Lioua Kolsi

Effect of an External Magnetic Field on the 3-D Unsteady Natural Convection in a Cubical Enclosure

A numerical study of natural convection in a differentially heated cubic cavity for Pr = 0.054 and in the presence of an external magnetic field orthogonal to the isothermal walls is carried out. The effect of this field on the three-dimensional spiraling transverse flow is analyzed. In addition to the damping and laminarization effects of the external magnetic field, an organization of the central three-dimensional motion is observed. Also, a depiction of the merging phenomena of the two central vortices is shown.

HYDROMAGNETIC DOUBLE-DIFFUSIVE LAMINAR NATURAL CONVECTION IN A RADIATIVELY PARTICIPATING FLUID

ABSTRACT Two-dimensional hydromagnetic double-diffusive convection of a radiatively participating fluid confined in a rectangular enclosure is studied numerically for fixed Prandtl, Rayleigh, and Lewis numbers, Pr = 13.6, Ra = 105, Le = 2. Uniform temperatures and concentrations are imposed along the vertical walls, while the horizontal walls are assumed to be adiabatic and impermeable. The damping and stabilization effects of an external horizontal magnetic field are studied for three different optical thicknesses of the semitransparent fluid as well as for an opaque medium. For moderate optical thickness, a steady compositionally dominated flow is observed for all values of Hartmann number considered, and the magnetic damping is remarkably lower than in the opaque medium, for which the flow is always thermally dominated. For optically thin and optically thick media, the thermally dominated flow is stabilized and becomes compositionally dominated as soon as the Hartmann number is increased.

Natural convection and entropy generation in a cubical cavity with twin adiabatic blocks filled by aluminum oxide–water nanofluid

ABSTRACT A finite volume-based three-dimensional numerical simulation on natural convection and entropy generation in a cubical cavity filled with a nanofluid of aluminum oxide–water is presented by vorticity–vector potential formalism. The blocks are adiabatic and the vertical walls are differentially heated unidirectionally. The variables considered are Ra, volumetric fraction of aluminum oxide particles, and block size. The results for fluid flow with a single-phase model are elucidated with iso-surfaces of temperature, Nusselt number, and Bejan number. The local entropy generated was due to friction surges when the volumetric fraction of nanoparticles was increased. The average Nusselt number rose with the increase in Ra and volumetric fraction of solid particles and declined with the increase in block size.

Effect of Heat and Mass Transfer Through Diffusive Walls on Three-Dimensional Double-Diffusive Natural Convection

A three-dimensional numerical study using vorticity–vector potential formulations based on the finite-volume method has been performed to investigate double-diffusive convection in a stack of cubic enclosures submitted to horizontal gradients of temperature and concentration. The flow is driven by conditions of constant temperature and concentration imposed along the two vertical side walls of each cubic enclosure, while the horizontal walls are diffusive in heat and mass. This numerical study is conducted for fixed Prandtl, Rayleigh, and Lewis numbers, Pr = 10, Ra = 105 and Le = 10, buoyancy ratio N in the range [−2, 0], and for a range of heat and mass transfer diffusion coefficients. The results show that the effect of heat and mass diffusive walls differs between the case of thermally dominated flow and the compositionally dominated one. For thermally dominated flow, considering heat and mass diffusive walls results in a change from a structure with one inner core to a multicore structure and reduces the transverse velocity. On the other hand, for solutally dominated flow, an “inverse transition” from a multicell pattern to a unicellular one occurs, and the transverse velocity increases considerably.

3D Buoyancy-Induced Flow and Entropy Generation of Nanofluid-Filled Open Cavities Having Adiabatic Diamond Shaped Obstacles

A three dimensional computational solution has been obtained to investigate the natural convection and entropy generation of nanofluid-filled open cavities with an adiabatic diamond shaped obstacle. In the model, the finite volume technique was used to solve the governing equations. Based on the configuration, the cavity is heated from the left vertical wall and the diamond shape was chosen as adiabatic. Effects of nanoparticle volume fraction, Rayleigh number (103 ≤ Ra ≤ 106) and width of diamond shape were studied as governing parameters. It was found that the geometry of the partition is a control parameter for heat and fluid flow inside the open enclosure.

