Representation of instantaneous flow velocity field during combustion oscillations in the combustion chamber of a model rocket engine. The figure shows that large-scale vortex rings are produced from the injector rim during combustion oscillations. (Image: Satomi Shima, Kosuke Nakamura, Hiroshi Gotoda, Yuya Ohmichi, and Shingo Matsuyama)

A vital piece of gas engines, combustors — the chambers in which the combustion powering the engine occurs — break down due to fatal high-frequency oscillations during the combustion process. Using advanced time-series analyses based on complex systems, researchers have found what causes them, opening up novel paths to solving the problem.

Rocket engines contain confined combustion systems that are essentially combustion chambers. In these chambers, nonlinear interactions among turbulent fuel and oxidizer flows, sound waves, and heat produced from chemical reactions cause an unstable phenomenon called combustion oscillations. The force of these oscillations on the body of the combustion chamber — the mechanical stress on the chamber — is high enough to threaten catastrophic failure of the engine.

The researchers revealed the physical mechanism behind the formation and sustenance of high-frequency combustion oscillations in a cylindrical combustor using analytical methods inspired by symbolic dynamics and complex networks. The combustor the team picked to simulate is one of model rocket engines. They were able to pinpoint the moment of transition from the stable combustion state to combustion oscillations and visualize it. They found that significant periodic flow velocity fluctuations in a fuel injector affects the ignition process, resulting in changes to the heat release rate. The heat release rate fluctuations synchronize with the pressure fluctuations inside the combustor and the whole cycle continues in a series of feedback loops that sustain combustion oscillations.

Additionally, by considering a spatial network of pressure and heat release rate fluctuations, they found that clusters of acoustic power sources periodically form and collapse in the shear layer of the combustor near the injection pipe’s rim, further helping to drive the combustion oscillations. These findings provide reasonable answers for why combustion oscillations occur, albeit specific to liquid rocket engines.

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