Effects of Flow Structure Dynamics on Thermoacoustic Instabilities in Swirl-Stabilized Combustion

Abstract

The thermoacoustic coupling caused by dynamic flow/flame interactions was investigated in a gas-turbine model combustor using high-repetition-rate measurements of the three-component velocity field, OH laser-induced fluorescence, and OH* chemiluminescence. Three fuel-lean, swirl-stabilized flames were investigated, each of which underwent self-excited thermoacoustic pulsations. The most energetic flow structure at each condition was a helical vortex core that circumscribed the combustor at a frequency that was independent of the acoustics. Resolving the measurement sequence with respect to both the phase in the thermoacoustic cycle and the azimuthal position of the helix allowed quantification of the oscillatory flow and flame dynamics. Periodic vortex/flame interactions caused by deformation of the helices generated local heat-release oscillations having spatially complex phase distributions relative to the acoustics. The local thermoacoustic coupling, determined by statistically solving the Rayleigh integral, showed intertwined regions of positive and negative coupling due to these vortices. In the quietest flame, the helical vortex created a large region of negative coupling that helped damp the oscillations. In the louder flames, the shapes of the oscillating vortices and flames were such that large regions of positive coupling were generated, driving the instability. From these observations, flame/vortex configurations that promote stability are identified. Nomenclature a = proper orthogonal decomposition temporal coefficient ap = doubly-phase-resolved mean oscillation D = dissipation rate of acoustic energy f = frequency M = proper orthogonal decomposition spatial eigenmode P th = thermal power p = pressure _ q = heat-release rate S = swirl number t = excess stochastic turbulent fluctuation _ v = volume flow rate = thermoacoustic phase shift = total thermoacoustic energy transfer # = local thermoacoustic energy transfer = azimuthal angle through helical vortex cores = proper orthogonal decomposition eigenvalue = flame surface density = phase angle = total doubly-phase-resolved thermoacoustic coupling = local doubly-phase-resolved thermoacoustic coupling ! = vorticity $ = long time average $ap = sum of long average and doubly-phase-resolved oscillation Subscripts a = acoustic h = helical vortex core qc = heat-release centroid

Experimental analysis of thermo-acoustic instabilities in a generic gas turbine combustor by phase-correlated PIV, chemiluminescence, and laser Raman scattering measurements

“…Those measurements include OH* and CH* chemiluminescence to identify heat release variations, OH, CH 2 O and CH PLIF to determine fluctuations in the flame surface density or local Rayleigh index and laser Raman measurements to assess changes in the thermo-chemical state of the flame [Bellows 2007, Biagioli 2008, Giezendanner 2003, Hubschmid 2008, Ishino 2001, Lee 2003, Schildmacher 2006. More recently, high-speed imaging techniques in combination with acoustic pressure tracing have been applied to study thermo-acoustic instabilities in GT-relevant combustors , Caux-Brisebois 2014, Durox 2013, Galley 2011, Gounder 2014, Steinberg 2010, Steinberg 2012, Steinberg 2013, Worth 2012, yielding deeper insight into the feedback mechanisms.…”

Thermo-acoustic velocity coupling in a swirl stabilized gas turbine model combustor

“…2.2.2, along with past research [73,101], have shown that both the azimuthal position of the HVC and the acoustic phase had an important inuence on the rate of heat release. Therefore, understanding the coupled ame and ow dynamics required compilation of statistics as a function of both acoustic (φ a ) and azimuthal position around the HVC (φ h ) [73,101], which was derived from the POD temporal coecient associated to the most energetic turbulent mode. That is, any quantity ζ was decomposed into the linear combination of a long time Table 3.1: List of the dierent cases of the experiment and corresponding equivalence ratio φ, themal power P th , RMS acoustic amplitude p , dominant acoustic frequency f ac , and dominant vortical frequency f v .…”

“…Possible auto-ignition events were also noted, though it was dicult to distinguish autoignition from through-plane, azimuthal convection of a quasi-stable ame. To address this problem, Steinberg et al [73] statistically decoupled the azimuthal position of the HVC and the acoustic cycle by resolving the time-accurate data with respects to the acoustic and HVC phases. This process is detailed in Sec.…”

Flame Response to Equivalence Ratio Fluctuations — Relationship between Chemiluminescence and Heat Release

“…We can rewrite Eq. (24) as: (27) The next step is to evaluate the fluctuating flame surface area using the knowledge of the instantaneous flame surface location [40]. For this purpose, the flame position is analyzed using the G-equation [41−44], consistent with our flamelet analysis: (28) Here the instantaneous flame surface is given by all points where G(r, θ, z, t) = 0.…”