Premixed Flame Dynamics in Response to Two-Dimensional Acoustic Forcing

ABSTRACT

Thermoacoustic instability in annular combustors involves feedback interaction between individual (acoustically compact) flames and – predominantly – azimuthal acoustic modes of the annular combustion chamber. During such an interaction, the flames experience acoustic pressure fluctuations in the combustor and transverse acoustic velocity fluctuations simultaneously. To investigate the dynamics of premixed swirl flames under the influence of transverse acoustics, experiments were performed on a single-burner atmospheric combustion rig with controlled simultaneous axial and transverse acoustic forcing, where the axial acoustics simulates the effect of acoustic pressure fluctuations in the combustor. In this paper, we present results on the descriptive analysis of spatially resolved flame surface fluctuations, acquired via OH-PLIF (OH* planar laser-induced florescence) diagnostics of the forced flame and establish the role of the interaction among coherent structures generated by axial and transverse velocity components in generating global flame response changes with respect to the phase difference between axial and transverse acoustic forcing as reported recently (Saurabh and Paschereit, Combustion Flame 182, 2017).

KEYWORDS:

• Thermoacoustic Instability
• Annular Combustors
• Premixed Swirl Flames
• Transverse Acoustics
• Flame Response
• OH PLIF

Acknowledgments

The authors thank Arthur Ehlert for assistance during experiments. This work was supported by the German Research Foundation (DFG) [grant number PA 920/16].

Notes

¹ Note that secondary and indirect effects such as changes in the mean flame shape due to transverse acoustics (O’Connor and Lieuwen, 2012), effect of neighboring flames (Fanaca et al., 2008), the effect of flame asymmetry (Acharya and Lieuwen, 2016), and nonlinear effects (Acharya and Lieuwen, 2015) that have been identified recently may require consideration of the transverse acoustic velocity component.

² Hauser et al. (2012) state that the effect of transverse forcing is only to reduce flame length and this change affects the FTF.

³ Note, however, that the ratio between the wavelength of the acoustic mode and the burner diameter was about 38 – small enough to consider the burner compact and gradients over the burner small.

⁴ Vortical flow structures leading to flame surface modulations would be manifested in OH-PLIF snapshots of the flame surface.

Additional information

Funding

This work was supported by the DFG [grant number PA 920/16].