Estimation of the Mean Thermal Conductivity of Anisotropic Materials

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 lh of anisotropic materials measured by the transient hot-wire method is equivalent to that of the isotropic materials which have the same bulk density r and specific heat c as the anisotropic materials, the thermal conductivity lh is shown to be equal to `lxly, which is a geometrical mean of the thermal conductivities in the direction of the plane lx and the thickness ly of the anisotropic materials. For an alumina silica blanket (r=125 kg ·m−3), the thermal conductivities lh , lx, and ly were measured in the temperature range between −140 and 300°C using the transient hot-wire method for lh and the cyclic heat method for lx and ly. In the same way, the thermal conductivities lh , lx, and ly of a rock wool (r=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 lx, it is confirmed that the proposed method can estimate the measured values reasonably well.