2 research outputs found
The Interaction of High-Speed Turbulence with Flames: Turbulent Flame Speed
(Abridged) Direct numerical simulations of the interaction of a premixed
flame with driven, subsonic, homogeneous, isotropic, Kolmogorov-type turbulence
in an unconfined system are used to study the mechanisms determining the
turbulent flame speed, S_T, in the thin reaction zone regime. High intensity
turbulence is considered with the r.m.s. velocity 35 times the laminar flame
speed, S_L, resulting in the Damkohler number Da = 0.05. Here we show that: (1)
The flame brush has a complex internal structure, in which the isosurfaces of
higher fuel mass fractions are folded on progressively smaller scales. (2)
Global properties of the turbulent flame are best represented by the structure
of the region of peak reaction rate, which defines the flame surface. (3) In
the thin reaction zone regime, S_T is predominantly determined by the increase
of the flame surface area, A_T, caused by turbulence. (4) The observed increase
of S_T relative to S_L exceeds the corresponding increase of A_T relative to
the surface area of the planar laminar flame, on average, by ~14%, varying from
only a few percent to ~30%. (5) This exaggerated response is the result of
tight flame packing by turbulence, which causes frequent flame collisions and
formation of regions of high flame curvature, or "cusps." (6) The local flame
speed in the cusps substantially exceeds its laminar value, which results in a
disproportionately large contribution of cusps to S_T compared with the flame
surface area in them. (7) A criterion is established for transition to the
regime significantly influenced by cusp formation. In particular, at Karlovitz
numbers Ka > 20, flame collisions provide an important mechanism controlling
S_T, in addition to the increase of A_T by large-scale motions and the
potential enhancement of diffusive transport by small-scale turbulence.Comment: 44 pages, 20 figures; published in Combustion and Flam