Abstract
Due to the environmental concerns, strict regulations and requirements enforced by legislative bodies, the aviation industry is actively looking to undertake a transition from kerosene fuelled aircraft to zero-emission aircraft. Numerous options have been evaluated to identify a suitable candidate to replace kerosene. Hydrogen was identified as a potential candidate to replace and assist kerosene in future aircraft powered by cryogenic propulsion systems. Hydrogen is an abundant element which is found naturally in gas form. Gaseous hydrogen (at 700 Bar and atmospheric temperature) has around three times higher gravimetric energy density compared to kerosene (at atmospheric conditions) [1] [2] [3]. However, the volumetric energy density of gaseous hydrogen (at 700 Bar and atmospheric temperature) is lower than kerosene (at atmospheric conditions) [1] [2] [3]. To overcome this, hydrogen must be stored as a liquid at 20 K. In liquid form (at atmospheric conditions), the volumetric energy density of hydrogen is around four times lower than kerosene (at atmospheric conditions) [1] [2] [3]. This means that compared to kerosene, to travel the same distance with a hydrogen fuelled aircraft, around four times larger storage tanks are required. The storage tanks must also be compatible to operate under cryogenic conditions. Moreover, sufficient insulation is required to prevent any heat entering the storage tanks leading to evaporation and loss of hydrogen. Finally, the tanks must be designed to prevent components apart from the fuel system encountering with hydrogen.This leads to two main problems which are weight and complexity of cryogenic propulsion systems within aircraft. The larger tanks and the additional insulative measures used, increase the overall weight of the aircraft reducing the maximum take-off weight and profitability. Secondly, all the fuel system components must be compatible and operational with the extremely cold conditions hydrogen is stored in. To overcome these two issues, novel propulsion architectures are proposed consisting of innovative materials. However, most of these materials have not been tested under cryogenic conditions. Hence the effects of long-term cryogenic exposure in addition to thermal cycling is not known.
For investigating these two subjects, a practical testing approach was selected, and two novel testing rigs were designed from scratch. The cryogenic fluid flow test rig was designed to get a novel understanding of the heat requirement to maintain Leidenfrost self-propulsion in a pipe. The mechanical testing cryostat was designed to provide the ability to test carbon fibre composites material and pipes at low temperatures to enable next generation aerospace fuel systems.
The cryogenic fluid flow rig replicated an idealised fuel pipe where waste heat from the propulsion system was used to maintain self-propelled cryogenic flow without any external pumping, by harvesting the Leidenfrost effect. This rig used liquid nitrogen as a working fluid. The flow was maintained using flow restrictors between 1.5-3.5 mm in diameter. The liquid nitrogen was directed into the test section using gravity which was monitored using thermocouples at the inlet and outlet. Copper coil wrapped around the pipe section was used to apply external heat replicating the hypothesised cryogenic fuel delivery system. The results showed that the heat applied and the flow restrictors used were not effective in maintaining the Leidenfrost regime. The type of flow was highly dependent on the restrictor size being used but the relationship between two factors were not linear. The reduction in the restrictor size affected the flow regime.
The mechanical testing cryostat must be able to reach below 20 K for hydrogen applications, while undertaking mechanical tests. The design utilised a double vessel (outer shell and sample space) principle where, two separate layers of vacuum were incorporated for insulation to overcome the additional heat load introduced due to the rigid connections established for mechanical testing. The vacuum between the outer shell and the sample space was conserved even if the sample space was accessed due to the inverted flange design on the outer shell. The system was cooled using a Gifford-McMahon (G-M) cryocooler.
A novel sliding seal system was used for the insertion and removal of the sample holder assembly to minimise the entry of the heat into the system and to avoid loss of vacuum. The sample holder assembly was equipped with a slotted design to prevent pre-loading of the sample prior to testing. In addition to this, the novel slotted design enabled the sample to be removed even after failure during mechanical testing. The design of the sample holder was modular to allow for different sample holders and jigs to be attached for various types of tests.
To secure the sample holder assembly in position, a novel threaded locking mechanism was used. A similar approach was used to attach the cryocooler onto the copper block. The threaded locking mechanism maximised the thermal contact and enhanced heat transfer between the cryocooler and the sample. Additionally, the threaded locking mechanism was designed such that when fully tightened, the viewing window on the sample holder was aligned to the viewing windows located on the outer shell and the sample space. This provided visual access to the sample for observation and measurements. The mechanical testing cryostat was commissioned successfully and was proven to reach below 20 K under high vacuum numerous times. Additionally, both mechanical and exposure tests were successfully completed at around 25 K using 3-point bending sample holders for a carbon fibre reinforced plastic (CFRP) sample.
| Date of Award | 10 Dec 2025 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Andrew Rhead (Supervisor) & Kei Takashina (Supervisor) |