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Research Papers: Gas Turbines: Combustion, Fuels, and Emissions

# Correspondence Between “Stable” Flame Macrostructure and Thermo-acoustic Instability in Premixed Swirl-Stabilized Turbulent Combustion

[+] Author and Article Information
Soufien Taamallah

Reacting Gas Dynamics Laboratory,
Mechanical Engineering Department,
MIT,
Cambridge, MA 02139

Zachary A. LaBry, Santosh J. Shanbhogue

Reacting Gas Dynamics Laboratory,
Mechanical Engineering Department,
MIT,
Cambridge, MA 02139

Mohamed A. M. Habib

Mechanical Engineering Department,
KFUPM,
Dhahran 34464, Saudi Arabia

Ahmed F. Ghoniem

Reacting Gas Dynamics Laboratory,
Mechanical Engineering Department,
MIT,
Cambridge, MA 02139

1Corresponding author.

Contributed by the Combustion and Fuels Committee of ASME for publication in the JOURNAL OF ENGINEERING FOR GAS TURBINES AND POWER. Manuscript received September 22, 2014; final manuscript received October 31, 2014; published online December 23, 2014. Editor: David Wisler.

J. Eng. Gas Turbines Power 137(7), 071505 (Jul 01, 2015) (12 pages) Paper No: GTP-14-1558; doi: 10.1115/1.4029173 History: Received September 22, 2014; Revised October 31, 2014; Online December 23, 2014

## Abstract

In this paper, we conduct an experimental investigation to study the link between the flame macroscale structure—or flame brush spatial distribution—and thermo-acoustic instabilities, in a premixed swirl-stabilized dump combustor. We operate the combustor with premixed methane–air in the range of equivalence ratio ($φ$) from the lean blowout limit to $φ=0.75$. First, we observe the different dynamic modes in this lean range as $φ$ is raised. We also document the effect of $φ$ on the flame macrostructure. Next, we examine the correspondence between dynamic mode transitions and changes in flame macrostructure. To do so, we modify the combustor length—by downstream truncation—without changing the underlying flow upstream. Thus, the resonant frequencies of the geometry are altered allowing for decoupling the heat release rate fluctuations and the acoustic feedback. Mean flame configurations in the modified combustor and for the same range of equivalence ratio are examined, following the same experimental protocol. It is found that not only the same sequence of flame macrostructures is observed in both combustors but also that the transitions occur at a similar set of equivalence ratio. In particular, the appearance of the flame in the outside recirculation zone (ORZ) in the long combustor—which occurs simultaneously with the onset of instability at the fundamental frequency—happens at similar $φ$ when compared to the short combustor, but without being in latter case accompanied by a transition to thermo-acoustic instability. Then, we interrogate the flow field by analyzing the streamlines, mean, and rms velocities for the nonreacting flow and the different flame types. Finally, we focus on the transition of the flame to the ORZ in the acoustically decoupled case. Our analysis of this transition shows that it occurs gradually with an intermittent appearance of a flame in the ORZ and an increasing probability with $φ$. The spectral analysis of this phenomenon—we refer to as “ORZ flame flickering”—shows the presence of unsteady events occurring at two distinct low frequency ranges. A broad band at very low frequency in the range ∼(1 Hz–10 Hz) associated with the expansion and contraction of the inner recirculation zone (IRZ) and a narrow band centered around 28 Hz which is the frequency of rotation of the flame as it is advected by the ORZ flow.

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## Figures

Fig. 1

Long and short geometries for acoustic-heat release decoupling

Fig. 2

Axial eight-vane swirler with 45 deg vane angle and a streamlined centerbody with a diameter of 9 mm and a 60 deg cone angle

Fig. 3

Schematic flow field in the swirl-stabilized combustor: ORZ, annular incoming swirling jet, and IRZ regions delimited by the outer and inner shear layers, in the mean flow sense

Fig. 4

Correspondence between the observed mean swirling flame configurations (roman numerals) and dynamic mode transitions in the long combustor for methane. The equivalence ratio was changed from φ = 0.5 to φ = 0.75 in upward increments of Δφ = 0.01. (a) Mean swirling flame configurations. (b) Pressure fluctuations signal. (c) Associated spectrogram. Solid line at φ = 0.47 represents the LBO limit. Dotted lines represent the transitions between mean flame configurations.

Fig. 5

Predicted longitudinal modes of the combustor (left: long, right: short) using one-dimensional acoustic calculation

Fig. 6

Experimental measurements of the combustors' natural frequencies, to be compared with the predicted frequencies in Fig. 5 (top: long combustor, bottom: short combustor)

Fig. 7

Observed swirling flame configurations as the equivalence ratio is raised from the LBO limit in the long (a) and short combustors (b). From top to bottom: φ: 0.51, 0.54, 0.57, 0.62, and 0.66.

Fig. 8

Sequence of high speed flame chemiluminescence images (200 fps, exposure time of 1/200 s) showing the intermittent appearance of flame in the ORZ in the short combustor. Here, dt is the time step between two images, φ = 0.62. Dotted circle shows flame in the ORZ.

Fig. 9

Correspondence between the observed mean swirling flame configurations and dynamic mode transitions in long (a) and short (b) combustors. Vertical solid lines represent the LBO limit and dotted lines represent the transitions between flame configurations.

Fig. 10

Mean velocity streamlines colored by velocity magnitude for nonreacting flow and flame macrostructures I (φ=0.51), III (φ=0.60), and IV (φ=0.65)

Fig. 11

Mean velocity streamlines colored by total rms velocity for nonreacting flow and flame macrostructures I (φ=0.51), III (φ=0.60), and IV (φ=0.65)

Fig. 12

Comparison of incoming jet angle, defined using the combined axial–radial maximum velocity magnitude, for nonreacting flow and flame macrostructures I (φ = 0.51), III (φ = 0.60), and IV (φ = 0.65)

Fig. 13

Flame image at φ = 0.62, recorded with an exposure time of 5 ms, showing a schematic of the ORZ analysis box and the shear layers around the incoming swirling jet

Fig. 14

ORZ flame intensity probability during transition from flame III to IV

Fig. 15

ORZ flame spectral analysis for φ = 0.60 (flame III), φ = 0.63 (flame III ↔ IV), and φ = 0.66 (flame IV): space-averaged IR filtered chemiluminescence intensity signal in ORZ (left) and its fast Fourier transform (right)

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