A Comparative Fluxmetric (CFM) Method for Apparent Thermal Conductivity Measurement of Insulating Materials at High Temperature

Abstract

This article presents a simple thermal characterization method, noted CFM, for the measurement of the apparent thermal conductivity of insulating materials at high temperature (i.e., up to 600°C). The CFM method is a steady-state relative measurement method which requires a calibration. The calibration of the experimental apparatus was done with a calcium silicate board of known thermal conductivity. Thermal conductivity measurements were carried out on a low-density compressible fibrous felt and a high-density calcium silicate board between 100 and 600°C. A good agreement was observed with the values obtained with the guarded hot-plate (GHP) method for the low-density fibrous felt and the parallel hot-wire (PHW) method for the high-density calcium silicate board. The measurement of the apparent thermal conductivities of low-density fibrous felts of different apparent densities, combined with a simple conducto-radiative model, allowed to estimate a mean specific extinction coefficient in good agreement with a value derived from transmittance/reflectance measurements. Keywords Insulating material. High temperature. Thermal conductivity 1 Introduction Knowledge of the thermal conductivity of insulating materials at high temperature is of great importance for the control of industrial processes. For example, the cooling rate of a liquid metal in a mold (which has a great influence on the mechanical properties of the molding) is highly dependent on the thermal resistance of the surrounding insulation. Thermal insulation materials are usually porous materials. Their thermal transport properties, at a macroscopic scale, are not intrinsic values since they depend on the local structure of the material (i.e., porosity, tortuosity) [1] and the contributions of several heat transfer modes (i.e., conduction, convection and/or radiation). The term apparent (or effective) thermal conductivity is thus used for porous materials. The characterization of the thermal transport properties (i.e., thermal conductivity and thermal diffusivity) of materials can be performed with a large variety of methods. The choice of the method depends on the following criteria: the nature of the material, the size and shape of the specimen, the temperature range, the thermal conductivity and thermal diffusivity range, the uncertainty of measurement [2].