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Ministère de l'Enseignement Supérieur, de la Recherche Scientifique et de la Technologie. Assises Nationales de la Recherche Scientifique et de lInnovation.

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Présentation au sujet: "Ministère de l'Enseignement Supérieur, de la Recherche Scientifique et de la Technologie. Assises Nationales de la Recherche Scientifique et de lInnovation."— Transcription de la présentation:

1 Ministère de l'Enseignement Supérieur, de la Recherche Scientifique et de la Technologie. Assises Nationales de la Recherche Scientifique et de lInnovation Technologique Tunis Novembre 2007 Unité de Recherche : Thermodynamique Appliquée وحدة البحث : ديناميكا حرارية مطبقة Research Unit : Applied Thermodynamics Code : 99/UR/11-21 الرمز : جامعة قابس المدرسة الوطنية للمهندسين بقابس المسؤول : عمار بن ابرهيم Responsable : Ben Brahim Ammar École Nationale d'Ingénieurs de Gabès, Rue Omar Ibn El Khattab, 6029 Gabès, Tunisie Code : 99/UR/11-21 Performances des mélanges LiBr-H 2 O, LiCl-CaCl 2 et LiCl-MgCl 2 -H 2 O en pompe à chaleur et en réfrigération B. Chaouachi *, S. Gabsi, A. Ben Brahim École Nationale d'Ingénieurs de Gabès, Rue Omar Ibn El Khattab, 6029 Gabès, Tunisie Abstract : Les cycles à absorption mettent en jeu au moins deux fluides: un solvant et un soluté. Bien que d'autres couples soient étudiés, les seuls qui soient utilisés en pratique pour la quasi-totalité des applications sont les deux couples LiBr-H 2 O et NH 3 -H 2 0. Ces mélanges présentent quelques inconvénients tels que les niveaux de pression, la corrosion, la toxicité, le prix élevé, le risque de cristallisation pour le cas du binaire LiBr-H 2 O. L'objectif de ce travail est d'analyser les deux mélanges LiCl-MgCl 2 -H 2 O et LiCl-CaCl 2 -H 2 O. Il s'agit d'étudier l'effet des divers variables opératoires sur le coefficient de performance, la puissance dégagée, la dépense d'énergie et la densité de stockage en mode de fonctionnement en pompe à chaleur et en réfrigération. Les résultats obtenus montrent que le mélange LiCl-MgCl 2 -H 2 O présente des caractéristiques intéressantes telles qu'un coefficient de performance qui peut atteindre 1,95 en pompe à chaleur et 0,9 en réfrigération, un effet de pompe à chaleur qui peut dépasser de 100 % celui du LiBr-H 2 O et un prix de revient plus bas que ce dernier. Quant à la densité de stockage, qui traduit le volume efficace de la machine, est favorable au mélange conventionnel LiBr-H 2 O en pompe à chaleur et au mélange LiCl-MgCl 2 -H 2 O en réfrigération ce qui nécessite une étude technico-économique pour décider de la possibilité de remplacement du système LiBr- eau par l'un des deux autres systèmes le moins coûteux. Keywords : absorption, pompe à chaleur, réfrigération, performances. Abstract : Les cycles à absorption mettent en jeu au moins deux fluides: un solvant et un soluté. Bien que d'autres couples soient étudiés, les seuls qui soient utilisés en pratique pour la quasi-totalité des applications sont les deux couples LiBr-H 2 O et NH 3 -H 2 0. Ces mélanges présentent quelques inconvénients tels que les niveaux de pression, la corrosion, la toxicité, le prix élevé, le risque de cristallisation pour le cas du binaire LiBr-H 2 O. L'objectif de ce travail est d'analyser les deux mélanges LiCl-MgCl 2 -H 2 O et LiCl-CaCl 2 -H 2 O. Il s'agit d'étudier l'effet des divers variables opératoires sur le coefficient de performance, la puissance dégagée, la dépense d'énergie et la densité de stockage en mode de fonctionnement en pompe à chaleur et en réfrigération. Les résultats obtenus montrent que le mélange LiCl-MgCl 2 -H 2 O présente des caractéristiques intéressantes telles qu'un coefficient de performance qui peut atteindre 1,95 en pompe à chaleur et 0,9 en réfrigération, un effet de pompe à chaleur qui peut dépasser de 100 % celui du LiBr-H 2 O et un prix de revient plus bas que ce dernier. Quant à la densité de stockage, qui traduit le volume efficace de la machine, est favorable au mélange conventionnel LiBr-H 2 O en pompe à chaleur et au mélange LiCl-MgCl 2 -H 2 O en réfrigération ce qui nécessite une étude technico-économique pour décider de la possibilité de remplacement du système LiBr- eau par l'un des deux autres