Design and Experimental Assessment of a Multi-spherical Composite- Overwrapped Pressure Vessel for Cryogenic Storage

In the field of cryogenic storage, the medium inside the pressure vessel is in a liquid state and thus is incompressible. Therefore the storage tank should be designed in a way, that makes the best possible use of the available space within an aircraft. A composite-overwrapped pressure vessel (COPV) based on intersecting spheres (multi-sphere) provides a volumetrically efficient solution and leads to weight savings, due to reduced hoop stresses and less required thermal insulation. The latter is the result of the minimization of passive heat in the cryogenic liquid, associated with the fact that spheres have the minimum surface area for a given volume. In the present work, a numerical and experimental study of a novel multi-spherical COPV with a plastic liner was performed. A thermo-mechanical model based on Finite Element Analysis (FEA) was developed to assess the effect of cryogenic operation at the structure. The model incorporated the dependency of engineering properties and coefficient of thermal expansion of the composite overwrap and liner materials to temperature, in order to describe the structural response to cryogenic temperatures more accurately. This dependency was determined through using approximation functions based on results from material coupon testing. The temperature profile and strain response of the tank were assessed through thermocouples and Fiber Bragg Gratings (FBGs) respectively throughout the cryogenic chill-down and pressure cycling test. The experimental results verified the accuracy of the involved stiffness and CTE functions and the FE analysis with average offset of 10 [%]. The most important outcome from the study is the absence of damage in the composite overwrap after cryonic chill-down and pressure cycling, which can be regarded as positive indication of the suitability of Type IV multi-spherical COPVs for cryogenic storage applications.