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

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 (/) from the lean blowout limit to / ¼ 0:75. First, we observe the different dynamic modes in this lean range as / is raised. We also document the effect of / 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 truncationwithout 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 / 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 /. 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.

Flame macrostructures and thermoacoustic instabilities in stratified swirling flames

“…For both flames, a large driving region can be observed in the flame-to-wall impingement zone. This is in line with previous studies [2,33,34], where the presence of flame in the OSL was found to enhance the flame-wall interaction, contributing to the driving mechanisms. Coherent structures are also clearly observable along the flame brushes in Fig.…”

Helical vortex core dynamics and flame interaction in turbulent premixed swirl combustion: A combined experimental and large eddy simulation investigation

“…The recorded signal includes the acoustic and aerodynamic pressure fluctuations. From previous investigations of the same combustor, 56 the fundamental longitudinal acoustic mode is expected to be around 120 Hz. The power spectral density (PSD) amplitude of the pressure signal for the nonreacting flow is shown in Fig.…”

Pressure Characteristics Study for the Configuration of Sudden Expansion with Central Restriction and Suction

“…For 10 deg inlet swirl, they have observed that AR = 1.91 diffuser gives highest performance. Taamallah et al [11] have experimentally studied the link between the flame macroscale structure and thermo-acoustic instabilities, in a premixed swirl-stabilized dump combustor. They have concluded that dynamic modes in the long combustor are mostly associated with specific mean flame brush spatial distributions.…”

“…As per brief review of literature, it is noted that a number of researchers have studied the flow characteristics of fluid passing through plain sudden expansion configuration. But, the numerical or experimental study on plain sudden expansion configuration with some modifications is very few [5]- [11]. Again, it is noted that systematic detailed study on the pressure characteristics in case of sudden expansion configurations with central restriction and suction is not addressed.…”