systèmes le moins coûteux. Keywords : absorption, pompe à chaleur, réfrigération, performances. Abstract : The physical problem studied in this paper is the influence of thermodiffusion effect on the entropy generation in steady state of thermosolutal convection in a square cavity filled with a binary gas mixture with one vertical wall heated and the other vertical wall is cooled, the remaining two walls are insulated. The analysis deals with a numerical determination of the entropy generation on 2D approximation using the finite volume method adapted to the staggered grid. In this study, the total entropy generation is treated a function of : (1) the buoyancy forces by report to the thermal buoyancy, (2) the thermal diffusion ratio K T, a parameter characterizing the importance of thermodiffusion effect phenomenon and (3) the well know thermal buoyancy forces dominate the solutal buoyancy forces, values are not sensible for the variation of K T. In this case, thermodiffusion effect has no influence on the entropy production. When the solutal buoyancy forces dominate, thermodiffusion effect leads to an increase of the entropy generation in both cases of opposite and cooperative buoyancy forces. It has been found also, that the total entropy generation takes a minimum value found also, that the total entropy generation takes a minimum value found also, that the total entropy generation takes a minimum ratio for higher values of the thermal Grashof number. This minimum is found for N=-1 and Gr T =10 5 and is minimum for K T =0.3. More details of the investigation is focused for the special case when N=-1. Keywords : Soret effect, heat and mass transfer, entropy generation, numerical methods, cavity Abstract : The physical problem studied in this paper is the influence of thermodiffusion effect on the entropy generation in steady state of thermosolutal convection in a square cavity filled with a binary gas mixture with one vertical wall heated and the other vertical wall is cooled, the remaining two walls are insulated. The analysis deals with a numerical determination of the entropy generation on 2D approximation using the finite volume method adapted to the staggered grid. In this study, the total entropy generation is treated a function of : (1) the buoyancy forces by report to the thermal buoyancy, (2) the thermal diffusion ratio K T, a parameter characterizing the importance of thermodiffusion effect phenomenon and (3) the well know thermal buoyancy forces dominate the solutal buoyancy forces, values are not sensible for the variation of K T. In this case, thermodiffusion effect has no influence on the entropy production. When the solutal buoyancy forces dominate, thermodiffusion effect leads to an increase of the entropy generation in both cases of opposite and cooperative buoyancy forces. It has been found also, that the total entropy generation takes a minimum value found also, that the total entropy generation takes a minimum value found also, that the total entropy generation takes a minimum ratio for higher values of the thermal Grashof number. This minimum is found for N=-1 and Gr T =10 5 and is minimum for K T =0.3. More details of the investigation is focused for the special case when N=-1. Keywords : Soret effect, heat and mass transfer, entropy generation, numerical methods, cavity Influence of thermodiffusion effect on entropy generation in thermosolutal convection N. HIDOURI, H. ABBASSI*, M. MAGHERBI + and A. Ben Brahim N. HIDOURI, H. ABBASSI*, M. MAGHERBI + and A. Ben Brahim Abstract : Optimization of entropy generation in an inclined square cavity filled with a fluid and submitted to thermal and concentration gradients in presence of the Soret effect is numerically analysed by solving the mass, the momentum, the energy and the species conservation equations. Entropy generation is evaluated as a function of: (1) the inclination angle of the cavity β, ranging between 0° and 180°, (2) the thermal diffusion ratio K T, a parameter characterizing the Soret effect phenomenon and ranging between 0 and 0.5, (3) the buoyancy ratio N, a parameter measuring the intensity of the solutal buoyancy forces by report to the thermal buoyancy forces and (4) the well known thermal Grashof number. Entropy generation