Evaluation of Direct Electrical Heating Methods for the Determination of Thermal Conductivity at Elevated Temperatures

摘要

The report discusses the progress on a program for evaluating direct electrical heating methods for high temperature thermal conductivity determinations. A multi-purpose apparatus used in the evaluation of these methods, has generated data simultaneously on thermal conductivity, electrical resistivity, total hemispherical emittance and spectral emittance (0.65 microns) to 1800 K under high vacuum. Problems associated with emittance are extremely important in thermal conductivity determinations at high temperatures using direct heating methods. Great care must be taken to insure that the emittance remains constant over the central portion of the test specimen. The mathematical techniques used to compute thermal conductivity from the experimental data were explored and considerably advanced. These techniques eliminated the need for mathematical approximations which severely limit the region of applicability. One of these techniques was easily extended to the general case which includes three conductivity methods as special cases. Therefore, the need to match certain experimental conditions was eliminated. The inclusion of temperature-dependent physical properties was also readily included in this technique. The mathematical techniques also yielded temperature profiles which could be compared to the experimental profiles. (Author)