Différents arrangements des modèles simplifiés pour prédire la conductivité de mousses à cellules ouvertes

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

Experimental analysis of flow boiling pressure drop through copper metallic foam

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%

Different arrangements of simplified models to predict effective thermal conductivity of open-cell foams

International audienceIt is often desirable to predict the effective thermal conductivity (ETC) of a homogenous material like open-cell foams based on its composition, particularly when variations in composition are expected. A combination of five fundamental simplified thermal conductivity bounds and models (Series, Parallel, Hashin-Shtrikman, Effective Medium Theory, and Reciprocity models) is proposed to predict ETC of open-cell foams. Usually

Thermal conductivity correlations of open-cell foams: Extension of Hashin-Shtrikman model and introduction of effective solid phase tortuosity

International audienceOpen-cell foams have emerged as promising materials for use in heat sink and heat exchanger applications. The thermal behavior of open-cell foams depends on their microscopic structure. The effective thermal conductivity of open-cell porous foams can be measured using experimental techniques, predicted from the 3-0 direct numerical simulations on reconstructed foam structures obtained from micro-computed tomography images or derived from idealized structure thermal analysis. Based on the tetrakaidecahedron unit cell and different strut morphologies, three dependent and interdependent empirical correlations for effective thermal conductivity were derived. They encompass all morphological parameters and ratios of constituent phases of foams of different materials. In this process, the Hashin-Shtrikman (HS) bounds model was first extended and applied to the resistor model. A correlation term, Omega was introduced to take into account the thermal conductivities of constituent phases and the morphological parameters of the foam structure. Secondly, a more complex effective model that is a combination of series and parallel models was derived by introducing effective solid phase tortuosity. Lastly, a simple model (KT-model) was derived that can be used to predict either effective thermal conductivity or intrinsic solid phase conductivity depending upon which one of these quantities is known. The present study clearly demonstrates that the proposed empirical correlations yield extremely accurate estimates of the effective thermal conductivity for all the experimental and numerical data of different foam materials reported in the literature. (C) 2015 Elsevier Ltd. All rights reserved

IMPACT OF ANISOTROPY ON GEOMETRICAL AND THERMAL CONDUCTIVITY OF METALLIC FOAM STRUCTURES

15th International Heat Transfer Conference, Kyoto, JAPAN, AUG 10-15, 2014International audienceThe thermo-physical properties of open cell metal foams depend on their microscopic structure. Various virtual ideal periodic isotropic foam samples having circular, square, hexagon, diamond, and star strut cross sections with various orientations are realized in the porosity range from 60 to 95%. The anisotropy of the original foam sample is then realized by elongating in one direction by a factor Omega, while a factor of 1/root Omega is applied along the two perpendicular directions to conserve the porosity of the original sample. A generalized analytical model of geometrical parameters has been proposed and all results are fully compared with the original measured data. Three-dimensional heat conduction numerical simulations at the pore scale have been performed, which allow determining the macroscale physical properties, such as the effective thermal conductivity, using the volume averaging technique. Two analytical models are derived simultaneously in order to predict the intrinsic solid phase conductivity (lambda(s)) and effective thermal conductivity (lambda(eff)). A modified correlation term (F) is introduced in the analytical resistor model to take into account the thermal conductivities of constituent phases and a modified Lemlich model is derived. The analytical results of the effective thermal conductivity are compared with the numerical data and excellent agreement is observed

State-of-the-Art of Pressure Drop in Open-Cell Porous Foams: Review of Experiments and Correlations

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Influence of Morphology on Flow Law Characteristics in Open-Cell Foams: An Overview of Usual Approaches and Correlations

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Simultaneous determination of intrinsic solid phase conductivity and effective thermal conductivity of Kelvin like foams

International audienceThe relationship of geometrical parameters with thermal properties such as effective thermal conductivity is critical for conductive heat transfer in metal foams. We have realized different virtual isotropic structures of tetrakaidecahedron shape having circular, square, hexagon, diamond and star strut shapes with various orientations using CAD modeling. The range of solid to fluid phase conductivity ratios (lambda(s)/lambda(f)) studied is from 10 to 30,000 for porosity range 60-95%. At pore scale, 3-D direct numerical simulations were performed to calculate effective thermal conductivity, lambda(eff) in local thermal equilibrium condition. A database of 2000 values of effective thermal conductivity is generated. Two models are derived in order to predict simultaneously intrinsic solid phase thermal conductivity, lambda(s) and effective thermal conductivity, lambda(eff). A modified correlation factor, F is introduced in analytical resistor model in order to take thermal conductivities of constituent phases into account and a new PF model based on Lemlich model is derived. An excellent agreement has been observed between the predicted results against numerical and experimental data. We have also shown the importance of intrinsic solid phase thermal conductivity (lambda(s)) in determining effective thermal conductivity (lambda(eff)). (C) 2014 Elsevier Ltd. All rights reserved

Investigation of fluid flow properties in open cell foams: Darcy and weak inertia regimes

International audienceKnowledge of flow properties induced by open cell foams is essential to the successful design and operation of high performance industrial systems. Flow properties are strongly dependent on the geometry of foams, but these relationships are not still fully characterized. This paper investigates the impact on flow properties of the geometrical characteristics of metal foams in the porosity range of 80-95%. An analytical model is proposed to characterize the foam matrix fully. 3-D direct numerical simulations at pore scale have been performed, and the Navier-Stokes equation has been solved for various Reynolds numbers (10(-5) < Re < 500) in the fluid space. We determine permeability in the Darcy regime and distinguish it from permeability obtained in the inertia regime (Forchheimer Equation). The inertia coefficient is determined using both the Darcy and inertia regimes. The two approaches are fully discussed and the critical importance of the permeability of the Darcy regime to the friction factor is shown. We further derive an analytical solution determining fluid flow characteristics in order to incorporate the resulting macroscopic pressure and velocity gradients in the Ergun-like equation. The analytical results are thoroughly compared with the numerical and experimental data and an excellent agreement is observed. (C) 2014 Elsevier Ltd. All rights reserved

Experimental Analysis of Transport Phenomena During the Convective Drying in Superheated Steam

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