takes a constant minimum value for all K T values at β = 0° and 180° for respectively, opposite, and cooperative buoyancy forces. The inclination angle for which entropy generation is maximum, is constant for all K T values. For a fixed inclination angle, Soret effect tends always to increase the entropy generation. More details of the influence of Soret effect on entropy generation for the special case when N = -1 is discussed. Keywords : double diffusive convection, entropy generation, inclined enclosure, Soret effect thermodynamics of irreversible processes. Abstract : Optimization of entropy generation in an inclined square cavity filled with a fluid and submitted to thermal and concentration gradients in presence of the Soret effect is numerically analysed by solving the mass, the momentum, the energy and the species conservation equations. Entropy generation is evaluated as a function of: (1) the inclination angle of the cavity β, ranging between 0° and 180°, (2) the thermal diffusion ratio K T, a parameter characterizing the Soret effect phenomenon and ranging between 0 and 0.5, (3) the buoyancy ratio N, a parameter measuring the intensity of the solutal buoyancy forces by report to the thermal buoyancy forces and (4) the well known thermal Grashof number. Entropy generation takes a constant minimum value for all K T values at β = 0° and 180° for respectively, opposite, and cooperative buoyancy forces. The inclination angle for which entropy generation is maximum, is constant for all K T values. For a fixed inclination angle, Soret effect tends always to increase the entropy generation. More details of the influence of Soret effect on entropy generation for the special case when N = -1 is discussed. Keywords : double diffusive convection, entropy generation, inclined enclosure, Soret effect thermodynamics of irreversible processes. Entropy generation optimization in double diffusive convection considering Soret effect M. Maghrebi*, N. Hidouri and A. Ben Brahim M. Maghrebi*, N. Hidouri and A. Ben Brahim Influence of Dufour effect on entropy generation in double diffusive convection Abstract: The influence of Dufour effect on entropy generation in double diffusive convection is numerically investigated in this paper. The dimensionless total entropy generation is evaluated as a function of : - The buoyancy ratio N, a parameter characterising the solutal buoyancy forces by report to the thermal ones ; - The Dufour Parameter K F, a parameter characterising the influence of the diffision- thermo effect ; - The thermal Grashof number. More details of the investigation is focused for the special case when N = -1. Abstract: The influence of Dufour effect on entropy generation in double diffusive convection is numerically investigated in this paper. The dimensionless total entropy generation is evaluated as a function of : - The buoyancy ratio N, a parameter characterising the solutal buoyancy forces by report to the thermal ones ; - The Dufour Parameter K F, a parameter characterising the influence of the diffision- thermo effect ; - The thermal Grashof number. More details of the investigation is focused for the special case when N = -1. M. Magherbi * Civil Engineering Department, High Institute of Applied Sciences and Technology, Avenue of Omar Ibn El Khattab, Gabès 6029, Tunisia Fax: *Corresponding author M. Magherbi * Civil Engineering Department, High Institute of Applied Sciences and Technology, Avenue of Omar Ibn El Khattab, Gabès 6029, Tunisia Fax: *Corresponding author H. Abbassi Sciences Faculty of Sfax, Department of Physics, P.B. 802, Sfax 3018, Tunisia N. Hidouri Chemical and Process Engineering Department, Engineers National School of Gabès, Avenue of Omar Ibn El Khattab, Gabès 6029, Tunisia N. Hidouri Chemical and Process Engineering Department, Engineers National School of Gabès, Avenue of Omar Ibn El Khattab, Gabès 6029, Tunisia A. Ben Brahim Chemical and Process Engineering Department, Engineers National School of Gabès, Avenue of Omar Ibn El Khattab, Gabès 6029, Tunisia Entropy generation in double diffusive convection in presence of the Soret effect Abstract: Influence of the Soret effect on entropy generation in a square cavity filled with a binary gas mixture in double diffusive convection