A rotating detonation engine has a shocking combustion process.
Unlike the conventional propulsion method of burning propellant and pushing fuel through the back of an engine, the rotating detonation engine, made of concentric cylinders, creates a wave of pressure.
With a rotating detonation engine, fuel flows in the gap between the cylinders. Upon ignition, the rapid heat release forms a strong pulse of gas, and the detonation wave travels around the circular channel, or annulus.
The resulting shockwave has a significantly higher pressure and temperature — one that moves faster than the speed of sound.
"These detonations have a mind of their own," said James Koch , a researcher at the University of Washington. "Once you detonate something, it just goes. It's so violent."
By building a series of experimental rotating detonation engines, however, engineers at the University of Washington have found patterns in the detonation behavior. The team, led by Koch, used the test results to develop a mathematical model that describes how the engines work.
The model demonstrated the characteristics of the waves, including speed, oscillations, creation, destruction, and synchronization.
With the wave information, engineers can, for the first time, develop tests to improve and stabilize the often-chaotic RDEs.
The combustion-driven process is literally a detonation — an explosion — but behind this initial start-up phase, Koch and his team saw a number of stable combustion pulses form that continue to consume available propellant.
"This produces high pressure and temperature that drives exhaust out the back of the engine at high speeds, which can generate thrust," said Koch.
Koch's experimental RDE allowed the control of different parameters, such as the size of the gap between the cylinders.
The team then monitored the combustion processes with a high-speed camera. Although each experiment took just a half-second to complete, the researchers recorded the experiments at 240,000 frames per second.
A mathematical model was then created to mimic what was shown in the slow-motion videos. The model enabled the researchers to determine, for the first time, the stability (or instability) of a rotating detonation engine. (Review the team's findings in the Jan. 10 issue of Physical Review E .)
The goal of the mathematical model, according to Koch, was solely to reproduce the behavior of the pulses seem in the high-speed camera videos — to confirm that the model output matched the experimental results.
"I have identified the dominant physics and how they interplay. Now I can take what I've done here and make it quantitative," said Koch. "From there we can talk about how to make a better engine."
In an edited interview with Tech Briefs below, Koch explains why it is so important to understand both the chaos and the order of a rotating detonation engine's behavior.
Tech Briefs: What is different about an RDE, compared to conventional engines?
James Koch: With engines, the natural instinct is to suppress instabilities rather than promote them. The RDE concept is a different way of thinking; instead of suppressing the instabilities, you try to amplify them so they saturate out. Make them grow as fast as strong as they can, and eventually they are going to reach a limit where they can’t grow any further. The RDE is the mechanism, or the physical implementation, of that.
Tech Briefs: What are the advantages of a rotating detonation engine?
James Koch: Fuel efficiency is a big advantage. Second, an RDE is dead-simple to make. I think anyone can put a cylinder inside of a cylinder.
The engine that I designed and built is a cylinder, and then inside that cylinder is another one. They form an annular gap, and that’s the combustion chamber.
Tech Briefs: How were you able to examine different variables through your experimental engine?
James Koch: The first variable that a test engineer has, based on a setup like this, is the amount of propellant that you are pumping into this thing. How fast can you shove in methane and oxygen?
The second variable is the ratio of methane to oxygen. Other variables that we can control are the exit pressure or inlet pressure.
You could be hoping to see one-wave, stable operation with high-performing metrics, but you might end up with 6 or 7 waves that are whirling around in their chamber and completely unstable.
Tech Briefs: What is the camera able to show you?
James Koch: It’s very clear that, even though it’s a very chaotic, violent process, there is some order to this system. Even though there’s so much going on, our end result seems to be a number of these traveling pulses going at certain speeds.
We directly image our combustion chamber. We are watching — slowed down, of course — the actual detonation waves traveling around the combustion chamber. The high-speed videos that are attached to that press release are filmed at 240,000 frames per second.
What that corresponds to are these traveling waves going around our annular combustion chamber. Those are the actual detonation waves going at km per second — super-fast.
Tech Briefs: What was the most interesting discovery through this process?
James Koch: I think that in retrospect the mathematical we put forward is pretty simple.
Our detonation waves exist in this chamber, and they’re all spinning around. As time goes on, the detonation waves synchronize or “mode-lock.” They seem to be communicating with each other in such a way that they all converge to the same behavior. They all mode-lock, or lock into position, and travel around very happily with each other. It’s a nice, ordered unit.
What’s most surprising is that the same phenomenon exists all through science. And it’s not specific to this engine, and it’s not specific to engineering. It’s not even specific to combustion. Fireflies, neurons — this same phenomenon of "mode-locking" exists all over the place. In all of those different systems, that exhibit this excitable behavior, they follow a really similar mathematical structure.
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