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Coquard R.,Societe Etude Conseils Calcul en Mecanique des Structures EC2MS | Rochais D.,CEA Le Ripault | Baillis D.,INSA Lyon | Baillis D.,University Claude Bernard Lyon 1
Fire Technology | Year: 2012

In addition to the multiple actual or possible applications of metal and ceramic foams in various technological fields, their thermal properties make them a good candidate for utilization as fire barriers. Several studies have shown experimentally their exceptional fire retardance due to their low apparent thermal conductivity. However, while the thermal properties of this porous material have been widely studied at ambient temperature and are, at present, well-known, their thermal behaviour at fire temperatures remains relatively unexplored. Indeed, at such temperatures, the major difficulties are not only due to the fact that thermal measurements are rendered fussy since heavy equipments are required but also stem from the fact that a significant part of the heat transfer occurs by thermal radiation which is much more difficult to evaluate than conductive heat transfer. Therefore, the present chapter is written with a view to report progress on the knowledge of heat transfer in open cell foams and to enlighten the reader on the mechanisms of heat transfer at high temperatures. A first part is devoted to the review of the prior published works on the experimental or theoretical characterisations of radiative and conductive heat transfers from ambient to high temperatures. By taking inspiration from the concepts and models presented in these previous works, we propose, in a second part, a model of prediction of the conductive and radiative contributions to heat transfer at fire temperatures. This analytical model is based on numerical simulations applied to real foams and takes into account the structure of the foam and the optical and thermal properties of the constituents. In a third part, we propose an innovative experimental technique of characterization of heat transfer in foams at high temperatures which permit to evaluate independently the radiative and conductive contributions from a unique and simple measurement. The experimental results obtained on several metal and ceramic foams are compared to the results predicted by our numerical model. The good adequacy between experimental and theoretical results show the consistency of both approaches. © 2010 Springer Science+Business Media, LLC.

Coquard R.,Societe Etude Conseils Calcul en Mecanique des Structures EC2MS | Coment E.,Societe NEOTIM | Flasquin G.,Societe NEOTIM | Baillis D.,INSA Lyon
International Journal of Thermal Sciences | Year: 2013

The hot-disk technique is a very practicable transient method of measurement of the thermal properties of solid materials. It has been applied successfully to a wide variety of materials. However, it is based on several approximations regarding the nature of the heat transfer. Notably, the probe is considered thermally neutral, and the heat transfer is assumed purely conductive. These two assumptions are questionable when dealing with low-density thermal insulators. In order to evaluate the accuracy of the method, we have generated numerically noised thermograms reproducing the thermal response that would be recorded when measurements are applied to those type of materials. Thereafter, the best-fitting procedure of the classical hot-disk technique was applied to these thermograms. The analysis of the identification results show that the presence of a radiative contribution do not affect the accuracy of the thermal properties identified. The conductivity measured actually corresponds to the equivalent conductivity. On the other hand, when the method is applied to materials with thermal inertia strongly different from the probe (≈2 order of magnitude lower or more), the accuracy of the method becomes questionable. This is notably the case for common insulators used in the building industry like polymer foam or mineral wools. The preceding conclusions have been validated by experimental measurements on a standard low-density XPS foam sample and a superinsulating silica areogel. © 2012 Elsevier Masson SAS. All rights reserved.

Coquard R.,Societe Etude Conseils Calcul en Mecanique des Structures EC2MS | Thomas M.,Airbus | Estebe B.,Airbus | Baillis D.,University Claude Bernard Lyon 1
Journal of Porous Media | Year: 2012

