The thermal conductivity of a high-temperature calcium silicate block insulation product was measured in gaseous environments at pressures up to 100 bar at room temperature. The thermal conductivity of the porous material was tested in nitrogen, argon, and carbon dioxide gaseous environments. These tests were performed in a newly-constructed pressure chamber integrated with a thermal conductivity testing device. A standardized testing method was employed in the design of the apparatus. The test method used was based on the ASTM c177, the guarded-hot-plate method [1]. Tests performed in a carbon dioxide pressure medium have produced data with thermal conductivity as a nonlinear function of pressure. The results of tests conducted using nitrogen and argon show that the variations of thermal conductivity of the porous silica insulating material are linear functions of pressure and specific heat (Cv) of the fill gas. Tests performed in a nitrogen gaseous environment have relatively higher thermal conductivity values than thermal conductivity values at corresponding pressures in argon gaseous environment. This trend is attributable to the higher thermophysical property values of nitrogen than those of argon. This observation suggests that the thermophysical properties of the fill gas have significant effect on the effective thermal conductivity of the porous material. Thermal conductivity data collected in both nitrogen and argon pressure media have coefficients of determination (r2) of 0.9955 and 0.9956, respectively. An exponential function fitted to the carbon dioxide data produced a coefficient of determination of 0.9175. A precision study for the newly-constructed steady-state thermal conductivity measuring apparatus was performed in atmospheric air. With a standard deviation of 0.00076 W/m · K and a mean thermal conductivity value of 0.07294 W/m · K, a 95% confidence interval was assumed for a sample space size of 13 for the baseline tests in air. This produced a precision error of ±0.00046 W/m · K (±0.63%), a mean bias error of ±0.00955 W/m · K (±13.09%), and a mean steady-state error of ±1.67%. Hence, the total uncertainty in the mean thermal conductivity value of the baseline tests in atmospheric air could be reported as 0.07294W/m · K ± 13.22% with 95% confidence. The result of the precision study is indicative of the reliability of the apparatus. The single-sample precision uncertainty in thermal conductivity values at varying pressures in the various fill gases were estimated based on the standard deviation of the repeated tests in atmospheric air as 0.001166W/m · K.
Maxwell-type models for the effective thermal conductivity of a porous material with radiative transfer in the voids