Multiscale analysis of head-on quenching premixed turbulent flames

Ahmed, Umair; Doan, Nguyen Anh Khoa; Lai, Jiawei; Klein, Markus; Chakraborty, Nilanjan; Swaminathan, Nedunchezhian

doi:10.1063/1.5047061

Cited by 37 publications

(30 citation statements)

References 42 publications

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“…This follows from the fact that the fluid velocity is affected by thermal expansion arising from heat release, and not by the intermediate steps of a chemical reaction. For example, the enstrophy transport characteristics obtained from simplified chemistry DNS [21,22] of turbulent premixed flames have been found to be qualitatively consistent with detailed chemistry results [23,24]. Thus, the findings regarding the SF statistics are likely to be qualitatively valid in the presence of detailed chemistry and transport.…”

Section: Mathematical Background and Numerical Implementationsupporting

confidence: 61%

Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime

Brearley

Ahmed

Chakraborty

et al. 2020

Fluids

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The second-order velocity structure function statistics have been analysed using a DNS database of statistically planar turbulent premixed flames subjected to unburned gas forcing. The flames considered here represent combustion for moderate values of Karlovitz number from the wrinkled flamelets to the thin reaction zones regimes of turbulent premixed combustion. It has been found that the second-order structure functions exhibit the theoretical asymptotic scalings in the dissipative and (relatively short) inertial ranges. However, the constant of proportionality for the theoretical asymptotic variation for the inertial range changes from one case to another, and this value also changes with structure function orientation. The variation of the structure functions for small length scale separation remains proportional to the square of the separation distance. However, the constant of proportionality for the limiting behaviour according to the separation distance square remains significantly different from the theoretical value obtained in isotropic turbulence. The disagreement increases with increasing turbulence intensity. It has been found that turbulent velocity fluctuations within the flame brush remain anisotropic for all cases considered here and this tendency strengthens towards the trailing edge of the flame brush. It indicates that the turbulence models derived based on the assumptions of homogeneous isotropic turbulence may not be fully valid for turbulent premixed flames.

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Section: Mathematical Background and Numerical Implementationsupporting

confidence: 61%

Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime

Brearley

Ahmed

Chakraborty

et al. 2020

Fluids

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“…As the present analysis focuses on the fluid-dynamical aspects of turbulent premixed combustion, the simplification in terms of chemistry is not expected to influence the results of this paper and the conclusions drawn from them. Previous studies demonstrated that the enstrophy statistics obtained from simple chemistry DNS data 12,14,42 remain in good qualitative agreement with the corresponding statistics obtained from detailed chemistry DNS. 12,39 In SENGA+, all the spatial derivates for the internal grid points are evaluated using a 10 th order central difference scheme and the order of accuracy gradually drops to a one-sided 2 nd order scheme at the non-periodic boundaries.…”

Section: Numerical Implementationsupporting

confidence: 62%

“…53,54 The qualitative nature of the findings of the current investigation is unlikely to be modified in the presence of detailed chemistry, as indicated in previous analyses. 15,42 However, three-dimensional detailed chemistry DNS data for high values of turbulent Reynolds number will be necessary for deeper understanding of vorticity and enstrophy evolutions in premixed turbulent flames under the influence of external body forces. Furthermore, the present analysis does not address the wall-induced shear effects, which are likely to have significant influences on the vorticity transformation mechanisms for vorticity dynamics in turbulent reacting flows.…”

Section: Discussionmentioning

confidence: 99%

Effects of body forces on vorticity and enstrophy evolutions in turbulent premixed flames

2021

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The effects of body forces (alternatively Froude number) on both vorticity and enstrophy evolutions within the flame brush have been analysed using Direct Numerical Simulations (DNS) data of freely propagating statistically planar turbulent premixed flames subjected to different turbulence intensities.The turbulence parameters are taken to represent the thin reaction zone regime of premixed turbulent combustion. The enstrophy has been found to decay significantly from the unburned to the burned gas side of the flame brush for high turbulence intensities and this trend is particularly prominent for the strengthening of the body force promoting unstable stratification. However, local instances of enstrophy generation have been observed and in some cases the decay of enstrophy is arrested across the flame brush for small turbulence intensities. This trend strengthens with the increasing magnitude of the body force promoting stable stratification. The enstrophy generation due to the baroclinic torque is primarily responsible for this local enstrophy generation for small turbulence intensities especially under the body force promoting stable stratification. This baroclinic torque contribution is also found to be responsible for anisotropic behaviour of vorticity components within the flame brush. The vortex stretching and viscous dissipation terms have been found to be the leading order source and sink terms, respectively, in the enstrophy transport for high turbulence intensities especially in the case of body force promoting unstable stratification. However, baroclinic torque, and the sink term due to dilatation rate continue to play significant roles even for high turbulence intensity cases considered here but their relative importance increases with decreasing turbulence intensity especially under the body force promoting stable stratification. The surface-weighted entrainment velocity has been found to be mostly unaffected by the body force in this analysis, and a minor influence can be discerned in the case of small turbulence intensities where an unstable stratification tends to promote high negative values of entrainment velocity only towards the unburned gas side of the flame brush.

