Response of generic swirl flames to transverse acoustic forcing
Final Report Abstract
State-of-the-art gas turbines employ the annular configuration of combustors, where several circumferentially arranged flames share a common annulus as the combustor. Thermoacoustic instability, a phenomenon involving feedback coupling between acoustics and unsteady heat release rate from flames in combustors, is still a concerning problem in such annular combustors. While results from previous studies on flame-acoustic interactions can be employed for modelling, prediction, and control of instabilities in the older generation longitudinal combustors; previous results—in particular, results on flame response to acoustics (flame transfer functions)—do not apply directly for annular combustors. This is because the change in the combustor configuration from longitudinal to annular combustors introduces new, previously unknown, aspects of flame-acoustic interaction, associated specifically to the response of flames to transverse acoustics. To address this knowledge gap, the present project was undertaken. It involved detailed experiments on the response of a generic swirl flame to a two-dimensional acoustic field involving transverse acoustics in the combustor and axial acoustics (induced either indirectly by transverse acoustics, or directly by axial acoustic forcing) in the burner tube. A few other research groups around the world had also simultaneously undertaken this research. While these works have focused on a largely qualitative description of the flame response, for limited configurations of transverse acoustic forcing; we looked at quantitative characterization supported by detailed diagnostics for a much wider range of two-dimensional acoustic forcing of the flame. In summary, the project has led to several key contributions: Firstly, the specialized rig suited for the experiments was developed. As of yet, the rig has the most advanced capabilities for the study of flame response to transverse acoustics. Subsequently, the subject of flame response to a pure transverse pressure antinode was addressed. Based on observations of self-excited instability involving transverse acoustics in the rig, symmetric transverse acoustic forcing, and by comparing the two with axial acoustic forcing we arrived at the conclusion that axial and transverse pressure forcing are qualitatively identical scenarios in terms of flame response. However, at low frequencies the response to transverse pressure acoustics is distinctly higher than the response to axial acoustics. The next aspect addressed was the response to transverse acoustic velocity, where the results corroborated the existing prediction that transverse acoustic velocity does not induce a global flame response. The most important contribution of the project involves the scenario of the flame response to simultaneous transverse acoustic pressure and velocity. Here, we find that the existing assumptions on flame response to simultaneous transverse acosutic pressure and velocity are invalid because the coherent structures that are responsible for flame response to the pressure and velocity components individually, interact with each other along the flame surface. Moreover, the dynamics of this interaction directly affects flame response. Thus, although transverse acoustic velocity in itself may not lead to a global flame response, flame response to axial or transverse acoustic pressure is distinctly modified in the presence of transverse acoustic velocity. In fact, this effect may have a more governing influence on flame response—hence on the instability characteristics—in comparison to other phenomena such as nonlinear amplitude saturation. We investigated this scenario in much detail. The approach followed was to quantify flame response in terms of the flame transfer function with axial velocity fluctuations as the reference, and subsequently compare different cases based on the features of the obtained transfer function. We also found that the inherent asymmetry of the flame is a critical parameter in the scenario of flame response to simultaneous forcing. The results and inferences drawn were supported by the analysis of planar flow field diagnostics, line-of-sight flame chemiluminescence, and planar flame diagnostics. These diagnostics in particular allowed for the identification of the physical mechanisms underlying the observed characteristics and trends of flame response to various acoustic forcing configurations. As a final note, to emphasize the importance of the results one may consider the following: Flame response is key to the phenomenon of thermoacoustic coupling. Accordingly, instability characteristics are sensitive to the accuracy of the flame response. As identified in this project, transverse acoustics introduce several effects as mentioned above, and detailed in the report, that govern flame response. As such, precise modelling, prediction, and control of instabilities in annular combustors is only possible if these effects are appropriately accounted for. The present study essentially provides critical inputs to the improvement of modeling and control strategies for the instability in annular systems, and directions to future studies involving the interaction of flames with transverse acoustics.
Publications
- Combustion Instability in a Swirl Flow Combustor With Transverse Extensions. ASME. Turbo Expo: Combustion, Fuels and Emissions1 , V01BT04A057, 201
Saurabh A., Paschereit C. O.
(See online at https://dx.doi.org/10.1115/GT2013-95732) - Swirl Flame Response to Traveling Acoustic Waves. ASME. Turbo Expo: Combustion, Fuels and Emissions, V04BT04A043, 2014
Saurabh A., Steinert R., Moeck J. P., Paschereit C. O.
(See online at https://dx.doi.org/10.1115/GT2014-26829) - Swirl Flow Response to Transverse and Axial Acoustic Forcing. Journal of Fluid Science and Technology 9(4), pp. JFST0059, 2014
Saurabh A., Paschereit C. O.
(See online at https://dx.doi.org/10.1299/jfst.2014jfst0059) - Dynamics of Premixed Swirl Flames under the Influence of Transverse Acoustic Fluctuations. Combustion and Flame 182, pp 298–312, 2017
Saurabh A., Paschereit C. O.
(See online at https://dx.doi.org/10.1016/j.combustflame.2017.04.014) - Swirl Flame Response to Simultaneous Axial and Transverse Velocity Fluctuations. Journal of Engineering Gas Turbines & Power 139(6), pp. 061502, 2017
Saurabh A., Moeck J. P., Paschereit C. O.
(See online at https://doi.org/10.1115/1.4035231)
