Physicists find a way to control the detonation wave in a promising new type of engine

Skoltech researchers theoretically predicted synchronization – a kind of self-regulation – in detonation waves. The discovery could help tame this inherently chaotic process to stabilize combustion in a spinning detonation engine. It is an experimental device potentially conserving large amounts of fuel compared to conventional rocket and ship engines. The study appeared in the journal of fluid mechanics.

Detonation is a type of combustion that involves reaction products propagating at supersonic speeds, which in theory would make better use of fuel. Researchers are exploring the concept of the detonation engine with the hope of eventually achieving a 25% increase in efficiency.

“In a rotary detonation engine, one cylinder fits inside another larger cylinder, creating a space between the two into which the combustible mixture must be injected. The mixture continuously explodes, the detonation wave traveling in circles around the smaller cylinder. However, “Due to the chaotic nature of the process, the detonation wave will not behave perfectly evenly from cycle to cycle. The very speed of its propagation is subject to unpredictable oscillations, making the engine unstable,” said the study’s lead researcher and Skoltech associate Professor Aslan Kasimov.

His team discovered a way to tame the detonation wave, by equalizing its oscillations. For this, the researchers deliver the first theoretical demonstration of synchronization in a detonation process.

What is synchronization?

Synchronization was originally discovered as a mechanical phenomenon by Huygens in the 17th century. He observed a pair of pendulum clocks hanging from the same beam and noticed that over time this extremely subtle connection between the clocks caused their pendulums to swing either in phase or out of phase. Since then, synchronization has been discovered in a wide range of fields in chemistry, medicine, biology and even sociology.

“For example, there are fireflies that blink with a certain frequency. When a large number of them gather in one place, they start blinking in sync despite a weak connection: each beetle can only see its nearest neighbors,” the paper’s first author, Skoltech Ph.D. student Andrei Goldin, said before giving more examples.

According to the researcher, a person’s natural biorhythm could have a different 24-hour periodicity, which is clear from experiments placing test subjects in an artificial environment with no night or day. The fact that periodic external stimuli in the form of the daily progression from sunrise to noon to sunset regulate the internal rhythms of humans and other animals to conform to the 24-hour cycle is also a case of synchronization.

A pacemaker is another example of a periodic external stimulus, which in this case regulates the internal oscillations of the heart, overcoming the arrhythmia.

Beyond that, the timing frame has been applied to the moon facing Earth with the same hemisphere at all times and even how the number of serial killer victims varies by date.

In their new paper, Skoltech scientists offer the first-ever demonstration of synchronization to a detonation wave.

Detonation synchronization

The nature of the detonation process is such that even in a perfectly homogeneous medium, a detonation wave propagates “in spurts” – with varying speed. This means that the wave itself is an oscillator analogous to the heart with arrhythmia in the example above. Arrhythmia in this case refers to the unpredictable way in which the speed of the wave oscillates. Recall that this is precisely the problem that makes the detonation engine unstable.

“It turns out that the detonation wave oscillations can be regularized with a periodic external stimulus, but it won’t be a stimulus in the conventional sense. Rather, it’s very regular inhomogeneities in the medium. That is- “that is, in the combustible mixture. Injected into the space between the cylinders of the engine. You can think of these inhomogeneities as a collection of areas – some filled with fuel, some with air – at regular intervals,” Kasimov said. . “By varying the engine design, such as the gaps between adjacent fuel injectors, you can vary the characteristic size of the inhomogeneities encountered by detonation wave propagation.”

Skoltech researchers have discovered that the complex internal oscillations of a detonation wave can be regulated through synchronization with “oscillations” (periodic inhomogeneities) in the medium. After examining a wide range of potential characteristic sizes of such inhomogeneities, the team discovered certain ranges in which the oscillations of a given detonation wave experience regularization. In other words, the wave always propagates in spurts, but these spurts become quite predictable.

Because of their particular shape on the graph, all of these regularization-promoting ranges are collectively referred to as Arnold’s tongues, and the article in the journal of fluid mechanics is the first to describe them with regard to detonation.

The discovery of timing and Arnold’s tongues in detonation waves laid the foundation for further research into engine design that would allow engineers to tame the detonation wave and control its speed of propagation. So far, researchers have performed the calculations in one dimension, but three-dimensional calculations are needed to understand the processes in a real engine.