Abdelkader Mojtabi - Profile on Academia.edu (original) (raw)

Papers by Abdelkader Mojtabi

Nahasketa bitarren Soret koefizientearen neurketa ingurune porotsuan

V. Ikergazte. Nazioarteko ikerketa euskaraz. Kongresuko artikulu bilduma. Ingeniaritza eta Arkitektura

Comptes Rendus. Mécanique

Recently, we proved that it was possible to significantly increase the importance of the species ... more Recently, we proved that it was possible to significantly increase the importance of the species separation for binary mixtures, under microgravity, using forced convection obtained by a uniform translational displacement of the horizontal isothermal walls of the cell maintained at T h and T c respectively, with ∆T = T h -T c . The present work is an extension of the study performed by Mojtabi (2020) with various mixtures, from binary to n-component mixtures. An application with a ternary mixture was presented. The mixture of tetralin, isobutyl benzene, n-dodecane with respectively 0.8-0.1-0.1 mass fractions was studied experimentally in weightlessness.

Soret-Driven Convection Inside Concentric Porous Spheres Saturated by Binary Fluid: Comparison with Parallelepipedic Vertical Column

Transport in Porous Media

HAL (Le Centre pour la Communication Scientifique Directe), 2015

L'effet de l'intensité et de l'orientation d'un champ magnétique uniforme sur la transition criti... more L'effet de l'intensité et de l'orientation d'un champ magnétique uniforme sur la transition critique au sein d'une couche fluide binaire, électriquement conductrice, stratifiée en température et en concentration, en prenant en considération l'effet Soret, est étudié numériquement. Pour une telle configuration, les résultats ont mis en évidence que les seuils critiques correspondant à un champ magnétique de direction quelconque peuvent se déduire de ceux obtenus pour un champ magnétique vertical et que les axes des rouleaux marginaux des modes instables sont désormais alignés avec la composante horizontale du champ magnétique. Par ailleurs, une étude analytique est menée pour étudier l'impact du champ magnétique sur les instabilités monocellulaires. L'effet du champ magnétique sur de telles instabilités a mis en évidence un phénomène inédit consistant à modifier profondément la nature des modes instables, qui perdent leur caractère monocellulaire pour retrouver leur aspect à plusieurs rouleaux qui les caractérisaient sans champ magnétique pour ψ < ψ 0 = 131 Le/(34 -131Le). Pour un fluide binaire caractérisé par un nombre de Lewis Le et un facteur de séparation ψ > ψ 0 , la valeur du nombre de Hartmann Ha (ψ, Le) correspondant à cette transition responsable d'une modification significative des transferts de masse et de chaleur peut être déterminée à partir des relations analytiques établies dans le cadre de ce travail.

On etudie l'influence des vibrations verticales harmoniques de hautes frequences et de faible... more On etudie l'influence des vibrations verticales harmoniques de hautes frequences et de faibles amplitudes sur la separation des especes d'un melange binaire saturant une couche poreuse horizontale de grande extension et soumise a une densite de flux de chaleur uniforme au niveau des parois horizontale. On utilise le formalisme des equations moyennees. Nous montrons qu'il est possible d'effectuer la separation des especes d'un melange binaire dans cette configuration geometrique, et que les vibrations peuvent etre utilisees pour retarder la perte de stabilite de l'ecoulement monocellulaire, ce qui permet d'assurer la separation pour des valeurs plus elevees du nombre de Rayleigh. La stabilite de la solution d'equilibre mecanique est etudiee analytiquement et numeriquement dans une cellule de grande extension horizontale. La perte de la solution d'equilibre conduisant a un ecoulement unicellulaire est obtenue pour une valeur critique du nombre de Ra...

In this work a theoretical study of the thermo-gravitational separation process of anN -component... more In this work a theoretical study of the thermo-gravitational separation process of anN -component mixture is presented. This study is focus on a bi-dimensional thermo-gravitational column. It is based on the parallel flow approximation and the theory developed by Furry Jones and Onsager. Using these hypotheses the velocity, the mass-fraction of each component and the temperature at the stationary state have been analytically determinate. Knowing the basis state in the column, the separation of each component in the mixture has been determinate. An analytical expression relating the separation and the transport coefficients has been obtained. The thermo-diffusion coefficients tabulated in the literature are in good agreement with the value analytically obtained. Mots clefs : écoulement convectif, thermo-diffusion, mélange de fluide multiconstituants 23ème Congrès Français de Mécanique Lille, 28 au 1er Septembre 2017

