Measurement limits and performance analysis of multilayer insulation under vacuum and cryogenic temperatures using a modified calorimeter-bar setup

High-performance cryogenic electric machines, such as superconducting motors, require exceptional thermal insulation to minimize heat leakage and maintain operational temperatures below 50 K. Multilayer insulation (MLI) is widely used in such systems due to its low thermal conductance, but its performance is highly sensitive to contact pressure, vacuum level, and structural configuration—making accurate experimental characterization essential. This thesis presents the design, development, and implementation of a modified ASTM D5470-based one-dimensional (1D) calorimeter bar apparatus to measure the through-thickness thermal conductance of MLI under vacuum at both ambient and cryogenic temperatures. The apparatus integrates mirror-polished metal bars, high-precision temperature sensing, and a high-vacuum chamber to enable controlled steady-state heat flow through MLI samples. Cryogenic conditions were achieved using both liquid nitrogen and a cryocooler-thermal strap system. Experimental results at ambient temperatures showed that stainless steel bars provided improved measurement reliability due to steeper axial temperature gradients. Tests at cryogenic temperatures demonstrated clear trends in conductance with respect to layer count, compression, and shielding, but failed to reach the ultra-low conductance values theoretically expected of high-performance MLI. Analytical modeling and uncertainty analysis revealed that lateral radiative losses and shallow temperature gradients led to significant overestimation of true conductance at low flux levels. The results indicate that while the adapted calorimeter bar method is suitable for moderate conductance characterization, its current configuration imposes a practical resolution limit near 1 W/m²·K. This study establishes both the capabilities and limitations of the ASTM D5470 method for cryogenic MLI evaluation and informs future designs for ultra-low conductance measurement systems.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Joshi, Saptarshi Yogesh

Contributors dc:contributor

• Miljkovic, Nenad

• Multilayer Insulation
• Thermal Characterization
• Cryogenics
• Thermal Conductivity Measurement

Statement dc:rights

• Copyright 2025 Saptarshi Joshi

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/130059

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F130059