is numerically studied. It was found that entropy generation increases with the increase of thermal diffusion and buoyancy ratios for fixed thermal Grashof number. Total irreversibility takes a minimum value at a special buoyancy ratio ranging between -1 and 0. Soret effect causes the increase of total irreversibility by more than 50 % at higher values of thermal Grashof number and buoyancy ratio. More details of the irreversibility localisation are discussed. Keywords : cavity; heat and mass transfer; entropy generation ; irreversible processes; numerical methods; Soret effect. N. Hidouri Engineers National School of Gabès, Gabès University, Avenue of Omar Ibn El Khattab, 6029 Gabès, Tunisia Fax: H. Abbassi Faculty of Sciences, Sfax University, 3018 Sfax, Tunisia M. Magherbi* High Institute of Applied Sciences and Technology, Gabès University, Avenue of Omar Ibn El Khattab, 6029 Gabès, Tunisia Fax: *Corresponding author A. Ben Brahim Engineers National School of Gabès, Gabès University, Avenue of Omar Ibn El Khattab, 6029 Gabès, Tunisia Fax: Second law analysis in convective heat and mass transfer Abstract : This paper reports the numerical determination of the entropy generation due to heat transfer. mass transfer and fluid friction in steady state for laminar double diffusive convection. In an inclined enclosure with heat and mass diffusive walls. by solving numerically the mass, momentum. species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, die thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number. Keywords : Heat transfer. Mass transfer. Cavity. Entropy generation Abstract : This paper reports the numerical determination of the entropy generation due to heat transfer. mass transfer and fluid friction in steady state for laminar double diffusive convection. In an inclined enclosure with heat and mass diffusive walls. by solving numerically the mass, momentum. species conservation and energy balance equations, using a Control Volume Finite-Element Method. The influences of the inclination angle, die thermal Grashof number and the buoyancy ratio on total entropy generation were investigated. The irreversibilities localization due to heat transfer, mass transfer and fluid friction is discussed for three inclination angles at a fixed thermal Grashof number. Keywords : Heat transfer. Mass transfer. Cavity. Entropy generation Magherbi Mourad 1, Abbassi Hassen 2, Hidouri Nejib 3, Ben Brahim Ammar 4 Cooling loads analysis of an endoreversible solar absorption refrigerator cycle Abstract : This paper deals with the analysis of the Cooling Loads (CL) of a solar absorption refrigerator. For this purpose, a new Process Conceptual Design (PCD) approach based on equivalent endoreversible model and hierarchical decomposition is developed. The influences of many cooling load aspects on the model performances are investigated. A sensitive study is made : performance coefficient (i.e., its inverse) vs, different cooling load aspects. New relationships between technical and economical criteria and optimal parameters stand out. An endoreversible new bound for the Inverse Specific Cooling Load (ISCL) is achieved. For every case, functional and practical domains are defined. Academic interest domains are also notified. Keywords : absorption cycle; conceptual design; cooling load; endoreversible; hierarchical decomposition. Abstract : This paper deals with the analysis of the Cooling Loads (CL) of a solar absorption refrigerator. For this purpose, a new Process Conceptual Design (PCD) approach based on equivalent endoreversible model and hierarchical decomposition is developed. The influences of many cooling load aspects on the model performances are investigated. A sensitive study is made : performance coefficient (i.e., its inverse) vs, different cooling load aspects. New relationships between technical and economical criteria and optimal parameters stand out. An endoreversible new bound for the Inverse Specific Cooling Load (ISCL) is achieved. For every case, functional and practical domains are defined. Academic interest domains are also notified. Keywords : absorption cycle; conceptual design; cooling load; endoreversible; hierarchical decomposition.


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