Experimental characterization of the coupled conductive and radiative heat transfer in ceramic foams with a flash method at high temperature

Abstract

During recent decades, numerous studies on ceramic foams improved their efficiency and made their use possible in a myriad of engineering applications. Some studies showed that a good understanding of the link between structural parameters (porosity, pore diameter, or tortuosity) and thermal properties could lead to a better system performance. Ceramic foams are often used at high temperatures, where radiation plays an important role. This paper explores, both conductive and radiative heat transfers in such media, using a flash method experiment. The direct model developed is based on the resolution of the Energy Balance Equation (EBE), solved by a Finite Volume Method and the Radiative Transfer Equation (RTE), computed with an Optimized Emission Reciprocity Method (OERM). An inversion procedure made it possible to simultaneously characterize both equivalent conductivity for conduction and equivalent optical thickness for radiation of structured and random foams with different pore size distributions at 800 ∘C. Results showed that the influence of each heat transfer mode can be characterized separately with single experiment and a good agreement was found between the developed model and the experimental measurements. Radiative transfers can be described by a single equivalent, Beerian and gray extinction coefficient dependings strongly on the size of the cells and the organization of the solid matrix. The equivalent conductivity depends on the porosity, the ceramic constituting the solid matrix, the organization of the porous medium and the internal porosity. We estimated the associated equivalent total and radiative conductivities, allowing the quantification of the conduction/radiation ratio, for a wide variety of structures.