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Detonating fuel to create a shockwave and subsequent ‘pulses’ of combustion could offer a fuel-efficient and lightweight form of rocket propulsion – if it was not so unpredictable.

Now researchers from the University of Washington (UW) have developed a mathematical model that describes how the ‘rotating detonation engine’ works, hoping to enable engineers to improve them and make them stable.

“The rotating detonation engine field is still in its infancy. We have tonnes of data about these engines, but we don't understand what is going on,” said lead author James Koch, a UW doctoral student in aeronautics and astronautics. “I tried to recast our results by looking at pattern formations, instead of asking an engineering question, such as how to get the highest-performing engine. Then ‘boom’, it turned out that it works.”

Rotating detonation engines combust propellant in a different way to conventional engines. Propellant flows into the gap between concentric cylinders. After ignition, the rapid heat release forms a shockwave, a strong pulse of gas with significantly higher pressure and temperature that is moving faster than the speed of sound.

“This combustion process is literally a detonation – an explosion – but behind this initial start-up phase we see a number of stable combustion pulses form that continue to consume available propellant,” said Koch. “This produces high pressure and temperature that drives exhaust out the back of the engine at high speeds, which can generate thrust.”

Conventional engines use a lot of machinery to direct and control combustion so that it generates thrust to propel the engine. In a rotating detonation engine, however, the researchers said the shockwave naturally does everything without needing additional help from engine parts.

“The combustion-driven shocks naturally compress the flow as they travel around the combustion chamber,” said Koch. “The downside of that is that these detonations have a mind of their own. Once you detonate something, it just goes. It's so violent.”

The team developed an experimental rotating detonation engine, in which they could control different parameters such as the size of the gap between cylinders. They then recorded the combustion processes with a high-speed camera. Each experiment took only half a second, but the researchers recorded them at 240,000 frames per second so they could see what was happening in slow motion.

From there, the researchers developed a mathematical model to mimic what they saw in the videos. The model allowed them to determine whether engines of this type would be stable or unstable and allowed them to assess performance of specific engines.

Koch now hopes to translate the behavioural results into quantitative information that engineers can use to build better engines.

The research was published in Physical Review E

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