Fakhreddine S. Oueslati

Heterogeneous nanofluids: natural convection heat transfer enhancement

Convective heat transfer using different nanofluid types is investigated. The domain is differentially heated and nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The obtained results, by solving numerically the full governing equations, are found to be in good agreement with the developed solution based on the scale analysis approach. The resulting convective flows are found to be dependent on the local particle concentration φ and the corresponding solutal to thermal buoyancy ratio N. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration but remains less than the enhancement previously underlined in forced convection case.

Multiple natural convection solution in porous media under cross temperature and concentration gradients

Double-diffusive natural convection is of interest in several natural and industrial fields, for example, oceanography, nuclear waste, transformation processes, and crystal growth techniques. This work focuses on double-diffusive natural convection in a square cavity filled with porous media heated and cooled along vertical walls by uniform heat fluxes when a solutal flux is imposed vertically. The formulation of the problem is based on the Darcy?Brinkman model, and the density variation is taken into account by the Boussinesq approximation. W e found three distinct regimes. The first is a fully thermal convective regime in which the flow is essentially due to thermal buoyancy forces. The second is a diffusive one where the solutal forces are strong enough to produce a stable solutal stratification with no significant convective flows. The third is an intermediate regime where competition between the two buoyancy forces takes place. In the intermediate regime a hysterisys is observed and two different sol...Double-diffusive natural convection is of interest in several natural and industrial fields, for example, oceanography, nuclear waste, transformation processes, and crystal growth techniques. This work focuses on double-diffusive natural convection in a square cavity filled with porous media heated and cooled along vertical walls by uniform heat fluxes when a solutal flux is imposed vertically. The formulation of the problem is based on the Darcy?Brinkman model, and the density variation is taken into account by the Boussinesq approximation. W e found three distinct regimes. The first is a fully thermal convective regime in which the flow is essentially due to thermal buoyancy forces. The second is a diffusive one where the solutal forces are strong enough to produce a stable solutal stratification with no significant convective flows. The third is an intermediate regime where competition between the two buoyancy forces takes place. In the intermediate regime a hysterisys is observed and two different solutions can be obtained depending on the initial state. The effects of Rayleigh, Lewis, and Darcy numbers are analyzed.

Thermosolutal Convection During Melting in a Porous Medium Saturated with Aqueous Solution

The thermosolutal convection in a porous medium saturated with an aqueous solution near the temperature of the density maximum is studied. The fixed temperatures applied to vertical walls include the density maximum. The formulation of the problem is based on the Darcy-Brinkman model and the density variation is governed by a nonlinear approximation. The equations are solved by a finite-volume method. The numerical model is validated through experimental results. We show that the nonlinear variation of the density influences strongly the flow structure and the heat transfer. The structures of this flow show that the density maximum generates a complex flow structure of two contrarotating cells of unequal importance.

Analytical and Numerical Solutions for Natural Convection in a Shallow Cavity Filled with Two Immiscible Fluids: Shear Stress Action

A shallow horizontal cavity filled with two layers of nonmiscible fluids, is studied both analytically and numerically. The upper free surface of the layer is subject to a shear stress. The analytical solution, based on the parallel flow approximation, is found to be in good agreement with a numerical solution. The existence of both natural and antinatural flows is demonstrated. Indeed, the critical Rayleigh number for the onset of motion is dependent on the side heating intensity. Multiple steady state solutions are possible in a range of the rate of fluids dynamics viscosity that depends strongly on the upper fluid one.

Natural convection in shallow cavity filled with nanofluids taking into account the Soret effect

Natural convection, in a shallow horizontal cavity filled with nanofluids, is studied both numerically and analytically. The horizontal surface of the enclosure are submitted to heat on the bottom and cooled on the upper. The vertical walls are subject to variable heat fluxes. The nanofluids are treated as heterogeneous mixtures with weak solutal diffusivity and possible Soret separation. Owing to the pronounced Soret effect of these materials in combination with a considerable solutal expansion, the resulting solutal buoyancy forces could be significant and interact with the initial thermal convection. A modified formulation taking into account the thermal conductivity, viscosity versus nanofluids type and concentration and the spatial heterogeneous concentration induced by the Soret effect is presented. The induced nanofluid heterogeneity showed a significant heat transfer modification. The heat transfer in natural convection increases with nanoparticle concentration. The analytical solution, based on the parallel flow approximation, is found to be in good agreement with a numerical solution. The existence of both natural and antinatural flows is demonstrated. Indeed, the critical Rayleigh number for the onset of motion is dependent on the concentration of nanoparticles and their nature.

Modeling of solar collectors and photovoltaic systems integrated with building assets

This study is within the framework of the promotion of solar energy, environmental protection and energy conservation. It is among the works that are intended for the use of solar energy for the production of energy to buildings by the use of photovoltaic and thermal hybrid collectors (photovoltaic/thermal). The finite volume method is used. Also, a change of coordinate is adopted to pass the physical space complex geometric configuration space simplest calculation. However, the model is developed it goes numerical investigation at the end of this study the effect of alternating the air flow and heat transfer inside the channel and analyze the performance and especially the brewing the outlet temperature and the cooling of the photovoltaic cells.

Study of transfer and flow in porous media in nonequilibrium

3D Natural convection in partially porous media. The Study focused on transfers and flow in porous media in nonequilibrium. Darcy-Brinkman models is adopted. Moreover the two-equation model is used to separately account for the local fluid and solid temperatures. A numerical method based on the finite volume method a full multigrid technique is employed. We find the impact of the conductivities ratio on the flow and the heat transfer.

Natural Convection in a Horizontal Cavity Partially Occupied by a Porous Media Saturated by a Coolant Liquid

The natural convection is studied in a cavity witch the lower half is filled with a porous media that is saturated with a first fluid (liquid), and the upper is filled with a second fluid (gas). The horizontal borders are heated and cooled by uniform heat fluxes and vertical ones are adiabatic. The formulation of the problem is based on the Darcy-Brinkman model. The density variation is taken into account by the Boussinesq approximation. The system of the coupled equations is resolved by the classic finite volume method. The numerical results show that the variation of the conductivity of the porous media influences strongly the flow structure and the heat transfer as well as in upper that in the lower zones. The effect of conductivity is conditioned by the porosity which plays a very significant roll on the heat transfer. The structures of this flow show that this kind of problem with specific boundary conditions generates a complex flow structure of several contra-rotating two to two cells, in the upper half of the cavity.Copyright