Combined experimental and numerical investigation on the radiative properties of ceramic open-cell foams

• Determination of radiative properties of ceramic open-cell foams. • Parametric study on material and geometric properties. • Experimental validation of parameter identification. Since ceramic open-cell foams are mostly utilized in high-temperature applications, investigation of their thermophysical properties, regarding radiative heat transfer, is crucial. This study comprises a comprehensive characterization of the radiative properties of various ceramic foams, considering their specific scattering behavior. Spectroscopic measurements performed with a Fourier-transform infrared spectrometer are combined with a numerical parameter identification procedure to establish extinction coefficients and scattering albedos using appropriate scattering phase functions. Fundamental differences in the radiation behavior of the foams are demonstrated, depending on whether the struts behave as semi-transparent or opaque. A parametric study revealed the influence of several material and structural parameters on the spectral or temperature-dependent extinction coefficients. At room temperature, when all ceramics behave opaquely, extinction coefficients are affected by the surface reflectivity, but mainly by the geometric properties of the foams (porosity, pore size). In contrast to carbon-containing foams with nearly constant radiative properties, extinction coefficients of foams made of oxidic ceramics show a nearly linear increase with increasing temperature. Validation is achieved by comparing model predictions, using Rosseland diffusion approximation and identified radiative properties, with measurement results at up to 700 °C of the effective thermal conductivity obtained from the transient plane source method (Hot Disk). Deviations largely amount to ±10 % for a pure alumina, as well as differently coated foams, when considering appropriate sample thickness and anisotropic scattering. Besides confirming the reasonability of the identified radiative properties, the suitability of both measurement devices and the simplified modelling procedure for ceramic open-cell foams is thus demonstrated.