Mohamed Naceur Borjini

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.

ANALYSIS OF RADIATIVE HEAT TRANSFER IN A PARTITIONED IDEALIZED FURNACE

In this article, the finite-volume method is applied, in conjunction with the blocked-off-region procedure, to solve the radiative heat transfer problem in a three-dimensional partitioned rectangular enclosure containing absorbing-emitting and isotropic scattering medium in the presence of heat generation. Three-dimensional test cases were chosen to benchmark this model against other methods, namely, the discrete transfer method, the discrete ordinates method, and the zone method. For all cases, excellent agreement is obtained using reasonable spatial and angular discretizations. Then the radiative heat transfer problem is solved for an idealized furnace with a finite-thickness opaque partition. The shadowing partition effect is discussed for different optical medium properties and partition surface emissivities.

Application of the Finite-Volume Method to Study the Effects of Baffles on Radiative Heat Transfer in Complex Enclosures

A finite-volume radiation model for participating gray media in 2-D and 3-D complex rectangular enclosures with obstacles is developed. The step and the bounded high-order resolution curved-line advection method (CLAM) schemes are examined. Using the blocked-off-region procedure, the present model is capable of predicting radiative heat transfer in enclosures with obstructions and baffles. In order to validate the formulations derived here a square cavity with one or three baffles and finned internal cylinder, then a three-dimensional complex heat recuperator of a pilot plant of biomass pyrolysis with obstructions and baffles, are studied. It should be pointed out that the developed code using the CLAM scheme is accurate and convenient for computational thermal calculations. For the considered heat recuperator, the presence of baffles enhances radiative heat flux and contributes to the increase of the mean medium temperature.

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.

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.

Prediction of unsteady natural convection within a horizontal narrow annular space using the control-volume method

ABSTRACT Oscillatory natural convection in a narrow horizontal space is investigated numerically using the control-volume method. A parametric study for Rayleigh number is carried out using the power-law scheme and a confrontation with the central-difference scheme is developed. At intermediate values of the Prandtl number, complex interaction between hydrodynamic and thermal instabilities exists. For Pr = 0.2, gradual transitions from monocellular to three-top cell regimes are registered, the phenomenon of coalescence and separation of like-rotating cells is observed, and a subharmonic instability, followed by an early chaotic flow, is depicted. A disagreement between the power-law and the central-difference schemes results exists; the latest ones are in good agreement with the finite-difference method computations. However, the central-difference scheme results suffer from the false diffusion phenomenon.