In this paper, the stationary flow induced at large scale by the propagation of ultrasonic waves ... more In this paper, the stationary flow induced at large scale by the propagation of ultrasonic waves in a viscous fluid has been studied. More precisely, the acoustic streaming under concern is the Eckart streamingwhich appears during the bulk wave propagation. In order to describe the fluid flow, impressible Navier-Stokes equations with a steady body force were solved using the industrial FEM code (COMSOL). The acoustic intensity was first calculated using the Rayleigh integral. The numerical results were compared to the experimental results obtained by Moudjed (2013). Mots clés : acoustic streaming, ondes ultrasonores, force acoustique, écoulement entraîné

$Research paper thumbnail of Theoretical and numerical analysis of Soret-driven convection in a horizontal porous layer saturated by an n-component mixture: Application to ternary hydrocarbon mixture tetralin, isobutyl benzene, n-dodecane with mass fractions 0.8-0.1-0.1$

International Journal of Heat and Mass Transfer, 2020

Theoretical and numerical analysis of Soret-driven convection in a horizontal porous layer satura... more Theoretical and numerical analysis of Soret-driven convection in a horizontal porous layer saturated by an n-component mixture: Application to ternary hydrocarbon mixture tetralin, isobutyl benzene, n-dodecane with mass fractions 0.8-0.1-0.1

Journal of Fluid Mechanics, Feb 25, 1999

International Journal of Heat and Mass Transfer, 2019

The coupling between shear-driven convection and thermo-diffusion is a complex phenomenon due to ... more The coupling between shear-driven convection and thermo-diffusion is a complex phenomenon due to the interactions between the different forces inside the fluid mixture. This paper studies the species separation that may appear in a parallelepipedic cell filled by a binary mixture. Dirichlet boundary conditions are imposed with differentially heated horizontal walls. Each wall can be animated with a uniform velocity. More precisely, two velocities 𝑈 𝑃 𝑒 𝑥 ⃗⃗⃗ , and 𝑓𝑈 𝑃 𝑒 𝑥 ⃗⃗⃗ are applied at the upper and the lower horizontal walls respectively. An analytical solution, based on the parallel flow approximation in the core region of the cavity of large aspect ratio, is obtained as a function of the dimensionless parameters of the problem. The species separation is optimized according to the multiple control parameters of the problem. The velocity field obtained is the superposition of the flow generated by the velocity of the walls under weightless conditions and thermoconvective flow under gravity only. The combination of these two flows leads to a mixed convection flow with a single convective cell for 𝑓 ∈ [-1, 0] and two superposed cells for 𝑓 ∈ [0, 1]. Only unicellular flow leads to a notable species separation. This study shows that the effective species separation admits a partial optimum as a function of 𝑓 and 𝑈 𝑝 . As in the case of thermogravitational columns, there is no optimum species separation with respect to parameters ∆𝑇 and 𝐻 but, for a fixed temperature difference ∆𝑇, there is a thickness 𝐻 that optimizes the separation and vice versa. 2𝐷 and 3𝐷 direct numerical simulations are performed using a finite element method in order to corroborate the analytical results. The influence of mass Péclet number on the diffusive relaxation time is determined numerically. Taking into account the confinement of the medium in the third dimension, perpendicular to a vertical plane, we show that the aspect ratio in the y-direction has an influence on the species separation.

Letters in Heat and Mass Transfer, 1978

The purpose of the present paper is to determine experimentally the axial limitations of various ... more The purpose of the present paper is to determine experimentally the axial limitations of various types of copper heat pipes operating at vertical inclination. For a vertical position, it appears, from our experimental study that the wick structure used over the evaporator does not affect the axial power limit. This power varies very little between this position and the 45 ° inclination to it. Moreover, after a drying out of the evaporator, the heat pipe, which is in vertical position and gravity assisted, re-starts fn operation.

Physics, 2022

In this paper, an analytical and numerical analysis of the species separation in a binary mixture... more In this paper, an analytical and numerical analysis of the species separation in a binary mixture is performed. The main objective is to study the influence of the thickness and the nature of the bounding plates of the thermogravitational column (TGC) on species separation. The theory of Furry, Jones and Onsager is extended to the cases where bounding conducting walls enclose the TGC. The governing 2-dimensional equations are solved numerically using COMSOL Multiphysics software. A good agreement is found between the analytical and the numerical results. It is shown that the determination of the thermal diffusion coefficient, DT, from the measurement of the vertical mass fraction gradient of binary solutions, does not depend on the temperature difference imposed on the vertical column either on the outer walls of the cavity or on the inner walls in contact with the binary solutions. However, it is found that this result is no longer valid in the case of a binary gas. To our knowledg...