Effect of Radiative Heat Transfer on Three-Dimensional Double Diffusive Natural Convection

This work presents a numerical study of the effect of the radiative heat transfer on the three-dimensional double diffusive convection in a differentially heated cubic cavity for different optical parameters of the medium. This numerical study is conducted for fixed Prandtl, Rayleigh, and Lewis numbers, Pr = 13.6, Ra = 105, Le = 2, and buoyancy ratio N in the range [–2, 0]. The natural convection equations, using the Boussinesq approximation for the treatment of buoyancy term in the momentum equation, are expressed using the vorticity–stream function formulation. These equations and the radiative transfer equation are discretized, respectively, with the control volume finite difference method and the FTn finite volume method. The influences of the optical thickness and the conduction–radiation parameter of the semitransparent fluid on heat and mass transfer are depicted. Results show different transitions of the structure of the main flow when varying the conduction–radiation parameter and the optical thickness.

Effects of Movable-Baffle on Heat Transfer and Entropy Generation in a Cavity Saturated by CNT Suspensions: Three-Dimensional Modeling

Convective heat transfer and entropy generation in a 3D closed cavity, equipped with adiabatic-driven baffle and filled with CNT (carbon nanotube)-water nanofluid, are numerically investigated for a range of Rayleigh numbers from 103 to 105. This research is conducted for three configurations; fixed baffle (V = 0), rotating baffle clockwise (V+) and rotating baffle counterclockwise (V−) and a range of CNT concentrations from 0 to 15%. Governing equations are formulated using potential vector vorticity formulation in its three-dimensional form, then solved by the finite volume method. The effects of motion direction of the inserted driven baffle and CNT concentration on heat transfer and entropy generation are studied. It was observed that for low Rayleigh numbers, the motion of the driven baffle enhances heat transfer regardless of its direction and the CNT concentration effect is negligible. However, with an increasing Rayleigh number, adding driven baffle increases the heat transfer only when it moves in the direction of the decreasing temperature gradient; elsewhere, convective heat transfer cannot be enhanced due to flow blockage at the corners of the baffle.

Lattice Boltzmann method based on Dual-MRT model for three-dimensional natural convection and entropy generation in CuO–water nanofluid filled cuboid enclosure included with discrete active walls

Abstract In the present study, the three-dimensional natural convection and entropy generation in a cuboid enclosure included with various discrete active walls is analyzed using lattice Boltzmann method. The enclosure is filled with CuO–water nanofluid. To predict thermo-physical properties, dynamic viscosity and thermal conductivity, of CuO–water nanofluid, the KKL model is applied to consider the effect of Brownian motion on nanofluid properties. In lattice Boltzmann simulation, two different MRT models are used to solve the problem. The D3Q7-MRT model is used to solve the temperature filed, and the D3Q19 is employed to solve the fluid flow of natural convection within the enclosure. The influences of different Rayleigh numbers 1 0 3 R a 1 0 6 and solid volume fractions 0 φ 0 . 04 and four different arrangements of discrete active walls on the fluid flow, heat transfer, total entropy generation, local heat transfer irreversibility and local fluid friction irreversibility are presented comprehensively.

Inclination effects of magnetic field direction in 3D double-diffusive natural convection

In this paper a numerical study which treats the effect of the magnetic field inclination on 3D double diffusive convection in a cubic cavity filled with a binary mixture is presented. The two vertical walls are maintained at different temperatures and concentrations. A particular interest is reserved to determine the effect of the magnetic field inclination on the flow structure and heat and mass transfer. The problem is formalized based on the vector potential vorticity procedure in its three-dimensional configuration and discretized based on the finite volume method. The results are given for Ra?=?105, Pr?=?1 and Le?=?2. This paper presents respectively the inclination effects of the magnetic field direction on the three-dimensional flow structure and on heat and mass transfer. The main results show that the increase of the inclination of the magnetic field direction damped the flow. A critical angle, which depending on Hartmann number, caused big change on the flow structure and accented the three dimensional aspect in the cavity.

Entropy generation analysis and heatline visualization of free convection in nanofluid (KKL model-based)-filled cavity including internal active fins using lattice Boltzmann method

Abstract Two-dimensional natural convection and entropy generation in a square cavity filled with CuO–water nanofluid is performed. The lattice Boltzmann method is employed to solve the problem numerically. The influences of different Rayleigh numbers 1 0 3 R a 1 0 6 and solid volume fractions 0 φ 0 . 05 on the fluid flow, heat transfer and total/local entropy generation are presented comprehensively. Also, the heatline visualization is employed to identify the heat energy flow. To predict the thermo-physical properties, dynamic viscosity and thermal conductivity, of CuO–water nanofluid, the KKL model is applied to consider the effect of Brownian motion on nanofluid properties. It is concluded that the configurations of active fins have pronounced effect on the fluid flow, heat transfer and entropy generation. Furthermore, the Nusselt number has direct relationship with Rayleigh number and solid volume fraction, and the entropy generation has direct and reverse relationships with Rayleigh number and solid volume fraction, respectively.