Abstract
A new laboratory scale pressurised annular combustion experiment is introduced and used to generate self-excited longitudinal and azimuthal instabilities. The experiments are operated at mean pressures ranging from approximately 2 to 3 atmospheres in order to maintain a well defined acoustic boundary at exit. A range of operating conditions is studied parametrically, and it is observed that at high equivalence ratios, the flame stabilisation location propagates upstream, significantly altering the flame structure. The change in flame stabilisation location promotes a transition from a dominant longitudinal to a dominant azimuthal instability. Investigation of the azimuthal instabilities highlights a rich array of frequency content, with significant amplitude pressure and heat release responses observed for not only the fundamental (n=1), but also higher harmonics (n=2,3). These higher harmonics are also shown to exhibit distinct characteristic modal dynamics, shown through probability density functions of the spin ratio. The flame dynamics for three distinct operating states, corresponding to longitudinal modes at two different stabilisation locations, and one corresponding to strong azimuthal modes are studied. These highlight the difference between longitudinal and azimuthal modes, and demonstrate the presence of significant higher harmonic content. The characterisation of both longitudinal and azimuthal modes in a pressurised laboratory scale annular combustor for the first time provides a unique opportunity for understanding the nature of such instabilities in practically relevant configurations.
Highlights
IntroductionThe ability to replicate azimuthal instabilities in both lab and industrial scale annular combustors has given way to a range of recent experimental [2,3,4,5,6,7], numerical [8,9], and theoretical studies [10,11,12,13,14], which have provided insight into their nature
Corresponding peak locations in this case raise the possibility of coupling between the dynamics of these components, and a more detailed investigation of the modal dynamics between harmonic components is warranted in future work
The frequency and amplitude of both longitudinal and azimuthal modes were characterised across a range of mass flow rates and equivalence ratios, resulting in a variation of pressure, power and flow rate
Summary
The ability to replicate azimuthal instabilities in both lab and industrial scale annular combustors has given way to a range of recent experimental [2,3,4,5,6,7], numerical [8,9], and theoretical studies [10,11,12,13,14], which have provided insight into their nature. A similar combustor height mismatch was employed by Bourgouin et al [4] to generate azimuthal modes, with strong acoustic activity in the plenum showing the importance of the upstream boundary conditions.
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