In the framework of the reduction of the weight of airplanes, porous honeycomb structures are increasingly used in the aircraft industry. They notably enter in the composition of the new generation composite fuselages as thermal insulating shields due to interest in combining, at the same time, high thermal insulating properties, low density, and sufficient mechanical resistance. However, their thermal properties remain relatively unexplored and the number of theoretical and experimental studies concerning the heat transfer through honeycomb structures is very limited. Therefore, the present study is interested in the modeling of the complete heat transfer through this type of porous material. Due to their low density, both conductive and radiative heat transfers have to be taken into account while the contribution of convection can be neglected. The coupled heat transfer is solved by a numerical resolution of the combined energy and radiative transfer equations. The equivalent radiative properties of the material are determined using ray-tracing procedures inside the idealized porous structure while the effective conductivity is estimated via simple, but nonetheless, realistic analytical formulas. The accuracy of the developed model is validated by comparing the heat transfer coefficient measured by different authors for various honeycomb structures with the theoretical results. Thereafter, a parametric study is conducted by varying the structural dimensions and physical properties of the constituents. This permits us to evaluate the contributions of radiative and conductive heat transfers and to highlight the parameters that strongly influence the thermal performance of the insulating shield. © 2012 by Begell House, Inc.

Baillis D.,University Claude Bernard Lyon 1 | Coquard R.,Societe Etude Conseils Calcul en Mecanique des Structures EC2MS | Randrianalisoa J.,University of Reims Champagne Ardenne
Journal of Physics: Conference Series | Year: 2012

Porous Honeycomb Structures present the interest of combining, at the same time, high thermal insulating properties, low density and sufficient mechanical resistance. However, their thermal properties remain relatively unexplored. The aim of this study is the modelling of the combined heat transfer and especially radiative heat transfer through this type of anisotropic porous material. The equivalent radiative properties of the material are determined using ray-tracing procedures inside the honeycomb porous structure. From computational ray-tracing results, simple new analytical relations have been deduced. These useful analytical relations permit to determine radiative properties such as extinction, absorption and scattering coefficients and phase function functions of cell dimensions and optical properties of cell walls. The radiative properties of honeycomb material strongly depend on the direction of propagation. From the radiative properties computed, we have estimated the radiative heat flux passing through slabs of honeycomb core materials submitted to a 1-D temperature difference between a hot and a cold plate. We have compared numerical results obtained from Discrete Ordinate Method with analytical results obtained from Rosseland-Deissler approximation. This approximation is usually used in the case of isotropic materials. We have extended it to anisotropic honeycomb materials. Indeed a mean over incident directions of Rosseland extinction coefficient is proposed. Results tend to show that Rosseland-Deissler extended approximation can be used as a first approximation. Deviation on radiative conductivity obtained from Rosseland-Deissler approximation and from the Discrete Ordinated Method are lower than 6.7% for all the cases studied.

Coquard R.,Societe Etude Conseils Calcul en Mecanique des Structures EC2MS | Rousseau B.,CNRS Nantes Thermocinetique Lab | Echegut P.,French National Center for Scientific Research | Baillis D.,INSA Lyon | And 2 more authors.
International Journal of Heat and Mass Transfer | Year: 2012

Metallic open cell foams are increasingly used in various applications where their thermal properties are of interest. Most of these applications concern relatively high temperatures and thus, radiation propagation is an important mode of heat transfer. Therefore, many studies have already been interested in the prediction of their radiative behaviour. Most of these works used analytical approaches which simplify considerably the porous architecture, notably by neglecting the pore size distribution. Recently, the progress in 3D imaging such as X-ray μ-tomography or Nuclear Magnetic Resonance has led to the development of numerical models which take into account much more realistic representations of the porous structure. However, in these models, strong assumptions are still used to treat the reflection at the solid surface. Moreover, experimental validations are lacking and the authors have never evaluated and highlighted the improvements brought by the use of tomography in comparison with previous analytical models. In the present study, we propose a rather comprehensive modelling of the radiative properties of Al-NiP foam samples using both X-ray tomographies to depict the porous architecture and stereoscopic images based on Scanning Electron Microscopy to deal with the reflection at the solid-fluid interface. The optical properties have been retrieved from literature data. The numerical results are compared with the radiative properties predicted by prior analytical models. Finally, the model is validated by comparison between measured and predicted hemispherical transmittances and reflectances. The agreement is quite satisfactory and demonstrates the superiority of the numerical modelling. © 2011 Elsevier Ltd. All rights reserved.

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