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- Lipatnikov, Andrei, 1961, et al.
(författare)
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A direct numerical simulation study of vorticity transformation in weakly turbulent premixed flames
- 2014
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 26:10
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Tidskriftsartikel (refereegranskat)abstract
- Database obtained earlier in 3D Direct Numerical Simulations (DNS) of statistically stationary, 1D, planar turbulent flames characterized by three different density ratios σ is processed in order to investigate vorticity transformation in premixed combustion under conditions of moderately weak turbulence (rms turbulent velocity and laminar flame speed are roughly equal to one another). In cases H and M characterized by σ = 7.53 and 5.0, respectively, anisotropic generation of vorticity within the flame brush is reported. In order to study physical mechanisms that control this phenomenon, various terms in vorticity and enstrophy balance equations are analyzed, with both mean terms and terms conditioned on a particular value c of the combustion progress variable being addressed. Results indicate an important role played by baroclinic torque and dilatation in transformation of average vorticity and enstrophy within both flamelets and flame brush. Besides these widely recognized physical mechanisms, two other effects are documented. First, viscous stresses redistribute enstrophy within flamelets, but play a minor role in the balance of the mean enstrophy Ω ¯ ¯ ¯ within turbulent flame brush. Second, negative correlation u ′ ⋅∇Ω ′ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ ¯ between fluctuations in velocity u and enstrophy gradient contributes substantially to an increase in the mean Ω ¯ ¯ ¯ within turbulent flame brush. This negative correlation is mainly controlled by the positive correlation between fluctuations in the enstrophy and dilatation and, therefore, dilatation fluctuations substantially reduce the damping effect of the mean dilatation on the vorticity and enstrophy fields. In case L characterized by σ = 2.5, these effects are weakly pronounced and Ω ¯ ¯ ¯ is reduced mainly due to viscosity. Under conditions of the present DNS, vortex stretching plays a minor role in the balance of vorticity and enstrophy within turbulent flame brush in all three cases.
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| 2. |
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| 3. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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Combustion-induced local shear layers within premixed flamelets in weakly turbulent flows
- 2018
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1089-7666 .- 1070-6631. ; 30:8, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- 3D Direct Numerical Simulation data obtained from statistically stationary, planar, weakly turbulent, premixed flames, which are characterized by two different density ratios (7.53 and 2.50) and are associated with the flamelet combustion regime, are analyzed to investigate differences between velocity and pressure variations (i) in flamelets in a weakly turbulent flow and (ii) in the counterpart laminar flame. Results show that while the thermo-chemical structure of the flamelets is weakly affected by turbulence under the studied conditions, the local velocity, vorticity, and pressure fields within the flamelets differ significantly from the velocity, vorticity, and pressure, respectively, fields within the laminar flame. In particular, local shear layers appear within flamelets in the turbulent flow, because acceleration of a reacting mixture by the local pressure gradient is inversely proportional to the mixture density and, hence, depends on the mixture state. The shear layers are characterized by large velocity gradients (both the tangential gradient of the normal velocity with respect to the flamelet surface and the normal gradient of the tangential velocity), whose magnitudes may be comparable with the magnitude of the velocity gradient across the laminar flame. In flamelet zones characterized by a relatively large magnitude of the locally normal gradient of the tangential velocity, the local vorticity magnitude is also large and such zones contribute substantially to the overall generation of vorticity due to baroclinic torque. These results cast doubts on the validity of a simple common modeling approach that consists in directly invoking expressions derived for the laminar flames in order to describe the influence of combustion-induced thermal expansion on weakly turbulent velocity and pressure fields.
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| 4. |
- Lipatnikov, Andrei, 1961, et al.
(författare)
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DNS assessment of a simple model for evaluating velocity conditioned to unburned gas in premixed turbulent flames
- 2015
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Ingår i: Flow, Turbulence and Combustion. - : Springer Science and Business Media LLC. - 1573-1987 .- 1386-6184. ; 94:3, s. 513-526
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Tidskriftsartikel (refereegranskat)abstract
- Recently, a simple model for evaluating turbulent scalar flux in premixed flames was developed and validated using six experimental data sets obtained from flames stabilized in impinging jets (Sabelnikov and Lipatnikov, Combust. Sci. Technol. 183, 588-613, 2011; Sabelnikov and Lipatnikov, Flow Turbulence Combust. 90, 387-400, 2013). The model addresses the flamelet regime of premixed turbulent combustion and yields an algebraic expression for the mean velocity conditioned to unburned mixture, while turbulent scalar flux is evaluated substituting this conditioned velocity into the well-known Bray-Moss-Libby (BML) expressions. The present work aims at further assessment of the aforementioned model against two well-known 3D DNS databases obtained from statistically planar, 1D premixed turbulent flames characterized by various density ratios (7.53, 5.0, 3.3, and 2.5). For the highest density ratio, an excellent agreement between the model and DNS data was obtained. This result is particularly encouraging, because the experimental data used earlier to test the model are associated with approximately the same (7-8) density ratios. However, the DNS data obtained for lower density ratios indicate a trend, not addressed by the original model, i.e. a model parameter is not a constant but decreases with decreasing density ratio, with the dependence of the model parameter on the density ratio being roughly linear for three flames addressed by one DNS database. Implementation of this linear fit into the model makes it consistent both with the DNS and with all experimental data used earlier to validate the original model.
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| 5. |
- Sabelnikov, Vladimir, 1946, et al.
(författare)
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A transport equation for reaction rate in turbulent flows
- 2016
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Ingår i: Physics of Fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 28:081701, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- New transport equations for chemical reaction rate and its mean value in turbulent flows have been derived and analyzed. Local perturbations of the reaction zone byturbulent eddies are shown to play a pivotal role even for weakly turbulent flows. The mean-reaction-rate transport equation is shown to involve two unclosed dominantterms and a joint closure relation for the sum of these two terms is developed. Obtained analytical results and, in particular, the closure relation are supported byprocessing two widely recognized sets of data obtained from earlier direct numerical simulations of statistically planar 1D premixed flames associated with bothweak large-scale and intense small-scale turbulence.
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