Thermogravitational separation in porous vertical and horizontal cylindrical annular cells saturated by a binary mixture

The European Physical Journal E

Comptes Rendus de l'Académie des Sciences - Series IIB - Mechanics-Physics-Chemistry-Astronomy, 1997

de régimes de convection thermosolutale dans une cellule rectangulaire poreuse. (1997) Comptes Re... more de régimes de convection thermosolutale dans une cellule rectangulaire poreuse. (1997) Comptes Rendus de l'Académie des Sciences -Series IIB -Mechanics-Physics-Chemistry-Astronomy, 324 (1). 9-17.

Transport in Porous Media, 2011

We consider convection in a uniform fluid-saturated porous layer which is bounded by conducting p... more We consider convection in a uniform fluid-saturated porous layer which is bounded by conducting plates and heated from below. The primary aim is to determine the identity of the postcritical convection planform as a function of the thicknesses and conductivities of the bounding plates relative to that of the porous layer. This work complements and extends an early paper by Riahi (1983) who considered a situation where the porous layer is bounded by infinitely thick conducting media. We present regions in parameter space wherein convection in the form of rolls is unstable and within which cells with square planform form the preferred pattern.

Physics of Fluids, 2004

The dynamic behavior of convective motion in a confined porous medium saturated by a pure fluid u... more The dynamic behavior of convective motion in a confined porous medium saturated by a pure fluid under the action of mechanical vibration is studied. A redefinition of vibrational Rayleigh number is proposed from which we distinguish the domain of validity of the mean flow. The weakly nonlinear stability analysis performed demonstrates, contrary to published results, that the bifurcation is of supercritical nature and the subcritical branch does not exist. It is emphasized that, in order to find the thermal behavior of the system for the onset of convection, we should separate the vibrational effect from the thermal effect involving the temperature difference.

During recent years considerable effort has been directed towards the study of complex body force... more During recent years considerable effort has been directed towards the study of complex body forces whereby convective heat transfer rates might be increased. In natural convection, the interaction of non-homogeneities of density field with body force produces convective motion. Space related technologies have demanded a profound knowledge of these kinds of body forces; for example vibration, magnetic, etc which under earth condition are of second order importance. The study of thermo-vibrational problems was originally inspired by its applications in cryogenic propellants in rocket system, Fashbaugh and Streeter [1]. This type of convection differs from natural convection induced by gravity in that it may be produced even under microgravity or space station conditions. The systematic study of this type of convection began in the early sixties, Schoenhals and Clark [2], and was later essentially focused on the mathematical aspect of body force modulation, for example see the review paper of Davis . There, he emphasizes the importance of the relevant scales of the problem, and how it is possible to control the hydrodynamic stability of convective motion by proper tuning of modulation parameters. The idea of using mechanical vibration as a mean for enhancing the heat transfer rate has also received equal attention from the early beginning. Forbes et al. [4] present an experimental study of the influence of vibration on heat transfer and fluid dynamic behavior of a differentially heated vertical slot. The direction of vibration is perpendicular to the temperature gradient. They find that there is a possibility of resonance, which leads to the heat transfer augmentation. This idea has been exploited in numerous papers in which the geometry is a rectangular cavity. However, it should be pointed out that this idea is limited to the cases in which the thermal system possesses a natural frequency. This means that the external acceleration field should be thought as the superposition of a constant acceleration and a periodic acceleration with zero mean where the constant acceleration produces the natural motion in enclosure. Therefore, logically, for the system under the effect of periodic acceleration with zero mean there is no possibility of resonance. The important issues related to the enhancement of heat transfer concern the type of thermal boundary conditions, relative orientation of vibration vector with the temperature gradient (non-parallelism) and the considered geometry. Here, we only review the papers under microgravity conditions in which the boundaries of enclosure are maintained at different but uniform temperatures and the direction of vibration is perpendicular to the temperature gradient. For other types of boundary conditions and vibrational direction, see Christov and Homsy [8] , Savino et al. and Gershuni and Lyubimov . Kamotani et al. [11] examine the influence of vibration with zero mean on the heat transfer behavior of the thermal system. Their results show that, in this case, vibration has no significant effect on the heat transfer rate. This finding may be explained by considering the range of frequency and amplitude used in that study; the amplitude and frequency were both very small. The fundamental question that we address here is how to select the frequency and amplitude of vibration under microgravity in order to obtain optimal geometry for which heat transfer is maximized. Of numerous possible combinations between amplitude and frequency of vibration, high frequency and small amplitude possesses particular characteristics, which make it attractive for heat transfer studies . The most important of these characteristics is that it provides us with a closed system of equations governing mean field, which does not contain periodic coefficients. Khallouf et al. [12] consider the influence of vibration on heat transfer and fluid flow behavior of the thermal system in the range of high frequency and small amplitude. Their study is limited to the case of a Prandtl number Pr = 1 and a vibrational Rayleigh number Ra v =10 5 with the objective of studying the fluid flow structures. They propose the possibility of the heat transfer augmentation with respect to aspect ratios, A.

