Analysis of the development of the flame brush in turbulent premixed spherical flames

Formal governing equations are derived for the mean orientation vector, 〈n〉f, and the mean curvature, 〈∇ · n〉f, in turbulent premixed combustion. Balance is checked to evaluate all component terms and to understand their physical implications in DNS of two test flames. The terms involving ∇T(vn + Sd) and n″(vn + Sd)″ are dominant to determine 〈n〉f through a flame brush and at the leading edge. All listed terms are relevant to determine 〈∇·n〉f, while those involving ∇T2νn+SdΣ′f and (∇·n)″ (vn + Sd)″ become important at the edges. Different trends are observed on the dominant terms for thicker flamelets with the Karlovitz number greater than unity. Further investigation may be required to clarify relative importance of the component terms in different regimes of realistic flame conditions.

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Flame Brush Structure of H2-Air Turbulent Premixed Flames at High Reynolds Numbers

ASME-JSME-KSME 2011 Joint Fluids Engineering Conference: Volume 1, Symposia – Parts A, B, C, and D ◽

10.1115/ajk2011-02020 ◽

2011 ◽

Author(s):

Youngsam Shim ◽

Shoichi Tanaka ◽

Masayasu Shimura ◽

Naoya Fukushima ◽

Mamoru Tanahashi ◽

...

Keyword(s):

Flame Front ◽

Layer Structure ◽

Three Dimensional ◽

Kinetic Mechanism ◽

Integral Length Scale ◽

Length Scale ◽

Elementary Reactions ◽

High Reynolds ◽

Flame Brush ◽

Turbulent Premixed

Three-dimensional direct numerical simulations (DNSs) of turbulent premixed planar, jet and V flames of hydrogen-air mixture have been conducted to investigate the flame brush and the local flame structures at high Reynolds number turbulences. The detail kinetic mechanism including 12 reactive species and 27 elementary reactions was used to represent the hydrogen-air reaction. For planar flame, flame front is highly fluctuating, and multi-layer structure, multiply-folded flame front and unburned mixture island which lead to corresponding increase of the flame brush thickness can be observed. The flame brush thickness of the planar flame is relatively uniform along the flame front, and is about 2∼3 times the integral length scale (l), which is defined from an energy spectrum. For the jet and V flames, the flame brush thicknesses grow with the streamwise direction from about 0.5∼1 times the integral length scale (l) to about 2∼3 times the integral length scale (l) due to the highly fluctuating flame front at the downstream region.

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Spatially localized multi-scale energy transfer in turbulent premixed combustion

Journal of Fluid Mechanics ◽

10.1017/jfm.2018.371 ◽

2018 ◽

Vol 848 ◽

pp. 78-116 ◽

Cited By ~ 14

Author(s):

J. Kim ◽

M. Bassenne ◽

C. A. Z. Towery ◽

P. E. Hamlington ◽

A. Y. Poludnenko ◽

...

Keyword(s):

Energy Transfer ◽

Kinetic Energy ◽

Pressure Gradient ◽

Energy Spectra ◽

Cumulative Energy ◽

Turbulent Premixed Flame ◽

Wavelet Space ◽

The Mean ◽

Flame Brush ◽

Turbulent Premixed

A three-dimensional wavelet multi-resolution analysis of direct numerical simulations of a turbulent premixed flame is performed in order to investigate the spatially localized spectral transfer of kinetic energy across scales in the vicinity of the flame front. A formulation is developed that addresses the compressible spectral dynamics of the kinetic energy in wavelet space. The wavelet basis enables the examination of local energy spectra, along with inter-scale and subfilter-scale (SFS) cumulative energy fluxes across a scale cutoff, all quantities being available either unconditioned or conditioned on the local instantaneous value of the progress variable across the flame brush. The results include the quantification of mean spectral values and associated spatial variabilities. The energy spectra undergo, in most locations in the flame brush, a precipitous drop that starts at scales of the same order as the characteristic flame scale and continues to smaller scales, even though the corresponding decrease of the mean spectra is much more gradual. The mean convective inter-scale flux indicates that convection increases the energy of small scales, although it does so in a non-conservative manner due to the high aspect ratio of the grid, which limits the maximum scale level that can be used in the wavelet transform, and to the non-periodic boundary conditions, which exchange energy through surface forces, as explicitly elucidated by the formulation. The mean pressure-gradient inter-scale flux extracts energy from intermediate scales of the same order as the characteristic flame scale, and injects energy in the smaller and larger scales. The local SFS-cumulative contribution of the convective and pressure-gradient mechanisms of energy transfer across a given cutoff scale imposed by a wavelet filter is analysed. The local SFS-cumulative energy flux is such that the subfilter scales upstream from the flame always receive energy on average. Conversely, within the flame brush, energy is drained on average from the subfilter scales by convective and pressure-gradient effects most intensely when the filter cutoff is larger than the characteristic flame scale.

