A DNS study of extreme and leading points in lean hydrogen-air turbulent flames - part II: Local velocity field and flame topology

Lee, Hsu Chew; Dai, Peng; Wan, Minping; Lipatnikov, Andrei

doi:10.1016/j.combustflame.2021.111712

Cited by 9 publications

(14 citation statements)

References 47 publications

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“…Since the present DNSs are basically similar to our simulations discussed in detail earlier [68,69,70,71,72,73], only a summary of the DNS attributes is given below.…”

Section: B Dns Attributesmentioning

confidence: 69%

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Turbulent burning velocity and thermo-diffusive instability of premixed flames

Lee¹,

Wu²,

Dai³

et al. 2023

Preprint

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Reported in the paper are results of unsteady three-dimensional direct numerical simulations of laminar and turbulent, lean hydrogen-air, complex-chemistry flames propagating in forced turbulence in a box. To explore the eventual influence of thermo-diffusive instability of laminar flames on turbulent burning velocity, (i) a critical length scale Λn that bounds regimes of unstable and stable laminar combustion is numerically determined by gradually decreasing the width Λ of computational domain until a stable laminar flame is obtained and (ii) simulations of turbulent flames are performed by varying the width from Λ < Λn (in this case, the instability is suppressed) to Λ > Λn (in this case, the instability may grow). Moreover, simulations are performed either using mixtureaveraged transport properties (low Lewis number flames) or setting diffusivities of all species equal to heat diffusivity of the mixture (equidiffusive flames), with all other things being equal. Obtained results show a significant increase in turbulent burning velocity UT when the boundary Λ = Λn is crossed in weak turbulence, but almost equal values of UT are computed at Λ < Λn and Λ > Λn in moderately turbulent flames characterized by Karlovitz number equal to 3.4 or larger. These results imply that thermo-diffusive instability of laminar premixed flames substantially affects burning velocity in weak turbulence only, in line with a simple criterion proposed by Chomiak and Lipatnikov (Phys. Rev. E 107, 015102, 2023).

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“…Since the present DNSs are basically similar to our simulations discussed in detail earlier [68,69,70,71,72,73], only a summary of the DNS attributes is given below.…”

Section: B Dns Attributesmentioning

confidence: 69%

“…Since the computed results agreed with Eq. ( 1), there was no need for running DNS by increasing u /S L , especially as DNS results obtained recently from highly turbulent flames propagating in wider boxes were already reported by us [68,69,70,71,72,73].…”

Section: A Key Pointmentioning

confidence: 81%

Turbulent burning velocity and thermo-diffusive instability of premixed flames

Lee¹,

Wu²,

Dai³

et al. 2023

Preprint

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“…2015; Lee et al. 2022 b ; Rieth et al. 2022), since the former rate is mainly controlled by reactions that consume H, but do not consume H, the highest may be reached in negatively curved low-temperature reaction zones due to a significant increase in the local concentration of atomic hydrogen diffused rapidly from surrounding combustion products.…”

Section: Resultsmentioning

confidence: 99%

“…Cases A, A1, C and C1 were addressed in earlier papers (Lee et al. 2021, 2022 a , b ). All other cases were studied by Lee et al.…”

Section: Direct Numerical Simulation Attributesmentioning

confidence: 99%

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Displacement speed, flame surface density and burning rate in highly turbulent premixed flames characterized by low Lewis numbers

et al. 2023

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Direct numerical simulation data obtained from four pairs of turbulent, lean hydrogen–air, complex-chemistry flames are analysed to explore the influence of molecular diffusion on flame surface density, displacement speed $S_d$ and the flame surface density transport equation terms. Each pair involves (i) a flame where mixture-averaged molecular diffusivities are adopted and Lewis number $Le$ is significantly less than unity and (ii) an equidiffusive flame where all molecular diffusivities are set equal to molecular heat diffusivity of the mixture and $Le=1$ , with other things being equal. Reported results show that significantly higher turbulent burning rates simulated in the former flames result mainly from an increase in the local fuel consumption rate, whereas an increase in flame surface area plays a secondary role, especially in more intense turbulence. The rate increase stems from (i) an increase in the peak local fuel consumption rate and (ii) an increase in a width of a zone where the rate is significant. The latter phenomenon is of more importance in richer flames and both phenomena are most pronounced in the vicinity of the flame leading edges, thus indicating a crucial role played by the leading edge of a premixed turbulent flame in its propagation. Moreover, mean displacement speed differs significantly from the laminar flame speed even in the equidiffusive flames, varies substantially across flame brush and may be negative at the leading edges of highly turbulent flames.

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