Jaona Randrianalisoa

Improved Inverse Method for Radiative Characteristics of Closed-Cell Absorbing Porous Media

Radiative characteristics such as the extinction coefficient, the scattering albedo, and the scattering phase function of fused quartz containing closed cells are determined by using an inverse method based on theoretical and experimental bidirectional transmittances. The theoretical transmittances are obtained by solving the radiative transfer equation with the discrete ordinate method. Improvements have been made over previously reported experimental determination of porous fused quartz radiative characteristics by using a more accurate phase function and an adaptive quadrature to compute more precisely the intensities in the measurement directions. In addition, a two-step inverse method to compute accurately and simultaneously the radiative parameters has been developed. The results are shown to be independent of samples thickness. Exhaustive comparison between experimental measurements of hemispherical transmittance and reflectance and computational results using the retrieved radiative characteristics shows good agreement. The retrieved absorption coefficient of porous fused quartz appears to be more realistic than that reported in our earlier publication.

Radiative Transfer in Dispersed Media: Comparison Between Homogeneous Phase and Multiphase Approaches

The radiative transfer in dispersed media in the geometric optic regime is investigated through two continuum-based approaches. The first one is the traditional treatment of dispersed media as continuous and homogeneous systems, referred here as the homogeneous phase approach (HPA). The second approach is based on a separate treatment of the radiative transfer in the continuous and dispersed phases, referred here as the multiphase approach (MPA). The effective radiative properties involved in the framework of the HPA are determined using the recent ray-tracing (RT) method, enabled to overcome the modeling difficulties such as the dependent scattering effects and the misunderstanding of the effective absorption coefficient. The two modeling approaches are compared with the direct Monte Carlo simulation. It is shown that (i) the HPA combined with effective radiative properties, such as those from the RT method, is satisfactory in analyzing the radiative transfer in dispersed media constituting of transparent, semitransparent, or opaque particles. Therefore, the use of more complex continuum models such as the dependence included discrete ordinate method (Singh, B. P., and Kaviany, M., 1992, “Modelling Radiative Heat Transfer in Packed Beds ,” Int. J. Heat Mass Transfer, 35, pp. 1397–1405) is not imperative anymore. (ii) The MPA, though a possible candidate to handle nonequilibrium problems, is suitable if the particle (geometric) backscattering is weak or absent. It is the case, for example, for dispersed media constituted of opaque particles or air bubbles. However, caution should be taken with the MPA when dealing with the radiative transfer in dispersed media constituted of nonopaque particles having refractive indexes greater than that of the continuous host medium.

Modeling radiation characteristics of semitransparent media containing bubbles or particles

Modeling of radiation characteristics of semitransparent media containing particles or bubbles in the independent scattering limit is examined. The existing radiative properties models of a single particle in an absorbing medium using the approaches based on (1) the classical Mie theory neglecting absorption by the matrix, (2) the far field approximation, and (3) the near field approximation are reviewed. Comparison between models and experimental measurements are carried out not only for the radiation characteristics but also for hemispherical transmittance and reflectance of porous fused quartz. Large differences are found among the three models predicting the bubble radiative properties when the matrix is strongly absorbing and/or the bubbles are optically large. However, these disagreements are masked by the matrix absorption during calculation of radiation characteristics of the participating medium. It is shown that all three approaches can be used for radiative transfer calculations in an absorbing matrix containing bubbles.

Extension of the FLASH Method to Semitransparent Polymer Foams

The classical photo-thermal FLASH method is a very practicable method for measurement of the conductive properties of solid materials due to its simplicity, rapidity, and to the limited size of the samples required. It has been applied successfully to a wide variety of materials. However, it is theoretically restricted to purely conductive media. Notably, it could, strictly speaking, not be used to measure the equivalent conductivity of low-density thermal insulators since a significant part of the heat transfer is due to the propagation of thermal radiation. This constitutes a major drawback of the method. Therefore, the present study investigates the possibility to extend the method to this kind of materials by estimating the errors made on the equivalent conductivity when the classical FLASH method is used. To this aim, FLASH experiments have been conducted at different temperatures on several low-density polymer foams whose radiative properties have been estimated from spectrometric measurements. By applying a least-square fit-method associated with a numerical simulation of the 1D coupled heat transfer, we managed to identify the phonic conductivities of the samples and to compute their equivalent conductivities. These values have been compared with the thermal conductivities obtained from classical FLASH method, i.e., assuming that the thermal transfer occurs only by heat conduction. It appears that the discrepancies between the conductivities stemming from the classical FLASH method and the equivalent conductivities computed are quite negligible at ambient temperature even for foams with very low densities. This demonstrates the applicability of the classical FLASH method to this type of materials for building applications. This conclusion is likely to interest foam manufacturers in view of reducing the time required for an accurate measurement of the insulating performances. On the other hand, at elevated temperatures, the errors become significant so that the method could not be considered satisfactory. [DOI: 10.1115/1.4004392]

Independent and Dependent Scattering for Semitransparent Media Containing Bubbles

The objective of this present work is to provide a new approach for the radiative characteristics computation of semitransparent media containing spherical bubbles. The bubble size is considred much larger than the wavelength. First, previous models from the literature based on the independent theory are reviewed and established in the Geometric optic limit. Second, a predictive model using the Monte Carlo method is developed. The results obtained from the independent theory models and the Monte Carlo approach are compared. In addition, by varying the bubbles volume fraction, we investigate the limit of validity of the independent theory in such medium.Copyright