Hydrogen has long been considered a possible route to reducing carbon dioxide emissions from aviation, but storing fuel at temperatures below −250°C creates some unusual engineering problems. One of them happens inside the metal itself.
Researchers at the University of Oxford have developed a model that could help engineers predict how hydrogen atoms move through metallic aircraft components when different parts of the material are at different temperatures.
Hydrogen molecules can split apart at a metal surface, allowing individual atoms to enter the material. Even in very small concentrations, they can alter its mechanical behaviour and contribute to hydrogen embrittlement, where a metal becomes more susceptible to damage and loses mechanical strength.
The problem is particularly relevant to aircraft using liquid hydrogen. Before reaching an engine, the extremely cold fuel must be converted into a gas, creating substantial temperature differences within components such as heat exchangers.
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The Oxford team has been investigating thermomigration – the movement of hydrogen in response to those temperature gradients. Its model suggests that hydrogen movement is influenced partly by changes in electrostatic fields and vibrational energy within the material, but also by an ‘electron-wind’ effect. As heat-carrying electrons move down a temperature gradient, they interact with mobile hydrogen atoms and influence their movement through the metal lattice.
Understanding where those atoms are likely to accumulate could help engineers assess which parts of a component are most vulnerable over its working life.
The researchers have incorporated the findings into a numerical framework for predicting hydrogen uptake and retention in metallic components. The work, carried out with Rolls-Royce, could eventually feed into assessments of components intended for hydrogen-powered aircraft.
‘It will not be possible to do this with empirical testing alone – accurate modelling of hydrogen transport and embrittlement will be essential,’ says Rolls-Royce materials specialist Louise Gale.
Oxford’s Professor Felix Hofmann says experimental measurements are also being developed to test and refine the theoretical model. It is another reminder that making hydrogen-powered flight practical will involve more than replacing one fuel with another. Engineers also need to understand how that fuel interacts with the aircraft around it.
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The research is published in the journal Physical Review Letters.