Development of a test apparatus for thermal conductivity measurements of insulation materials with adsorbed nitrogen between 20 and 77 K

Abstract

Understanding the effective thermal conductivity of insulations utilized in vacuum-jacketed liquid hydrogen vessels is critical to predicting heat leak and boil-off characteristics. More specifically, the ability to predict the transient heat flux during a loss of vacuum event is a critical metric that will impact the design of safety systems and hold time of these tanks. There is a lack of measurements at liquid hydrogen boundary temperatures due to the majority of insulation measurements occurring at liquid nitrogen cold boundary temperatures (77 K and above). Boil-off calorimetry has been a typical method for measuring thermal conductivities but proves challenging for liquid hydrogen temperatures (20 K). In this study, a cryogenic refrigerator enables thermal conductivity measurements of three common insulations (glass microbubbles, aerogel, and multilayer insulation) with an accuracy better than ± 1 mW/m-K at liquid hydrogen boundary temperatures. Comparisons are made to prior measurements. Additionally, the experiment is capable of uniformly introducing gas in the insulation space, simulating a vacuum failure. Transient heat fluxes can be determined from these experiments and utilized to design safety systems.