Flow topology and alignments of scalar gradients and vorticity in turbulent spray flames: A Direct Numerical Simulation analysis

Proceedings of the Combustion Institute

Ketterl

et al. 2019

Self Cite

The Favre-averaged scalar dissipation rate transport conditional on local flow topologies in premixed turbulent flames has been analysed based on a detailed chemistry Direct Numerical Simulation database of statistically planar turbulent hydrogen-air premixed flames with an equivalence ratio of 0.7 representing the corrugated flamelets, thin reaction zones and broken reaction zones regimes of premixed turbulent combustion. The local flow topologies have been categorised by the values of the three invariants of the velocity gradient tensor and the statistical behaviour of the Favre-averaged scalar dissipation rate conditional on these flow topologies has been analysed in detail for different choices of reaction progress variable. The qualitative behaviour of the scalar-turbulence interaction term in the Favre-averaged scalar dissipation rate transport equation has been found to be affected by the regime of combustion, whereas the chemical reaction rate gradient contribution to the scalar dissipation rate transport has been found to be affected by the choice of the reaction progress variable. The topologies, which exist for all values of dilatation rate, contribute significantly to the Favre-averaged scalar dissipation rate transport in premixed turbulent flames for all regimes of combustion. In addition, the flow topologies, which are obtained only for positive values of dilatation rate, contribute significantly to the Favre-averaged scalar dissipation rate transport in the case representing the corrugated flamelets regime combustion. An unstable nodal flow topology, which is representative of a counter-flow configuration, has been found to be a dominant contributor to the Favre-averaged scalar dissipation rate transport for all regimes of combustion irrespective of the choice of reaction progress variable.

supporting

confidence: 89%

“…1 [12,13] (S2 topology is schematically shown for later discussion). Relatively limited effort has been directed to the analysis of flow topology distributions in turbulent combustion [14][15][16][17][18][19][20][21]. This analysis considers a detailed chemistry…”

Section: Introductionmentioning

confidence: 99%

Scalar dissipation rate transport conditional on flow topologies in different regimes of premixed turbulent combustion

Proceedings of the Combustion Institute

Ketterl

et al. 2019

Self Cite

“…Recently, Cifuentes and his co-workers [18,19] analysed the distribution of flow topologies across the flame front using simple chemistry DNS database of premixed turbulent flames with unity Lewis number representing the flamelets regime of combustion under decaying turbulence and reported that the probabilities of finding focal (nodal) flow topologies decrease (increase) across the flame front. Flow topology distributions in turbulent spray flames were analysed by Wacks and Chakraborty [20] using DNS data, which demonstrated that the flow topology distribution within the spray flames shows some resemblance to the findings by Cifuentes et al [18] and Grout et al [17]. Recently, Wacks et al [21] analysed flow topology distributions for the different regimes of turbulent premixed combustion and it has been found that the weakening of dilatation rate (in other words weakening of P) from the corrugated flamelets to the thin reaction zones to the broken reaction zones regimes of premixed turbulent combustion plays a key role in the behaviours of the invariants of the velocity gradient tensor and their components, which in turn affects the distribution of flow topologies and their contributions to the evolutions of enstrophy and scalar dissipation rate.…”

Section: Introductionmentioning

confidence: 99%

Flow topology distribution in head-on quenching of turbulent premixed flame: A Direct Numerical Simulation analysis

Lai

2018

Fuel

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“…Cifuentes and co-workers 18 , 19 demonstrated that the probability of finding focal topologies decreases from the unburned to the burned gas side of the flame front, which was also observed by Wacks et al . 20 for droplet combustion. A recent analysis by Wacks et al .…”

Section: Introductionmentioning

confidence: 99%

Enstrophy transport conditional on local flow topologies in different regimes of premixed turbulent combustion

Papapostolou

et al. 2017

Sci Rep

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Enstrophy is an intrinsic feature of turbulent flows, and its transport properties are essential for the understanding of premixed flame-turbulence interaction. The interrelation between the enstrophy transport and flow topologies, which can be assigned to eight categories based on the three invariants of the velocity-gradient tensor, has been analysed here. The enstrophy transport conditional on flow topologies in turbulent premixed flames has been analysed using a Direct Numerical Simulation database representing the corrugated flamelets (CF), thin reaction zones (TRZ) and broken reaction zones (BRZ) combustion regimes. The flame in the CF regime exhibits considerable flame-generated enstrophy, and the dilatation rate and baroclinic torque contributions to the enstrophy transport act as leading order sink and source terms, respectively. Consequently, flow topologies associated with positive dilatation rate values, contribute significantly to the enstrophy transport in the CF regime. By contrast, enstrophy decreases from the unburned to the burned gas side for the cases representing the TRZ and BRZ regimes, with diminishing influences of dilatation rate and baroclinic torque. The enstrophy transport in the TRZ and BRZ regimes is governed by the vortex-stretching and viscous dissipation contributions, similar to non-reacting flows, and topologies existing for all values of dilatation rate remain significant contributors.