Similarity phenomena of lean swirling flames at different bulk velocities with acoustic disturbances

In this study, flame responses to acoustic disturbances with different frequencies and amplitudes were experimentally investigated in a lean premixed swirl-stabilized combustor operat-ing at different bulk velocities. The total heat release rate fluctuations and spatial CH* chemilumi-nescence distributions were captured using a photomultiplier tube and high-speed camera, respectively. The results indicate that the heat release rate exhibits a relatively drastic oscillation and high-order harmonics for low-frequency disturbances. When the bulk velocity and forcing fre-quency were doubled simultaneously, similar flame structures were observed in the CH* chemilu-minescence distributions. As the bulk velocity increases, the gain of the Flame Describing Function (FDF) extends toward the higher frequencies, and the delay time of the flame response decreases. The similarity among FDFs at different bulk velocities was effectively captured by intro-ducing a non-dimensional parameter, defined as the ratio of the flame response delay to the forcing time scale, to replace the dimensional forcing frequency. Furthermore, the availability of the newly defined non-dimensional parameter was verified for flames with different swirl numbers, as this played an important role in determining the flame structures and associated unsteady heat release rate.(c) 2023 Production and hosting by Elsevier Ltd. on behalf of Chinese Society of Aeronautics and Astronautics. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).

Automated summary

This study experimentally investigated the flame responses to acoustic disturbances with different frequencies and amplitudes in a combustor operating at different bulk velocities. The results showed that low-frequency disturbances caused drastic oscillations in the heat release rate and high-order harmonics. Increasing the bulk velocity extended the frequency range for flame response and reduced the delay time. A non-dimensional parameter was introduced to capture the similarity among flame responses at different bulk velocities.