9 research outputs found
The Interaction of High-Speed Turbulence with Flames: Global Properties and Internal Flame Structure
We study the dynamics and properties of a turbulent flame, formed in the
presence of subsonic, high-speed, homogeneous, isotropic Kolmogorov-type
turbulence in an unconfined system. Direct numerical simulations are performed
with Athena-RFX, a massively parallel, fully compressible, high-order,
dimensionally unsplit, reactive-flow code. A simplified reaction-diffusion
model represents a stoichiometric H2-air mixture. The system being modeled
represents turbulent combustion with the Damkohler number Da = 0.05 and with
the turbulent velocity at the energy injection scale 30 times larger than the
laminar flame speed. The simulations show that flame interaction with
high-speed turbulence forms a steadily propagating turbulent flame with a flame
brush width approximately twice the energy injection scale and a speed four
times the laminar flame speed. A method for reconstructing the internal flame
structure is described and used to show that the turbulent flame consists of
tightly folded flamelets. The reaction zone structure of these is virtually
identical to that of the planar laminar flame, while the preheat zone is
broadened by approximately a factor of two. Consequently, the system evolution
represents turbulent combustion in the thin-reaction zone regime. The turbulent
cascade fails to penetrate the internal flame structure, and thus the action of
small-scale turbulence is suppressed throughout most of the flame. Finally, our
results suggest that for stoichiometric H2-air mixtures, any substantial flame
broadening by the action of turbulence cannot be expected in all subsonic
regimes.Comment: 30 pages, 9 figures; published in Combustion and Flam
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
Models for Type Ia supernovae and related astrophysical transients
We give an overview of recent efforts to model Type Ia supernovae and related
astrophysical transients resulting from thermonuclear explosions in white
dwarfs. In particular we point out the challenges resulting from the
multi-physics multi-scale nature of the problem and discuss possible numerical
approaches to meet them in hydrodynamical explosion simulations and radiative
transfer modeling. We give examples of how these methods are applied to several
explosion scenarios that have been proposed to explain distinct subsets or, in
some cases, the majority of the observed events. In case we comment on some of
the successes and shortcoming of these scenarios and highlight important
outstanding issues.Comment: 20 pages, 2 figures, review published in Space Science Reviews as
part of the topical collection on supernovae, replacement corrects typos in
the conclusions sectio