Influence of radiation heat transfer on parallel hot-wire thermal conductivity measurements of semi-transparent materials at high temperature

This paper presents a study of the influence of the radiation transfer on the thermal conductivity measurement in a parallel hot-wire method at high temperature. Simulations of the temperature evolution are first carried out with COMSOL Multiphysics (R) using the coupled conduction-radiation module based on the P1 approximation for the radiation calculation. A wide range of conditions were investigated: a low and a high density insulator were considered with various radiation characteristics (purely scattering, absorbing and scattering, purely absorbing) and with temperature varying up to 1500 degrees C. This study enables the determination of the validity limit of the modeling of the temperature by a purely conductive model with an equivalent conductivity based on the Rosseland approximation. When this assumption is valid, a new estimation process was proposed to improve the estimation accuracy of the thermal properties. A calibration process of the distance between the hot-wire and the thermocouple has also been proposed and validated, enabling a more accurate estimation of the volume heat capacity. An experimental study carried out on three insulating materials with densities ranging from 581 kg m(-3) to 910 kg m(-3) and at temperatures ranging from 20 degrees C to 1200 degrees C confirms the results of the theoretical study. Finally, a method enabling the estimation of the extinction coefficient from thermal conductivity measurements at various temperatures is presented and successfully applied to the three tested materials.