Understanding thermal and mechanical behaviors and their inter-dependencies of complex tank systems is crucial to making proper design decisions. Low-maintenance, high-performance systems are becoming more important as global energy demands and efficiency requirements increase.
A new apparatus and method were constructed for the purpose of testing cryogenic tanks under real-world conditions, including mechanical vibration and thermal cycling. Prior test equipment and methods have been limited to testing only specific or individual physical and environmental attributes. A cryogenic tank is a complex system of thermal and mechanical interactions. A full and adequate understanding of the performance of such tanks cannot therefore be obtained through separate tests.
The new device and method includes mechanical vibration, thermal cycling, thermal heat-leak performance, and vacuum retention, all in one test apparatus and methodology. It allows for testing and evaluation of both thermal and mechanical performance of cryogenic tanks. The thermal performance includes heat leakage rate by cryogen boil-off (weight scale and/or flow meter). The mechanical performance includes vibration (related to settling) and thermal cycling (related to compaction). All of these factors work together to produce the overall system tank performance. This new technology allows all these factors to be reliably and repeatedly tested in concert together or separately as required by the design requirements.
The vibration and thermal cycling effects on different bulk-fill insulation materials were investigated. Compared to perlite powder for the high-vacuum tests, the heat leak rate is 37% less for glass bubbles and 47% more for aerogel beads. Both glass bubbles and aerogel beads were found to have a consistent and repeatable bulk density. The main feature among the compaction level trends is that the glass bubbles settle out at around the original fill level after a few cycles, whereas the perlite powder continues to settle with each additional cycle. Aerogel beads were found to attain their final level after only a few thermal/vibration cycles, whereas the perlite powder was seen to be slowly compacting more and more as the cycles progressed. Glass bubbles have the lowest thermal conductivity in a high-vacuum environment and offer advantages in ease of handling and consistent density. The glass bubbles insulation system has been proven for vacuum-jacketed cryogenic tanks in a number of field demonstrations.
An additional set of thermal performance tests was performed under the novacuum (ambient pressure) condition. Compared to perlite powder in the no-vacuum tests, the heat leak rate through the glass bubbles is 24% less, while the heat leak rate through the aerogel beads is 49% less. Aerogel beads have the lowest thermal conductivity under ambient pressure and, with advantages over perlite in ease of handling and minimal mechanical settling, can be considered for double-wall, non-vacuum jacketed tanks or cold-box applications.
This work was done by James Fesmire of Kennedy Space Center; and Zoltan Nagy and Stanislaw Augustynowicz of Sierra Lobo, Inc. KSC-13046