Gazing into the flame – DLR and Rolls-Royce advance research into hydrogen combustion

The potential of hydrogen as a fuel of the future is enormous. Its low energy density, low-emission combustion and the freedom it offers from reliance on dwindling fossil fuels make it an enticing solution for commercial aviation. In addition, existing conventional turbine engines can be converted to run on hydrogen or a combination of fuels – a decisive economic factor in the rapid introduction and spread of this technology.

Extensive tests will be carried out before hydrogen fuel technology is launched into the market, to better understand and predict the behaviour of the flame in the combustion chamber of an aircraft engine.

"Society's mandate to us is clear: aviation needs to become climate compatible in the second half of this century," says Florian Herbst, Head of the DLR Institute of Propulsion Technology. "In view of the long product life cycles and high investment costs in aviation, we must therefore bring new propulsion technologies to market maturity within the next ten to 15 years, but we can only reach the required pace of innovation through high-resolution measurement data under real conditions. Our new test set-up, developed through a combination of industrial and scientific excellence, is a world first and will provide exactly those conditions."

Simulating realistic engine conditions

Following a series of tests on individual combustion chamber sections, coupled with a first test in 2023 of a full combustion chamber ring, trials happening this month will test a flight-capable combustion chamber at maximum operating pressure. For the first time, optical measurement technology from endoscopes – long, thin tubes carrying small cameras – are being integrated into a combustion chamber. DLR's High Pressure Combustion Chamber Test Rig 5 (HBK 5) in Cologne can simulate conditions in the combustion chambers of a real aircraft engine, including the extreme pressure and temperatures. Up to five tonnes of hydrogen fuel will flow through the rig during testing.

"The ground-based test is the decisive criterion for assessing the technological maturity of a combustion chamber fuelled with 100 percent hydrogen. Together with the engine manufacturer Rolls-Royce, we have succeeded in setting up a versatile and globally unique test infrastructure at HBK 5," says Christian Fleing, Project Manager and Head of the Combustion Chamber Test department.

Optical measurements through the keyhole

To understand, predict and ultimately control combustion processes, scientists and engineers have traditionally relied on data on pressure, temperature, power output and emissions. The exact behaviour of the flame within the combustion chamber however remained a mystery, particularly when it comes to hydrogen as its flame only emits ultraviolet radiation and is not visible to the naked eye.

Extreme conditions inside the combustion chamber, including pressures of 40 bar and temperatures of 2000 kelvins (around 1726 degrees Celsius), are an additional challenge for Guido Stockhausen and his team from the Engine Measurement Technology department. The endoscope technology specially developed by DLR, with its image guides and essential thermal insulation, records and documents flame behaviour within the chamber, while Rolls-Royce manufactured an additional component behind the combustion chamber ring to ensure that the measurement instrumentation does not affect the flow.

"With this new measurement technology, which is unique under such conditions, we are increasing the knowledge gained from our industrial partner's extremely complex and cost-intensive high-pressure combustion chamber tests," explains Stockhausen. "Previously, hydrogen tests under these conditions had too many unknowns, but now we get an immediate visualisation of hydrogen's UV flame during combustion and can draw the appropriate conclusions. We are particularly proud of the fact that the most important development steps in the realisation of the optical probe technology were all carried out in-house within our department."

Further developments

The current tests are being used to qualify combustion chambers in aircraft engines operating with 100 percent hydrogen fuel. They provide scientists and engineers with a vital database for the further development of climate-friendly aviation engines. "The current hydrogen combustion tests are a major milestone for the CAVENDISH EU Clean Aviation Project led by Rolls-Royce," says Carsten Clemen, the responsible project manager at Rolls-Royce Germany. "The introduction of optical endoscope technology will support the development of new, low-emission combustion chambers, and whether that is with sustainable aviation fuels, hydrogen or a mixture of both, the technology offers unique insights into the combustion process."