This study focuses on the understanding of the thermal and structural behavior of an innovative Type IV multi-spherical composite-overwrapped pressure vessel through an experimental assessment that consists of hydrostatic testing at ambient conditions and pressure cycling with a cryogenic medium (LN₂). During hydro-burst testing at a high displacement rate, the strain and damage progression is monitored with Digital-Image-Correlation (DIC) and Acoustic Emission (AE) techniques respectively. The effect of filling with LN₂, pressure cycling and draining on the composite overwrap temperature gradient and strain evolution is additionally obtained with Fiber Bragg Gratings (FBGs) and thermocouples. Utilization of AE helped to reveal the different damage mechanisms occurring and enabled the evaluation of the pressure window of the multi-sphere. The experimental measurements in the cryogenic regime verified the suitability of the involved stiffness and coefficient of thermal expansion (CTE) fitting functions developed in [32] that enable to establish of a relationship between strain and temperature during cryogenic chill-down and pressure cycling. This study provides a framework about the suitability of conformal Type IV multi-spherical COPVs for cryogenic storage. ... Read more

In the field of cryogenic storage, the medium inside the pressure vessel is in a liquid state and therefore cannot be further compressed. As a result, the storage tank should be designed in such a way, that it makes the best possible use of the available space (under a minimum weight) where it will be placed (e.g. within a reusable flight vehicle). Unlike conventional cylindrical pressure vessels, conformable pressure vessels provide an effective solution for this application in terms of volumetric and gravimetric efficiency. More specifically, conformable structures in the form of intersecting spheres (multi-sphere)-manufactured from composite materials- would be a beneficial configuration, since they can lead to weight savings associated with equal membrane strains when subjected to uniform pressure. Furthermore, because spheres have the minimum surface area for a given volume, they result in the minimization of passive heat in the tank and fuel boil-off, thus reducing the weight penalty associated with required thermal insulation thickness in cryogenic environments. Therefore a vessel configuration that incorporates partially merged spheres overwrapped with uni-directional (UD) carbon fiber straps applied at the merging points to introduce a uniform strain field would lead to a high volumetric efficiency at a low weight penalty... ... Read more

Understanding of the thermal and mechanical behaviour of conformal tanks when utilized in cryogenic fuel storage is considered crucial in the hypersonic aircraft sector. This behaviour is strongly dependent on the way the tank itself is designed. This study focuses on the effect of design on the performance of an innovative Type IV multi-spherical composite-overwrapped pressure vessel at both ambient and cryogenic conditions. A method to evaluate the required number of reinforcement rings at the intersections and thus avoid damage in those regions under pressurization is outlined. A thermo-mechanical FE-based model coupled with a progressive failure analysis (PFA) algorithm enables to evaluate the pressure window of the multi-sphere at ambient conditions. Additionally, a transient analysis -included in this study-is used to determine the different heat transfer mechanisms, temperature and strain evolution at the tank wall throughout cryogenic operation (chill-down, pressure cycling and purging). The temperature dependency of the tank wall materials is obtained by coupon testing and fitting functions and is hereby incorporated in the analysis. The most important outcome here is the absence of damage in the composite overwrap at cryogenic environments; this may be considered as a positive indication about the suitability of the Type IV multi-spherical COPVs for cryogenic storage. ... Read more

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. ... Read more

This paper outlines the structural performance of a conformable pressurizable tank consisting of intersecting spherical shells (multi-cell tank). Multi-cell tanks outrival conventional multiple cylindrical tanks in volumetric efficiency when required to fit in a rectangular envelope in the automotive industry. When pressurized, the multi-cell (or multi-bubble) tank experiences high stress concentrations at the vicinity of the junctions, and thus the concept of effectively reinforcing those regions without adding significant excess weight becomes crucial. Furthermore, when applied for cryogenic medium storage, the heat transfer between different bodies and the generation of respective thermal stresses in such vessels makes the solution more complicated. In this paper the effect of the i) fiber-reinforced materials at the membrane and ii) unidimensional carbon tows at the intersections on the structural integrity of the tank is analysed for different loading scenarios. An operating window for the proposed tank configuration under the given loading scenario is established indicating the safe zone where the tank can operate. ... Read more