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Colloque avec actes et comité de lecture. Internationale.; International audience; Une combinaison de cinq modèles simplifiés de conductivité effective thermique (modèles série, parallèles, Hashin-Shtrikman, Théorie de milieu effectif, et réciprocité) a été disposée en types de moyenne arithmétique et géométrique pour calculer les moyennes pondérées. Basées sur l'arrangement des bornes génériques minimales et maximales, il a été démontré que d'excellentes prédictions peuvent être estimées en types de moyennes arithmétiques ou géométriques. Diverses corrélations de conductivité thermique effective de mousses à cellules ouvertes pour des fluides arbitraires ainsi que pour des matériaux solides de faible à grande conductivité ( =10-3 ; 0) sur une large gamme de porosité (0.600.95) ont été proposées. Un arrangement des plus simples modèles série et parallèle pour les types de moyenne arithmétiques et géométriques a été trouvé pour prédire les meilleurs résultats parmi toutes les combinaisons génériques.

Colloque avec actes et comité de lecture. Internationale.; International audience; This work concerns the flow boiling pressure drop inside a channel filled with metallic foam soldered to its lateral walls. Notice that Open cell metallic foams are increasingly used in the heat and mass exchangers. This is due to the considerable opportunity that this material could offer in terms of hydrodynamic characteristics namely porosity and permeability which are larger than 0.88 and 10-7 m2, respectively. The state of the art shows that the works in this field are still in their early stages. In this study the tested metallic foam is from copper with 97% porosity and 36 PPI (Pore per Inch Linear). The working fluid is n-pentane. The independent variables are: the mass flux in the range from 10 to 100 kg/m2s and heating power which varies between 0 and 25 W/cm2. The dependent variables are: the static pressure and exit quality. The hydrodynamics results represented in terms of pressure drop are qualitatively similar to those found for plain tube. The two-phase friction multiplier is used to compare results of the present work with those in the literature. Hence results are compared to correlations in the literature for metal porous media. As metallic foam occupies only 5% of the channel entire volume, the obtained results are also compared to those given by the homogeneous and separated models for two-phase flow in plain tube. The results concerning pressure drop through metallic foam found in this work are in good agreement with the homogeneous model with a discrepancy of about 25%, for the other models this discrepancy is upper to 50%.

A transient model of the cathode catalyst layer of a proton exchange membrane fuel cell is presented. The catalyst layer structure can be described as a superposition of the polymer membrane, the backing layer, and some additional platinum particles. The model, which incorporates some of the features of the pseudo-homogeneous models currently present in the literature, considers the kinetics of the electrochemical reaction taking place at the platinum surface, the proton transport through the polymer agglomerates, and the oxygen and water transport within the pores as well as the membrane material of the catalyst layer. Due to the lower porosity of this region and the higher liquid water content, the catalyst layer can be current limiting in the fuel cell. Furthermore, since the cost of the catalyst material is critical, it is important to have a model predicting the effective utilization of this catalyst layer as well as one, which gives insights into how it might be improved. Equations are presented for the mass conservation of reactants and products, the electrical and ionic currents, and the conservation of energy. A discussion of a number of the closure relations such as the Butler-Volmer equation employed is included as is a discussion of the initial and boundary conditions applied. The mathematical model is solved using a finite elements approach developed at the I.U.S.T.I. Master of Science