Steady-State Thermal Test Methodology

John F. Kennedy Space Center, Florida

This innovation provides the following thermal properties data from a single steady-state test run: the effective thermal conductivity value (k_e) for the full temperature difference, and multiple thermal conductivity values (λ) for intermediate temperatures. The test specimen (or material) is instrumented with one or more intermediate temperature sensors to allow the calculation of the multiple λ data points within the material and through its thickness. The methodology is particularly effective when coupled with any of the cryogenic boil-off calorimetry instruments (cryostats) developed by the Cryogenics Test Laboratory at NASA-KSC.

The methodology is described by three examples: carbon composite panels testing using Cryostat-500 flat plate, aerogel blanket testing using Cryostat- 400 flat plate, and thin aerogel blanket testing using Cryostat-100 cylindrical. In all three cases, the test specimen consisted of six layers of material stacked together (six stack). The outer surfaces of the stack define the hot and cold boundary temperatures of the system as is normal for steady-state thermal testing principles. In between each layer of material is a temperature sensor. The individual layer thicknesses, as well as the stack thicknesses, are measured so that the position (depth) of each temperature sensor is known. The sensors used in these cases were thermocouples of small gauge wire (30 to 36 gauge, for example). The sensors are placed in the middle portion of the stack, and the lead wires are brought straight out, or spiraled around for additional length, depending on the level of heat transfer associated with a given test specimen. Cryostat boil-off testing is performed per normal in-house laboratory procedures.

When steady-state conditions are achieved, the steady-state heat flow (Watts) is known for the prescribed boundary temperatures and test environment. The layer temperatures are also recorded. From one single test, the heat flux and k_e are calculated (as previously described for cryostats), and large quantity of individual λ points (up to the factorial of one less than the number of sensors — (n–1)!) to generate a plot of λ versus temperature.

The methodology for multiple data points from a single test can be extended to any steady-state thermal measurement instrument. The key factor is knowing the specific in situ location of the intermediate temperature sensors. The sensor placement and lead wire routing has been developed for either flat plate or cylindrical test geometries. The software to perform these calculations is still under development at the time of this reporting. Current EXCEL macros and manipulation provide for plotting and reporting results.

This work was done by James Fesmire and Wesley Johnson of Kennedy Space Center. KSC-13811