Journal of Fluid Mechanics, 1999

This work brings new insight to the question of heat transfer in near–critical fluids under Earth... more This work brings new insight to the question of heat transfer in near–critical fluids under Earth gravity conditions. The interplay between buoyant convection and thermoacoustic heat transfer (piston effect) is investigated in a two-dimensional non-insulated cavity containing a local heat source, to reproduce the conditions used in recent experiments. The results were obtained by means of a finite-volume numerical code solving the Navier–Stokes equations written for a low-heat-diffusing near-critical van der Waals fluid. They show that hydrodynamics greatly affects thermoacoustics in the vicinity of the upper thermostated wall, leading to a rather singular heat transfer mechanism. Heat losses through this wall govern a cooling piston effect. Thus, the thermal plume rising from the heat source triggers a strong enhancement of the cooling piston effect when it strikes the middle of the top boundary. During the spreading of the thermal plume, the cooling piston effect drives a rapid th...

International Journal of Heat and Mass Transfer, 2013

We investigate the onset of convection in a uniform, constant-thickness, horizontal porous layer ... more We investigate the onset of convection in a uniform, constant-thickness, horizontal porous layer which is heated from below. The layer is bounded above and below by thermally conducting but impermeable layers. Our aim is to determine the effect on the onset of convection of the interaction between the presence ofthese outer conducting layers and a horizontal background flow. A linear stability analysis is performed and a dispersion relation is derived from which the stability characteristics of the layer are computed. Convection ce lis are found move along the layer at a speed which is lower than that of the imposed flow due to a thermal drag caused by the presence of the bounding solid layers. Neutra! curves and streamline/ isotherm patterns are presented in order to understand the physical rote played by the governing nondimensional parameters. Wh en the diffusivity of the solid layers is much lower than the diffusivity of the porous layer there exists a regime where the neutra! curve can exhibit two minima, and at one point in parameter space there exists a neutra! curve with a quartic minimum.

International Journal of Heat and Mass Transfer, 2004

The generation of two-dimensional thermal convection induced simultaneously by gravity and high-f... more The generation of two-dimensional thermal convection induced simultaneously by gravity and high-frequency vibration in a bounded rectangular enclosure or in a layer is investigated theoretically and numerically. The horizontal walls of the container are maintained at constant temperatures while the vertical boundaries are thermally insulated, impermeable and adiabatic. General equations for the description of the time-averaged convective flow and, within this framework, the generalized Boussinesq approximation are formulated. These equations are solved using a spectral collocation method to study the influence of vibrations (angle and intensity). Hence, a theoretical study shows that mechanical quasi-equilibrium (i.e., state in which the averaged velocity is zero but the oscillatory component is in general non-zero) is impossible when the direction of vibration is not parallel to the temperature gradient. In the other case, it is proved that the mechanical equilibrium is linearly stable up to a critical value of the unique stability parameter, which depends on the vibrational field. In this paper, it is shown that high-frequency vertical oscillations can delay convective instabilities and, in this way, reduce the convective flow. The isotherms are oriented perpendicular to the axis of vibration. In the case where the direction of vibration is perpendicular to the temperature gradient, small values of the Grashof number, the stability parameter, induce the generation of an average convective flow. When the aspect ratio is large enough, the character of the bifurcation is practically the same as in the limiting case of an infinitely long layer.