A device to measure bulk effective thermal conductivity of powder insulation at cryogenic temperatures has been designed and tested. The design consists of two concentric cylinders which are suspended inside of a cryostat. The insulation being tested is filled into the annular space between the cylinders. In the first test method the insulation space is evacuated, and the cryostat is filled with LN2 until the outer cylinder is completely submerged. The ensuing temperature drop of the inner cylinder over time is recorded. The rate of temperature decrease is used to calculate the bulk effective thermal conductivity of the insulation. For the second test method, a Gifford-McMahon single stage cryocooler is used to cool the apparatus to a desired temperature range. The insulation is evacuated, and a heater mounted on the inside cylinder creates a temperature difference between the two cylinders. Fourier’s law of heat conduction is used to relate that temperature difference to a bulk effective thermal conductivity of the powder insulation.
The insulation specimen used for this experiment was aerogel bead insulation. The first test method covered an average temperature range of 85-130 Kelvin. The calculated bulk effective thermal conductivity from the first method for this range increases from 0.333 to 0.714 mW/m*K. Six data points have been produced by the second test method. The average temperature for the second method ranged from 30-80 Kelvin. Over this temperature range the thermal conductivity shows an increase from 0.142 to 0.377 mW/m*K.
Feasibility of use of the concentric cylinder thermal conductivity measurement apparatus as a standard measurement technique is discussed. The implementation of a cryocooler gives the ability to reach lower temperature ranges and create smaller temperature differences than previous research has produced. However, a few difficulties such as electrical problems and an unexpected temperature gradient through the outer cylinder must be resolved prior to further implementation of the apparatus. Plans for further research and development of the apparatus are presented.