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“…However, in case BR, premixed combustion takes place in the thin reaction zones regime for small values of , and therefore in this case vorticity alignment with local principal strain rates resembles the corresponding behaviour of non-reacting flows. It was demonstrated recently in a number of studies (Leung et al 2012;Doan et al 2017;Ahmed et al 2018) that vorticity aligns preferentially with the most extensive principal strain rate for both non-reacting flows and premixed turbulent flames when the self-induced vorticity by the vortex tube is excluded (i.e. only the non-local effects are considered).…”

Section: Effects Of Heat Release On Alignment Of Vorticity and Reactimentioning

confidence: 99%

Influence of Thermal Expansion on Fluid Dynamics of Turbulent Premixed Combustion and Its Modelling Implications

Chakraborty

2021

Flow Turbulence Combust

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The purpose of this paper is to demonstrate the effects of thermal expansion, as a result of heat release arising from exothermic chemical reactions, on the underlying turbulent fluid dynamics and its modelling in the case of turbulent premixed combustion. The thermal expansion due to heat release gives rise to predominantly positive values of dilatation rate within turbulent premixed flames, which has been shown to have significant implications on the flow topology distributions, and turbulent kinetic energy and enstrophy evolutions. It has been demonstrated that the magnitude of predominantly positive dilatation rate provides the measure of the strength of thermal expansion. The influence of thermal expansion on fluid turbulence has been shown to strengthen with decreasing values of Karlovitz number and characteristic Lewis number, and with increasing density ratio between unburned and burned gases. This is reflected in the weakening of the contributions of flow topologies, which are obtained only for positive values of dilatation rate, with increasing Karlovitz number. The thermal expansion within premixed turbulent flames not only induces mostly positive dilatation rate but also induces a flame-induced pressure gradient due to flame normal acceleration. The correlation between the pressure and dilatation fluctuations, and the vector product between density and pressure gradients significantly affect the evolutions of turbulent kinetic energy and enstrophy within turbulent premixed flames through pressure-dilatation and baroclinic torque terms, respectively. The relative contributions of pressure-dilatation and baroclinic torque in comparison to the magnitudes of the other terms in the turbulent kinetic energy and enstrophy transport equations, respectively strengthen with decreasing values of Karlovitz and characteristic Lewis numbers. This leads to significant augmentations of turbulent kinetic energy and enstrophy within the flame brush for small values of Karlovitz and characteristic Lewis numbers, but both turbulent kinetic energy and enstrophy decay from the unburned to the burned gas side of the flame brush for large values of Karlovitz and characteristic Lewis numbers. The heat release within premixed flames also induces significant anisotropy of sub-grid stresses and affects their alignments with resolved strain rates. This anisotropy plays a key role in the modelling of sub-grid stresses and the explicit closure of the isotropic part of the sub-grid stress has been demonstrated to improve the performance of sub-grid stress and turbulent kinetic energy closures. Moreover, the usual dynamic modelling techniques, which are used for non-reacting turbulent flows, have been shown to not be suitable for turbulent premixed flames. Furthermore, the velocity increase across the flame due to flame normal acceleration may induce counter-gradient transport for turbulent kinetic energy, reactive scalars, scalar gradients and scalar variances in premixed turbulent flames under some conditions. The propensity of counter-gradient transport increases with decreasing values of root-mean-square turbulent velocity and characteristic Lewis number. It has been found that vorticity aligns predominantly with the intermediate principal strain rate eigendirection but the relative extents of alignment of vorticity with the most extensive and the most compressive principal strain rate eigendirections change in response to the strength of thermal expansion. It has been found that dilatation rate almost equates to the most extensive strain rate for small sub-unity Lewis numbers and for the combination of large Damköhler and small Karlovitz numbers, and under these conditions vorticity shows no alignment with the most extensive principal strain rate eigendirection but an increased collinear alignment with the most compressive principal strain rate eigendirection is obtained. By contrast, for the combination of high Karlovitz number and low Damköhler number in the flames with Lewis number close to unity, vorticity shows an increased collinear alignment with the most extensive principal direction in the reaction zone where the effects of heat release are strong. The strengthening of flame normal acceleration in comparison to turbulent straining with increasing values of density ratio, Damköhler number and decreasing Lewis number makes the reactive scalar gradient align preferentially with the most extensive principal strain rate eigendirection, which is in contrast to preferential collinear alignment of the passive scalar gradient with the most compressive principal strain rate eigendirection. For high Karlovitz number, the reactive scalar gradient alignment starts to resemble the behaviour observed in the case of passive scalar mixing. The influence of thermal expansion on the alignment characteristics of vorticity and reactive scalar gradient with local principal strain rate eigendirections dictates the statistics of vortex-stretching term in the enstrophy transport equation and normal strain rate contributions in the scalar dissipation rate and flame surface density transport equations, respectively. Based on the aforementioned fundamental physical information regarding the thermal expansion effects on fluid turbulence in premixed combustion, it has been argued that turbulence and combustion modelling are closely interlinked in turbulent premixed combustion. Therefore, it might be necessary to alter and adapt both turbulence and combustion modelling strategies while moving from one combustion regime to the other.

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Multiscale analysis of head-on quenching premixed turbulent flames

Cited by 37 publications

References 42 publications

Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime

Scaling of Second-Order Structure Functions in Turbulent Premixed Flames in the Flamelet Combustion Regime

Effects of body forces on vorticity and enstrophy evolutions in turbulent premixed flames

Influence of Thermal Expansion on Fluid Dynamics of Turbulent Premixed Combustion and Its Modelling Implications

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