A Comparison of Two Transient Methods of Measuring Thermal Conductivity of Particulate Samples

Abstract

A comparison is made of the line source (LS) method and the differential line source (DLS) method of measuring thermal conductivity of particulate materials in vacuum. The DLS method requires more instrumentation in the measuring circuitry (an additional amplifier and a differentiating circuit), but since it does not require a stable temperature to initiate a test, it does not need a sample temperature control system. DLS tests can be taken as the temperature in the samples is rising from liquid nitrogen temperature to room temperature. This eliminates the practice of extrapolating thermal conductivity over this large temperature range. Also, the advantages of reduced test time, data reduction time, and small sample temperature rise enable the experimenter to take about 7–12 DLS tests in the time of 2 LS tests. Test data from the two methods agree very well. The DLS method gives slightly lower conductivity values for the small particles tested. The difference between the two methods is smaller at low temperatures than at high temperatures. The agreement in the large particle tests at room temperature is good. An error analysis shows that the LS and the DLS have total relative errors of 18 and 20%, respectively.

Thermal conductivity measurements of particulate materials 2. Results

“…Since "worst" case conditions apply only to the smallest size fractions at the lowest pressures, and to the largest size fractions at the highest pressures, typical maximum instrumental errors for the measurements in this study are in the range 10-15%. This agrees well with the 10% error calculated by Scott et al [1973] for a variation of the line-heat source method.…”

Thermal conductivity measurements of particulate materials: 4. Effect of bulk density for granular particles

“…Typical instrumental errors for the measurements in this study are thus in the range of 5–15%. This measurement error is commensurate to those reported by previous investigators [e.g., Fountain and West , 1970; Cremers and Birkebak , 1971; Scott et al , 1973]. The improvements in the resolution of the DAQ board made collection of the data easier, but did not contribute significantly to the precision in the overall calculation of the thermal conductivity.…”

“…Most of the error is therefore likely to be due to heat loss along the heating wire itself. Scott et al [1973] estimate that the deviations from these assumptions would add another 10% to the uncertainty in the measurements.…”