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Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime

Fluids ◽

10.3390/fluids5020089 ◽

2020 ◽

Vol 5 (2) ◽

pp. 89

Author(s):

Peter Brearley ◽

Umair Ahmed ◽

Nilanjan Chakraborty ◽

Markus Klein

Keyword(s):

Structure Function ◽

Isotropic Turbulence ◽

Premixed Flames ◽

Structure Functions ◽

Separation Distance ◽

Second Order ◽

Order Structure ◽

Turbulent Premixed Flames ◽

Flame Brush ◽

Turbulent Premixed

The second-order velocity structure function statistics have been analysed using a DNS database of statistically planar turbulent premixed flames subjected to unburned gas forcing. The flames considered here represent combustion for moderate values of Karlovitz number from the wrinkled flamelets to the thin reaction zones regimes of turbulent premixed combustion. It has been found that the second-order structure functions exhibit the theoretical asymptotic scalings in the dissipative and (relatively short) inertial ranges. However, the constant of proportionality for the theoretical asymptotic variation for the inertial range changes from one case to another, and this value also changes with structure function orientation. The variation of the structure functions for small length scale separation remains proportional to the square of the separation distance. However, the constant of proportionality for the limiting behaviour according to the separation distance square remains significantly different from the theoretical value obtained in isotropic turbulence. The disagreement increases with increasing turbulence intensity. It has been found that turbulent velocity fluctuations within the flame brush remain anisotropic for all cases considered here and this tendency strengthens towards the trailing edge of the flame brush. It indicates that the turbulence models derived based on the assumptions of homogeneous isotropic turbulence may not be fully valid for turbulent premixed flames.

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Simultaneous 2-D Imaging Measurements of Reaction Progress Variable and OH Radical Concentration in Turbulent Premixed Flames: Experimental Methods and Flame Brush Structure

Combustion Science and Technology ◽

10.1080/00102200108952180 ◽

2001 ◽

Vol 167 (1) ◽

pp. 131-167 ◽

Cited By ~ 23

Author(s):

YUNG-CHENG CHEN ◽

ROBERT W. BILGER

Keyword(s):

Premixed Flames ◽

Experimental Methods ◽

Oh Radical ◽

Turbulent Premixed Flames ◽

Reaction Progress ◽

Progress Variable ◽

Radical Concentration ◽

Flame Brush ◽

Turbulent Premixed

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Influence of flame geometry on turbulent premixed flame propagation: a DNS investigation

Journal of Fluid Mechanics ◽

10.1017/jfm.2012.328 ◽

2012 ◽

Vol 709 ◽

pp. 191-222 ◽

Cited By ~ 25

Author(s):

T. D. Dunstan ◽

N. Swaminathan ◽

K. N. C. Bray

Keyword(s):

Turbulent Flame ◽

Leading Edge ◽

Propagation Speed ◽

Algebraic Model ◽

Mean Flow ◽

Continuous Growth ◽

Turbulent Premixed Flame ◽

Turbulent Flame Speed ◽

Flame Brush ◽

Turbulent Premixed

AbstractThe sensitivity of the turbulent flame speed to the geometry of the flame is investigated using direct numerical simulations of turbulent premixed flames in three canonical configurations: freely propagating statistically planar flames, planar flames stabilized in stagnating flows, and rod-stabilized V-flames. We consider both the consumption speed, which measures the integrated rate of burning, and the propagation speed, which measures the speed of an isosurface within the flame brush. An algebraic model for the propagation speed of the leading edge of the flame brush, which is blind to flame geometry, is also applied to the data for the purposes of establishing its range of validity and the causes of its failure. The turbulent consumption speed is found to be strongly geometry dependent, primarily due to the continuous growth of the flame brush thickness. Changes in the structure and consumption speed of instantaneous flame fronts are found to be only weakly sensitive to flame geometry. The turbulent propagation speed is analysed in terms of its reactive, diffusive and turbulent flux components. All three terms are shown to be significant, both through the flame brush and along the leading edge. The leading-edge propagation speed is found to be sensitive to flame geometry only in the V-flames under certain conditions. It is suggested that this apparent geometry dependence, which the model cannot capture, results from the relation between the turbulence and mean flow time scales in these particular cases, and is not intrinsic to the flame geometry itself.

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Effects of pressure gradients on turbulent premixed flames

Journal of Fluid Mechanics ◽

10.1017/s0022112097007556 ◽

1997 ◽

Vol 353 ◽

pp. 83-114 ◽

Cited By ~ 64

Author(s):

DENIS VEYNANTE ◽

THIERRY POINSOT

Keyword(s):

Pressure Gradient ◽

Turbulent Transport ◽

Turbulent Flame ◽

Premixed Flames ◽

Flame Speed ◽

Pressure Gradients ◽

Turbulent Premixed Flames ◽

Turbulent Flame Speed ◽

Flame Brush ◽

Turbulent Premixed

In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations.A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale lt.A simple model for the turbulent flux u″c″ is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.

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