Surface morphology and inner fractal cutoff scale of spherical turbulent premixed flames in decaying isotropic turbulence
Autor: | Fabrizio Bisetti, Tejas Kulkarni |
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Rok vydání: | 2021 |
Předmět: |
Length scale
Physics Scale (ratio) Turbulence Mechanical Engineering General Chemical Engineering Kolmogorov microscales Reynolds number Mechanics 01 natural sciences Fractal dimension 010305 fluids & plasmas Physics::Fluid Dynamics symbols.namesake Fractal 0103 physical sciences symbols Physical and Theoretical Chemistry 010306 general physics Scaling |
Zdroj: | Proceedings of the Combustion Institute. 38:2861-2868 |
ISSN: | 1540-7489 |
Popis: | The surface of turbulent premixed flames is fractal within a finite range of scales and the fractal dimension and inner cutoff scale are key components of fractal turbulent combustion closures. In such closures, the estimate for the surface area is sensitive to the value of the inner fractal cutoff scale, whose modeling remains unclear and for which both experimental and numerical contradictory evidence exists. In this work, we analyze data from six direct numerical simulations of spherically expanding turbulent premixed flames at varying Reynolds and Karlovitz numbers. The flames propagate in decaying isotropic turbulence and fall in the flamelet regime. Past an initial transient, we find that the fractal dimension reaches an asymptotic value between 2.3 and 2.4 in good agreement with previous results at similar conditions. A minor dependence of the fractal dimension on the Reynolds and Karlovitz numbers is observed and explained by the relatively low values of the Reynolds number and narrow inertial and fractal ranges. The inner fractal cutoff scale Δ* is found to scale as Δ * / l ∼ Re λ − 1.14 , where l is the integral scale of turbulence and Reλ is the Reynolds number based on the Taylor micro-scale computed in the turbulence on the reactants’ side. The scaling is robust and insensitive to the Karlovitz number over the range of values considered in this study. An important implication is that the ratio Δ*/η grows with Reynolds number (η is the Kolmogorov scale), albeit at a rather slow rate that may explain the widespread observation that 4 ≤ Δ*/η ≤ 10. This suggests that Δ*, although smaller than λ, is not a dissipative length scale for the flame surface and scaled solely by η. Finally, a dissipative threshold scale that remains constant once normalized by η is identified. |
Databáze: | OpenAIRE |
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