Physical space analysis of cross-scale turbulent kinetic energy transfer in premixed swirl flames

Kazbekov, Askar; Steinberg, Adam M.

doi:10.1016/j.combustflame.2021.111403

Cited by 11 publications

(6 citation statements)

References 45 publications

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“…It has been shown in the past [15,16,41] that the phenomenon of counter-gradient transport or counter-gradient stresses, as theoretically predicted by Bray and co-workers [28], is closely related to the axisymmetric expansion turbulent state. Its existence in the context of LES has been confirmed by very recent and advanced measurement techniques [42] that result in the conclusion that LES models should allow for upscale energy transfer in the vicinity of the flame. In addition, in the context of unsteady RANS or hybrid RANS/LES, the present results suggest the invalidity of the Boussinesq assumption and the need for an anisotropic correction, which will depend on how much of the turbulence kinetic energy can be resolved, as revealed by the multiscale analysis.…”

Section: Resultsmentioning

confidence: 81%

Multiscale Analysis of Anisotropy of Reynolds Stresses, Subgrid Stresses and Dissipation in Statistically Planar Turbulent Premixed Flames

et al. 2022

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The characterisation of small-scale turbulence has been an active area of research for decades and this includes, particularly, the analysis of small-scale isotropy, as postulated by Kolmogorov. In particular, the question if the dissipation tensor is isotropic or not, and how it is related to the anisotropy of the Reynolds stresses is of particular interest for modelling purposes. While this subject has been extensively studied in the context of isothermal flows, the situation is more complicated in turbulent reacting flows because of heat release. Furthermore, the landscape of Computational Fluid Dynamics is characterised by a multitude of methods ranging from Reynolds-averaged to Large Eddy Simulation techniques, and they address different ranges of scales of the turbulence kinetic energy spectrum. Therefore, a multiscale analysis of the anisotropies of Reynolds stress, dissipation and sub-grid scale tensor has been performed by using a DNS database of statistically planar turbulent premixed flames. Results show that the coupling between dissipation tensor and Reynolds stress tensor is weaker compared to isothermal turbulent boundary layer flows. In particular, for low and moderate turbulence intensities, heat release induces pronounced anisotropies which affect not only fluctuation strengths but also the characteristic size of structures associated with different velocity components.

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Section: Resultsmentioning

confidence: 81%

Multiscale Analysis of Anisotropy of Reynolds Stresses, Subgrid Stresses and Dissipation in Statistically Planar Turbulent Premixed Flames

et al. 2022

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“…[3][4][5][6][7][8][9][10][11][12][13][14][15] Numerical studies reviewed elsewhere [16][17][18][19] indicate that the influence of combustion-induced thermal expansion on turbulence within a premixed flame brush is well (hardly) pronounced in weakly (highly) turbulent flames. Recent Direct Numerical Simulation 11,[20][21][22][23] (DNS) and experimental 24,25 investigations further support this view. However, criteria for finding domains of importance of such an influence have not yet been well established.…”

Section: Introductionmentioning

confidence: 92%

“…𝑢 𝐾 = (𝜈 𝑢 𝜀̅ ) 1 4 ⁄ and 𝜂 𝐾 = (𝜈 𝑢 3 𝜀̅ ⁄ ) 1 4 ⁄ are Kolmogorov velocity and length scales 25 , respectively; 𝜈 is the kinematic viscosity of unburnt mixture; 𝜀̅ = 2𝜈𝑆 𝑗𝑘 𝑆 𝑗𝑘 ̅̅̅̅̅̅̅̅̅ is a mean dissipation rate; 𝑆 𝑗𝑘 = 0.5(𝜕𝑢 𝑗 𝜕𝑥 𝑘 ⁄ + 𝜕𝑢 𝑘 𝜕𝑥 𝑗 ⁄ ) is the rate-of-strain tensor; and summation convention applies to repeated indices. Note that (i) under conditions of a low Mach number, as in the case under study, dilatational contribution to the mean dissipation rate is commonly neglected if turbulence characteristics in 𝐾𝑎 are evaluated in the incompressible flow of unburnt reactants; and (ii) to properly characterize the dilatation magnitude, the laminar flame thickness should be evaluated as follows: 𝛿 𝐿 = (𝑇 𝑏 − 𝑇 𝑢 ) max|∇𝑇| ⁄ , where 𝑇 is the temperature.…”

