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 lambda(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 rho and specific heat c as the anisotropic materials, the thermal conductivity lambda(h) is shown to be equal to rootlambda(x)lambda(y), which is a geometrical mean of the thermal conductivities in the direction of the plane lambda(x) and the thickness lambda(y) of the anisotropic materials. For an alumina silica blanket (rho = 125 kg.m(-3)), the thermal conductivities lambda(h),lambda(x), and lambda(y) were measured in the temperature range between -140 and 300degreesC using the transient hot-wire method for lambda(h) and the cyclic heat method for lambda(x) and lambda(y). In the same way, the thermal conductivities lambda(h), lambda(x), and lambda(y) of a rock wool (rho = 121 kg.m(-3)) insulation were also measured in the temperature range, 100 to 600degreesC. From a comparison of the measured results with the estimated values of lambda(x), it is confirmed that the proposed method can estimate the measured values reasonably well.