An experimental study of fuel–air mixing section on unstable combustion in a dump combustor

• Instability modes have been accurately analyzed by multi-channel p′ sensing system.
• Major instability parameters are known to be φ, vmix and combustor, plenum geometry.
• The 1st mode was associated with fundamental longitudinal mode of combustor section.
• The 2nd mode was acoustically coupled with the combustor and inlet mixing section.
• The Instability was strongly affected by acoustic coupling with combustor and plenum.

The main objective of this study is effect of the various fuel–air mixing section geometries on the unstable combustion. For the purpose of observing the combustion pressure oscillation and phase difference at each of the dynamic pressure results, the multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section. By using an optically accessible quartz-type combustor, we were able to OH* measurements to characterize the flame structure and heat release oscillation with the use of a high-speed ICCD camera. In this study, we observed two dominant instability frequencies. Lower frequencies were measured around 240–380 Hz, which were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were measured around 410–830 Hz. These were related to the secondary longitudinal mode in the combustion chamber and the secondary quarter-wave mode in the inlet mixing section. These second instability mode characteristics are coupled with the conditions of the combustor and inlet mixing section acoustic geometry. Also, these higher combustion instability characteristics include dynamic pressure oscillation of the inlet mixing section part, which was larger than the combustor section. As a result, combustion instability was strongly affected by the acoustically coupling of the combustor and inlet mixing section geometry.