Correspondence Between Uncoupled Flame Macrostructures and Thermoacoustic Instability in Premixed Swirl-Stabilized Combustion

Abstract

In this paper, we conduct an experimental investigation of a confined premixed swirl-stabilized dump combustor similar to those found in modern gas turbines. We operate the combustor with premixed methane-air in the lean range of equivalence ratio ϕ ∈ [0.5–0.75]. First, we observe different dynamic modes in the lean operating range, as the equivalence ratio is raised, confirming observations made previously in a similar combustor geometry but with a different fuel [1]. Next we examine the correspondence between dynamic mode transitions and changes in the mean flame configuration or macrostructure. We show that each dynamic mode is associated with a specific flame macrostructure. By modifying the combustor length without changing the underlying flow, the resonant frequencies of the geometry are altered allowing for decoupling the heat release fluctuations and the acoustic field, in a certain range of equivalence ratio. Mean flame configurations in the modified (short) combustor and for the same range of equivalence ratio are examined. It is found that not only the same sequence of flame configurations is observed in both combustors (long and short) but also that the set of equivalence ratio where transitions in the flame configuration occur is closely related to the onset of thermo-acoustic instabilities. For both combustor lengths, the flame structure changes at similar equivalence ratio whether thermo-acoustic coupling is allowed or not, suggesting that the flame configuration holds the key to understanding the onset of self-excited thermo-acoustic instability in this range. Finally, we focus on the flame configuration transition that was correlated with the onset of the first dynamically unstable mode ϕ ∈ [0.61–0.64]. Our analysis of this transition in the short, uncoupled combustor shows that it is associated with an intermittent appearance of a flame in the outer recirculation zone (ORZ). The spectral analysis of this “ORZ flame flickering” — based on flame chemiluminescence data — shows the presence of unsteady events occurring at two distinct frequency ranges. A broad band at low frequency in the range ∼[1 Hz – 10 Hz] and a narrow band centered around 28 Hz.

Impact of Equivalence Ratio on the Macrostructure of Premixed Swirling CH4/Air and CH4/O2/CO2 Flames

“…Configuration III, where the flame is stabilized on the inner shear layer only is found in the air-flame at f = 0.60, whereas configuration IV is found in the oxy-flame at the same equivalence ratio. This is surprising since the laminar burning velocity of the air flame is higher than the oxy flame as shown in Table 1, and hence it should have been expected to be present in the air-flame according to the dependence of the flame configuration on the burning velocity [7][8][9][10]. Meanwhile, when the equivalence ratio is below 0.58, the flame length significantly increases, and a conical flame (configuration III) appears in both air and oxy-flames.…”

“…Recently, Shroll et al studied dynamic stability characteristics of premixed CH 4 /O 2 /CO 2 mixtures in the MIT swirl stabilized combustor, and found that the transition between instability modes correlated with the adiabatic flame temperature [6]. Further work in the same group [7,8] showed that the correlation between the flame structure and the dynamic mode, with flames stabilized on the inner shear layer around the inner recirculation zone being stable while flames stabilized in the inner shear layer as well as the outer shear layer being unstable.…”

“…The adiabatic temperatures of the oxy-flames were almost the same as those of the air-flames at the same f, whereas the laminar burning velocities of oxy-flames were consistently lower than those of the air-flames. All cases were calculated at atmospheric pressure, and the inlet reactant temperature was 300 K. In our previous work [7][8][9][10], we analyzed premixed air flames in the swirl-stabilized combustor, and observed four different shapes/configurations that appear as the equivalence ratio is raised or the laminar burning velocity of the corresponding flame increases: columnar flame (I); a bubble+columnar flame (II); a single conical flame (III); and a double conical flame (IV). In flame III (e.g.…”

“…A premixed flame is stabilized using a combination of swirl and sudden expansion. The experimental setup has been described in detail in our previous publications [6][7][8][9][10]. Premixed CH 4 /O 2 /CO 2 or premixed CH 4 /air enters the combustor through a 38 mm diameter inlet pipe.…”

Transition From a Single to a Double Flame Structure in Swirling Reacting Flows: Mechanism, Dynamics, and Effect of Thermal Boundary Conditions

“…The main reason remains its close link with the thermo-acoustic stability of the combustor under some conditions [1][2][3]. Recently, our group highlighted the strong correlation between these flame macrostructures, their transitions and the different thermo-acoustic stability modes observed in a canonical swirl-stabilized combustor.…”