Approximate expressions are obtained for the apparent ther mal conductivity and thermal resistance, in the case of combined conduc tive and radiative heat transfer through low-density insulation. These ex pressions are obtained in the regimes of intermediate and large optical depth, and they depend on only two parameters to be determined by experiment, namely the "conduction" thermal conductivity, kc, and the extinction coefficient, β. An expression is obtained for the apparent ther mal resistance of an insulation consisting of layers with distinct densities. The applicability of these expressions to the interpretation of experimental results and to the improvement of ASTM Test Methods is discussed.
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