Optimizing combustion systems to mitigate thermoacoustic instabilities

Lean premixed combustion, in which there is a higher concentration of air to fuel than is typical, can significantly reduce flame temperature and therefore lower greenhouse gas emissions. However, such lean combustors are especially susceptible to undesirable thermoacoustic instabilities. Characterizing and minimizing the instabilities, which arise from the conversion of sound to energy and vice versa, is crucial for preventing noise and problems in the combustor.

Sun et al. performed 3D large-eddy simulations in a premixed methane-fueled swirling combustor with and without an outlet nozzle. They considered different turbulence and combustion models, chemical reactions, boundary conditions, and numerical schemes to obtain the optimal combination for reducing both greenhouse gas emissions and thermoacoustic instabilities.

“The generation of thermoacoustic instabilities is due to the constructive interaction of unsteady flow-flame acoustics,” said author Dan Zhao. “It may lead to undesirable flame blowoff or flashback, overheating, structural vibrations, and even system failures.”

By including an outlet nozzle in the combustor design, the team created an acoustic damper that mitigated the fuel instabilities. They explored different diameter nozzles and found the smaller outlet further dampened thermoacoustic oscillations.

“The present paper contributes to a better understanding of the generation mechanism of longitudinal self-sustained thermoacoustic oscillations in a swirling combustor with an outlet nozzle,” said Zhao. “It also suggests an alternative passive control method for damping thermoacoustic oscillations.”

The authors plan to extend the current model, which simulates longitudinal oscillations, to incorporate azimuthal instabilities. Other renewable fuels, such as hydrogen and ammonia, could also be explored.

Source: “Large eddy simulations of self-excited thermoacoustic instability in a premixed swirling combustor with an outlet nozzle,” by Yuze Sun, Dan Zhao, Chenzhen Ji, Tong Zhu, Zhuming Rao, and Bing Wang, Physics of Fluids (2022). The article can be accessed at https://doi.org/10.1063/5.0087055 .