| Popis: |
Hydrogen is currently considered as a promising alternative fuel for the future. In transport, hydrogen seems to be very promising for greenhouse gas reduction achievement, provided that the transport segment comprises about one-third of the total CO2 emissions in the European Union. Specifically for the aviation sector, hydrogen when produced carbon-free, presents several advantages, as it allows for the elimination of CO2 emissions in flight. Its usage in fuel cells allows for zero-emission propulsion, including NOx and particles. When burnt in a turbine engine, very low particle emissions can be expected, as well as reduced NOx emissions, provided that the combustion system is optimised. The significant advantage of liquid hydrogen as an aircraft fuel lies on the high specific heat of combustion, as it has a gravimetric energy density of 33.3 kWh/kg compared to kerosene having 12 kWh/kg, i.e. hydrogen is about 2.8 times more efficient compared to conventional jet fuel. A typical investigation of the feasibility of liquid hydrogen as aviation fuel is, among others, the Cryoplane project [1]. However, the mass advantage of the liquid hydrogen fuel may result in a mass disadvantage for the entire fuel system, if the liquid hydrogen tank and its insulation mass are not minimised. Therefore, a big challenge lies in the design and manufacturing a tank that meets the mass requirements, while insulating the cryogenic liquid hydrogen well enough to prevent excessive heat leak and boil off. Provided that key elements of the liquid hydrogen aircraft technology are still at very low Technology Readiness Level, advanced parametric modelling is required to support an optimised tank design. In this direction, the present work refers to the development of a detailed thermo-mechanical simulation model of a small-scale liquid hydrogen fuel tank for aviation applications. A tank of a typical capacity equal to 150 kg of liquid hydrogen at 2 bar design pressure, 10 bar design overpressure and 20K design temperature is investigated. Two different tank geometry variations are considered, namely a spherical tank and a cylindrical one with hemispherical heads, assuming a mean density of the liquid hydrogen equal to 70 kg/m3. The liquid hydrogen tank insulation must meet conflicting requirements, as it has to demonstrate very low heat losses in order to meet the boil off requirements, while at the same time to be a light-weight system, fulfilling the tank mass requirements. Taking into account the low operating pressure, aluminium alloy materials are used in the tank construction, as composites could lead to marginal mass reductions, at a high-cost penalty. Such a selection is in line with NASA studies e.g. [2], suggesting that aluminium alloy 2219 fulfils all requirements of a LH2 tank design. For the insulation system a spray on foam based on polyurethane materials is currently considered. The developed thermo-mechanical simulation methodology accounts for all the distinctive and critical parameters regarding the simulation of cryogenic metallic hydrogen tanks. The model comprises a thermo-mechanical module to account for temperature history effects on the mechanical response and a structural module to account for thermo-mechanical stress analysis. Modelling approaches combining solid elements with thin-shell element approaches are applied and parametric studies for different tank variations having variable wall thickness of the double-walled aluminium vessel and variable spacing between the walls for introduction of the foam thermal insulation have been successfully performed. The developed model is used to draw conclusions about the structural performance of the tank and its mass efficiency as function of its sizing parameters. |
