al 2 O 3 nanofluid with volume fractions of 0, 0.3 and 0.06 was investigated inside a rectangular microchannel. Jet injection of nanofluid was used to enhance the heat transfer under a homogeneous magnetic field with the strengths of Ha = 0, 20 and 40. Both slip velocity and no-slip boundary conditions were used. Design/methodology/approach The laminar flow was studied using Reynolds numbers of 1, 10 and 50. The results showed that in creep motion state, the constricted cross section caused by fluid jet is not observable and the rise of axial velocity level is only because of the presence of additional size of the microchannel. By increasing the strength of the magnetic field and because of the rise of the Lorentz force, the motion of fluid layers on each other becomes limited. Findings Because of the limitation of sudden changes of fluid in jet injection areas, the magnetic force compresses the fluid to the bottom wall, and this behavior limits the vertical velocity gradients. In the absence of a magnetic field and under the influence of the velocity boundary layer, the fluid motion has more variations. In creeping velocities of fluid, the presence or absence of the magnetic field does not have an essential effect on Nusselt number enhancement. Originality/value In lower velocities of fluid, the effect of the jet is not significant, and the thermal boundary layer affects the entire temperature field. In this case, for Hartmann numbers of 40 and 0, changing the Nusselt number on the heated wall is similar.

Improving the melting performance of PCM thermal energy storage with novel stepped fins

Abstract: Numerical investigation on latent heat thermal energy storage (LHTES) systems with phase change materials (PCMs) vertically heated from one side with novel stepped fins is presented. Transient numerical simulation by using the enthalpy-porosity method is performed to investigate the heat transfer rate and melting behaviors, while the natural convection is considered. To improve the PCM melting process, different upward and downward stepped fins with the step ratios (b/c) of 0.66, 1, 1.5, 2.33 and 4 are employed. The melt fraction and temperature contours by consideration the natural convection effects are presented. The results show that at the beginning of the melting process, the fins in the downward direction with b/c = 0.66 improve the PCM melting rate than the other cases, as the part of the heat is well transferred to the bottom of container along the fins and heat is trapped between the heated wall and the fins. The results show that the melting process in all of the tested stepped fins is faster than the conventional horizontal fins. The results show that by using downward stepped fins (b/c = 0.4) instead of conventional horizontal fins, the melting process could be enhanced up to 56.3% at t = 800 s and 65.5% at t = 3600 s.

Magnetohydrodynamic natural convection and entropy generation analyses inside a nanofluid-filled incinerator-shaped porous cavity with wavy heater block

Abstract: The aim of the current study is natural convection analysis conjugated with entropy generation analysis in an incinerator shaped permeable enclosure loaded with Al2O3–H2O nanofluid subjected to the magnetic field with a rectangular wavy heater block positioned on the bottom of the cavity wall. The bottom and top horizontal walls are adiabatic; the inclined and vertical walls are thought to be cooled. Firstly, the governing expressions and standard k–e turbulence model are rewritten from dimensional form to non-dimensional form using dimensionless parameters such as vorticity and stream function. In the next step, the equation of entropy generation is written in dimensionless form. Then, the system of non-dimensional governing equations is solved by the finite volume method (FVM) conjugated with a non-dimensionalization scheme using ANSYS Fluent. Fine grids (wall y+ < 2) with inflated layers have been used for the higher Rayleigh number. The effects of the Rayleigh number in the laminar region (Ra = 103, 104, and 105) and turbulent region (Ra = 108, 0.5 × 109, and 109), Darcy number (Da = 0.01 and 100), Hartmann number (Ha = 0 and 40), and the nanoparticles ( $$ \phi = 2{{\% }} $$ ) on the entropy generation number and natural convection are investigated. The validation results were in good agreement with those of the literature. The results demonstrate that for the laminar region, the Nusselt number and entropy generation number increase as the Rayleigh number and the Darcy number grow, whereas both of them abate as Hartmann number increases. In the turbulent region, the average Nusselt number decreases by ascending the Darcy number. Also, for turbulent region (Ra = 109), convection flow strength decreases 6.28% when Hartmann number increases from 0 to 40, whereas the entropy generation number increases 31.5% at Da = 0.01.

Natural convection and entropy production in hybrid nanofluid filled-annular elliptical cavity with internal heat generation or absorption

Abstract: This study is an attempt to understand the characteristics of heat transfer by natural convection, flow, and entropy generation of Cu-Al2O3/H2O based hybrid nanoliquid filled-annulus delimited by two elliptic cylinders considering internal heat generation or absorption (IHG/A) phenomenon The buoyancy-driven flow is induced by a thermal gradient between isothermal and differentially heated inner and outer cylinders A numerical solution of the governing equations in the dimensionless and non-primitive form is performed using the technique of finite volume discretization Impacts of diverse parameters of the study such as copper-alumina nanoparticles volumic concentration, Rayleigh number, and dimensionless internal heat generation or absorption parameter on the thermohydrodynamic characteristic and entropy generation are examined An analysis of the results showed that the combined effects of internal heat generation/absorption and hybrid nanoliquid significantly alter the hydrothermal characteristics, heat transfer rate, and entropy generation within the annulus

MHD natural convection of Sodium Alginate Casson nanofluid over a solid sphere

Abstract: MHD free convection flow of Sodium Alginate nanofluid on a solid sphere with prescribed wall temperature is investigated. Tiwari and Das’s nanofluid model is applied to analyze the influence of nanoparticles and magnetic field on a natural convective flow. Titanium dioxide (TiO2), Silver (Ag), and Graphite oxide (GO) Sodium Alginate-based nanofluid has been considered. The Keller-box method is employed to solve the transformed system of partial differential equations. The impact of Casson fluid parameter, magnetic parameter, nanoparticles volume fraction on local skin friction coefficient, local Nusselt number, temperature, and velocity are plotted and analyzed. Our graphical results revealed that the (GO)- Sodium Alginate based Casson nanofluid has the highest local skin friction, local Nusselt number and velocity profiles as compared to the other nanoparticles Sodium Alginate based Casson nanofluid.

Second law analysis of magneto-natural convection in a nanofluid filled wavy-hexagonal porous enclosure

Abstract: Natural convection heat transfer analysis can be completed using entropy generation analysis. This study aims to accomplish both the natural convection heat transfer and entropy generation analyses for a hexagonal cavity loaded with Cu-H2O nanoliquid subjected to an oriented magnetic field.,Control volume-based finite element method is applied to solve the non-dimensional forms of governing equations and then, the entropy generation number is computed.,The results portray that both the average Nusselt and entropy generation numbers boost with increasing aspect ratio for each value of the undulation number, while both of them decrease with increasing the undulation number for each amplitude parameter. There is a maximum value for the entropy generation number at a specified value of Hartmann number. Also, there is a minimum value for the entropy generation number at a specified value of angle of the magnetic field. When the volume fraction of nanoparticles grows, the average Nusselt number increases and the entropy generation number declines. The entropy generation number attains to a maximum value at Ha = 14 for each value of aspect ratio. The average Nusselt number ascends 2.9 per cent and entropy generation number decreases 1.3 per cent for Ha = 0 when ϕ increases from 0 to 4 per cent.,A hexagonal enclosure (complex geometry), which has many industrial applications, is chosen in this study. Not only the characteristics of heat transfer are investigated but also entropy generation analysis is performed in this study. The ecological coefficient of performance for enclosures is calculated, too.