“…We measure the fluctuations of the flame edge and the underlying velocity field within a 2-D plane located at the mid-section of the wedge-shaped flame surface, using Mie scattering and high-speed particle image velocimetry (PIV). This experimental set-up has been employed previously for studying the effects of harmonic forcing and turbulence on premixed flames (Humphrey, Emerson & Lieuwen 2018; Roy & Sujith 2019). Figure 1.Turbulent V-flame facility.…”
Premixed turbulent flames, encountered in power generation and propulsion engines, are an archetype of a randomly advected, self-propagating surface. While such a flame is known to exhibit large-scale intermittent flapping, the possible intermittency of its small-scale fluctuations has been largely disregarded. Here, we experimentally reveal the inner intermittency of a premixed turbulent V-flame, while clearly distinguishing this small-scale feature from large-scale outer intermittency. From temporal measurements of the fluctuations of the flame, we find a frequency spectrum that has a power-law subrange with an exponent close to
$-2$
, which is shown to follow from Kolmogorov phenomenology. Crucially, however, the moments of the temporal increment of the flame position are found to scale anomalously, with exponents that saturate at higher orders. This signature of small-scale inner intermittency is shown to originate from high-curvature, cusp-like structures on the flame surface, which have significance for modelling the heat release rate and other key properties of premixed turbulent flames.
“…We measure the fluctuations of the flame edge and the underlying velocity field within a 2-D plane located at the mid-section of the wedge-shaped flame surface, using Mie scattering and high-speed particle image velocimetry (PIV). This experimental set-up has been employed previously for studying the effects of harmonic forcing and turbulence on premixed flames (Humphrey, Emerson & Lieuwen 2018; Roy & Sujith 2019). Figure 1.Turbulent V-flame facility.…”
Premixed turbulent flames, encountered in power generation and propulsion engines, are an archetype of a randomly advected, self-propagating surface. While such a flame is known to exhibit large-scale intermittent flapping, the possible intermittency of its small-scale fluctuations has been largely disregarded. Here, we experimentally reveal the inner intermittency of a premixed turbulent V-flame, while clearly distinguishing this small-scale feature from large-scale outer intermittency. From temporal measurements of the fluctuations of the flame, we find a frequency spectrum that has a power-law subrange with an exponent close to
$-2$
, which is shown to follow from Kolmogorov phenomenology. Crucially, however, the moments of the temporal increment of the flame position are found to scale anomalously, with exponents that saturate at higher orders. This signature of small-scale inner intermittency is shown to originate from high-curvature, cusp-like structures on the flame surface, which have significance for modelling the heat release rate and other key properties of premixed turbulent flames.
“…In particular, the propagation speed of a flame is directly modulated by free-stream turbulence (Driscoll 2008; Chowdhury & Cetegen 2017). The modulation of flame speed by local flow oscillations is of particular significance in acoustically excited flames, as the burnout of large-scale flame wrinkles occurs faster at higher flame speeds (Roy & Sujith 2019). Shin & Lieuwen (2013) showed, using a level-set approach, that the ensemble-averaged turbulent burning velocity is modulated by harmonic forcing with an inverse dependence upon ensemble-averaged flame curvature.…”
The presence of large-scale coherent structures can significantly impact the dynamics of a turbulent flow field and the behaviour of a flame stabilized in that flow. The goal of this study is to analyse how increasing free-stream turbulence can change the response of the flow to longitudinal acoustic excitation of varying amplitudes. We study the flow in the wake of a cylindrical bluff body at both non-reacting and reacting conditions, as the presence of a flame can significantly alter the global stability of the flow. The frequency of longitudinal acoustic excitation is set to match the natural frequency of anti-symmetric vortex shedding for this configuration and we vary the free-stream turbulence using perforated plates upstream of the bluff body. The results show that varying the level of free-stream turbulence can influence not only the amplitude of the coherent flow response, but also the symmetry of vortex shedding in the presence of longitudinal acoustic excitation. Increasing the turbulence intensity can fundamentally change the structure of the time-averaged flow and can directly impact the coherent flow response in two ways. First, increasing turbulence intensity can enhance the amplitude of the natural anti-symmetric vortex shedding mode in the wake. Second, increasing turbulence intensity weakens the symmetric response of the flow to the longitudinal acoustic excitation. In the non-reacting and reacting conditions, both symmetric and anti-symmetric modes are present and are characterized using a spectral proper orthogonal decomposition. We see evidence of interaction between the symmetric and anti-symmetric modes, which leads to an interference pattern in the coherent vorticity response in the shear layers. We conclude by presenting a conceptual model for the influence that turbulence has on these flows.
“…Our experimental facility consists of a premixed CH 4 −air conical V-flame stabilized on an oscillating flame holder, a configuration often employed in the study of laminar and turbulent premixed flames [37][38][39]. Turbulence is generated using a series of statorrotor plates, and the flame holder, which is an electrically heated nichrome wire, is vibrated at a frequency of f f = 1250 Hz.…”
We experimentally reveal the inner intermittency of a turbulent premixed flame surface while clearly distinguishing this small-scale feature from large-scale outer intermittency. From temporal measurements of the fluctuations of the flame, we find a frequency spectra that has a power-law subrange with an exponent close to -2, which we derive from Kolmogorov phenomenology. Crucially, however, the moments of the temporal increment are shown to scale anomalously with exponents that saturate at higher orders, demonstrating that the flame fluctuates intermittently with rapid extreme events.
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