An Optimal Guarding Scheme for Thermal Conductivity Measurement Using a Guarded Cut-Bar Technique, Part II: Guarding Mechanism

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Applied Thermal Engineering





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In measuring thermal conductivity using the guarded cut-bar technique, three guarding schemes have been recommended in literature including isothermal, linear-matched, and continuous-matched. However, no comprehensive evaluation or guidance is given to describe their influence on measurement accuracy. Using finite-element simulations, each of these guarding conditions is analyzed in detail. Depending on system parameters, the recommended guarding schemes are shown to result in systematic errors ranging from 4% to 49% for isothermal, −5% to 17% for linear-matched, and up to 6% for continuous-matched, with potential for greater error outside the parameters used in this study. A newly proposed, optimal guarding scheme is described while providing a comparison of the heat transfer mechanisms behind each of the guarding methods. Various system geometries, including ratios of sample to meter bar length, insulation thickness to sample diameter, and sample aspect ratio, have been studied to verify the validity of the optimal guarding method and show the influence of each parameter on the optimal condition. The practical insignificance of the average temperature difference between guard and sample column is verified and explained showing that the optimal guarding condition is really related to the guard temperature gradient. Direction is given for implementing the optimal guarding scheme to any configuration with the creation of an optimal guarding chart which is a function of sample to meter bar thermal conductivity ratio. The chart is verified experimentally using results from measurements on certified Pyroceram 9606 (<1.3% deviation), stainless steel (<1% deviation), and iron (<3.2% deviation). Following the optimal guarding scheme for the experimental system studied, the temperature gradient ratios between guard and sample column are around 1.8 for Pyroceram, unity for stainless steel and 0.55-0.6 for iron, respectively, at the studied temperatures (also see Part 1 of this work).

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