Section: Accepted To Phys Fluids 101063/50123211mentioning

confidence: 99%

Effects of thermal expansion on moderately intense turbulence in premixed flames

et al. 2022

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The study aims at analytically and numerically exploring the influence of combustion-induced thermal expansion on turbulence in premixed flames. In the theoretical part, contributions of solenoidal and potential velocity fluctuations to the unclosed component of the advection term in the Reynolds-averaged Navier-Stokes equations are compared and a new criterion for assessing the importance of the thermal expansion effects is introduced. The criterion highlights a ratio of the dilatation in the laminar flame to the large-scale gradient of root-mean-square (rms) velocity in the turbulent flame brush. To support the theoretical study, direct numerical simulation (DNS) data obtained earlier from two complex-chemistry, lean H2-air flames are analyzed. In line with the new criterion, even at sufficiently high Karlovitz numbers, results show significant influence of combustion-induced potential velocity fluctuations on the second moments of the turbulent velocity upstream of and within the flame brush. In particular, the DNS data demonstrate that (i) potential and solenoidal rms velocities are comparable in the unburnt gas close to the leading edge of the flame brush and (ii) potential and solenoidal rms velocities conditioned to unburnt gas are comparable within the entire flame brush. Moreover, combustion-induced thermal expansion affects not only the potential velocity, but even the solenoidal one. The latter effects manifest themselves in a negative correlation between solenoidal velocity fluctuations and dilatation or in the counter-gradient behavior of the solenoidal scalar flux. Finally, a turbulence-in-premixed-flame diagram is sketched to discuss the influence of combustion-induced thermal expansion on various ranges of turbulence spectrum.

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“…For instance, recent experimental data 22,23 obtained from highly turbulent lean methane-air swirl flames show importance of thermal expansion effects such as vorticity generation due to baroclinic torque 22 or backscatter. 23 Thus, there is a clear need for development of an advanced model of turbulence in flames, which could predict phenomena revealed recently and reviewed elsewhere. [10][11][12][13] However, no comprehensive modeling framework to represent distinct roles of thermal expansion on turbulence exists today.…”

Section: Introductionmentioning

confidence: 99%

“…Secondly, weak influence of the thermal expansion on certain turbulence characteristics does not prove that all other turbulence characteristics are also weakly affected by the thermal expansion under the same conditions. For instance, recent experimental data 22,23 obtained from highly turbulent lean methane-air swirl flames show importance of thermal expansion effects such as vorticity generation due to baroclinic torque 22 or backscatter. 23 Thus, there is a clear need for development of an advanced model of turbulence in flames, which could predict phenomena revealed recently and reviewed elsewhere.…”

Section: Introductionmentioning

confidence: 99%

Conditioned structure functions in turbulent hydrogen/air flames

et al. 2022

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Direct numerical simulation data obtained from two turbulent, lean hydrogen-air flames propagating in a box are analyzed to explore the influence of combustion-induced thermal expansion on turbulence in unburned gas. For this purpose, Helmholtz-Hodge decomposition is applied to the computed velocity fields. Subsequently, the second-order structure functions conditioned to unburned reactants are sampled from divergence-free solenoidal velocity field or irrotational potential velocity field, yielded by the decomposition. Results show that thermal expansion significantly affects the conditioned potential structure functions not only inside the mean flame brushes, but also upstream of them. Upstream of the flames, firstly, transverse structure functions for transverse potential velocities grow with distance r between sampling points more slowly when compared to the counterpart structure functions sampled from the entire or solenoidal velocity field. Secondly, the former growth rate depends substantially on the distance from the flame-brush leading edge, even at small r. Thirdly, potential root-mean-square (rms) velocities increase with decreasing distance from the flame-brush leading edge and are comparable with solenoidal rms velocities near the leading edge. Fourthly, although the conditioned axial and transverse potential rms velocities are always close to one another, thus, implying isotropy of the potential velocity field in unburned reactants; the potential structure functions exhibit a high degree of anisotropy. Fifthly, thermal expansion effects are substantial even for the solenoidal structure functions and even upstream of a highly turbulent flame. These findings call for development of advanced models of turbulence in flames, which allow for the discussed thermal expansion effects.

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Physical space analysis of cross-scale turbulent kinetic energy transfer in premixed swirl flames

Cited by 11 publications

References 45 publications

Multiscale Analysis of Anisotropy of Reynolds Stresses, Subgrid Stresses and Dissipation in Statistically Planar Turbulent Premixed Flames

Multiscale Analysis of Anisotropy of Reynolds Stresses, Subgrid Stresses and Dissipation in Statistically Planar Turbulent Premixed Flames

Effects of thermal expansion on moderately intense turbulence in premixed flames

Conditioned structure functions in turbulent hydrogen/air flames

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