Estimation of the Mean Thermal Conductivity of Anisotropic Materials

Abstract

An estimation method of the plane directional thermal conductivity of fibrous insulations using the cyclic heat method and the transient hot-wire method is proposed. By assuming that the thermal conductivity λ _h of anisotropic materials measured by the transient hot-wire method is equivalent to that of the isotropic materials which have the same bulk density ρ and specific heat c as the anisotropic materials, the thermal conductivity λ _h is shown to be equal to $\sqrt {\lambda _x \lambda _y }$, which is a geometrical mean of the thermal conductivities in the direction of the plane λ _x and the thickness λ _y of the anisotropic materials. For an alumina silica blanket (ρ=125 kg·m⁻³), the thermal conductivities λ _h , λ _x , and λ _y were measured in the temperature range between −140 and 300°C using the transient hot-wire method for λ _h and the cyclic heat method for λ _x and λ _y . In the same way, the thermal conductivities λ _h , λ _x , and λ _y of a rock wool (ρ=121 kg·m⁻³) insulation were also measured in the temperature range, 100 to 600°C. From a comparison of the measured results with the estimated values of λ _x , it is confirmed that the proposed method can estimate the measured values reasonably well.