Dynamics and kinematics of the reactive scalar gradient in weakly turbulent premixed flames

In turbulent flames, chemical species mixing rates are controlled to a large extent by the geometric alignment of the species composition gradients with the local velocity gradients. This alignment indeed characterizes the turbulence–scalar interaction (TSI), which is one of the leading-order source...

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Bibliographic Details
Published in:Combustion and flame 2018-12, Vol.198, p.436-454
Main Authors: Zhao, Song, Er-raiy, Aimad, Bouali, Zakaria, Mura, Arnaud
Format: Article
Language:English
Subjects:
Acoustics
Alignment
Automatic
Biomechanics
Budgeting
Chemical Sciences
Combustion
Compressibility
Computational fluid dynamics
Computer simulation
Direct numerical simulation
Electric power
Electromagnetism
Emissions
Engineering Sciences
Engines
Evolution
Flame retardants
Fluid mechanics
Fluidized bed combustion
Geometrical statistics
Heating
Inspection
Isotropic turbulence
Kinematics
Material chemistry
Materials and structures in mechanics
Mathematical analysis
Mathematical Physics
Mechanics
Motors
Organic chemistry
Orientation
Orientation analysis
Physics
Polymers
Premixed flames
Quantum Physics
Reactive fluid environment
Rotation
Strain rate
Tensors
Thermics
Thickening
Turbulence–scalar interaction
Turbulent flames
Turbulent premixed flame
Vibrations
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Summary:In turbulent flames, chemical species mixing rates are controlled to a large extent by the geometric alignment of the species composition gradients with the local velocity gradients. This alignment indeed characterizes the turbulence–scalar interaction (TSI), which is one of the leading-order source terms in the scalar dissipation rate (SDR) evolution. In situations featuring density variations, which may be relevant either to (i) non-reactive but multiphase or compressible flows, or to (ii) reactive flows such as those considered herein, it should be acknowledged that there is still some controversy about the role of dilatational effects and its connection with the rotation of the strain-rate tensor principal axes. These two issues are analyzed by considering direct numerical simulation databases of flame kernel growth in homogeneous isotropic turbulence (HIT). Two distinct conditions of turbulence–combustion interaction (TCI) are considered: the first is a quasi-laminar reference case while the second displays some local thickening of the preheating zone. Special emphasis is placed on the kinematics of the scalar gradient orientation, which is studied through the direct inspection of the reactive scalar gradient orientation budget. To the best of the authors’ knowledge, this is first time that such a budget is scrutinized in premixed combustion conditions. The analysis shows that, for the flow conditions that are presently investigated, the role associated to the rotation of the strain-rate eigenframe is of paramount importance: it is the leading-order term in the scalar gradient orientation budget. This study also confirms that, in the vicinity of the flame, the resulting evolution opposes the rise of the reactive scalar gradient norm (i.e., the rise of the SDR) but the corresponding effects are lessened as the normalized root mean square (RMS) of the velocity fluctuations is increased.
ISSN:0010-2180
1556-2921
DOI:10.1016/j.combustflame.2018.10.002