Effects of Karlovitz number variations on thermodiffusive instabilities in lean turbulent hydrogen jet flames

Lukas Berger*, Antonio Attili, Michael Gauding, Heinz Pitsch

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract / Description of output

Direct numerical simulations of turbulent lean hydrogen flames in a slot burner configuration have been performed for different Karlovitz numbers of Ka=20, 50, and 230 at constant jet Reynolds number of Re=11,000. All flames feature strong thermodiffusive effects, which arise from the significant disparity of the thermal and molecular diffusive fluxes due to the low Lewis number of hydrogen and have a leading order effect on the flame dynamics. In turbulent flows, these effects are even amplified and prevail over a large range of Karlovitz numbers. Thus, in this work, the effects of thermodiffusive instabilities are quantitatively assessed at different Karlovitz numbers with a particular focus on the turbulent flame speed. The latter is remarkably high due to the presence of instabilities and is decomposed into its contributions of flame wrinkling and effects on the local reactivity. For the surface area wrinkling, a monotonic reduction of the impact of instabilities is observed, which is analyzed by means of the local stretch rate and flame morphology. Relevant contributions of thermodiffusive instabilities are observed up to a Karlovitz number of Ka=50. In contrast, the variations of the local reactivity, which are directly linked to the fluctuations of local equivalence ratio within the flame, reveal a non-monotonic trend and peak for the intermediate Karlovitz number case. Most noteworthy, the total effect of reactivity, referred to as the stretch factor, is found to monotonically increase with increasing Karlovitz number. This results from an increase of the mean equivalence ratio within the flame due to turbulent strain, which compensates the reduction of equivalence ratio fluctuations for high Karlovitz numbers. The strain effect is discussed by means of laminar counterflow flamelets that are exposed to the same mean strain rate as the turbulent flames, yielding a good representation of strain rate effects in turbulent flames.

Original languageEnglish
Article number105219
JournalProceedings of the Combustion Institute
Volume40
Issue number1-4
Early online date5 Jul 2024
DOIs
Publication statusE-pub ahead of print - 5 Jul 2024

Keywords / Materials (for Non-textual outputs)

  • Direct numerical simulations
  • Hydrogen
  • Karlovitz number
  • Thermodiffusive instabilities
  • Turbulent flame speed

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