A comparison of entrainment velocity and displacement speed statistics in different regimes of turbulent premixed combustion

Embargo End Date
2022-09-01

Type
Article

Authors
Chakraborty, Nilanjan
Klein, M.
Im, Hong G.

KAUST Department
Clean Combustion Research Center
Computational Reacting Flow Laboratory (CRFL)
Mechanical Engineering Program
Physical Science and Engineering (PSE) Division

Online Publication Date
2020-09-01

Print Publication Date
2020-09

Date
2020-09-01

Submitted Date
2019-11-06

Abstract
The statistics of entrainment velocity, defined as the displacement speed of an enstrophy isosurface, which can be taken to be the interface between turbulent/non-turbulent regions, have been analysed using a Direct Numerical Simulation database of statistically planar H2-air flames with a range of different Karlovitz numbers. It has been found that the component of the entrainment velocity arising from molecular dissipation plays a leading order role for all values of Karlovitz number, whereas the relative importance of the baroclinic torque and dilatation rate weakens with increasing Karlovitz number. By contrast, the relative contribution of the entrainment velocity component arising from vortex-stretching strengthens with increasing Karlovitz number Ka. The mean entrainment velocity remains positive for the case representing the corrugated flamelets regime (i.e. Ka < 1), whereas it assumes negative values in the cases with large values of Karlovitz number (i.e. Ka ≫ 1). The magnitude of the ratio of the mean values of entrainment velocity to the mean values of flame displacement speed conditional upon non-dimensional temperature within the flame front remains of the order of unity irrespective of Karlovitz number. However, the probability density functions of entrainment velocity exhibit considerably higher probabilities of finding large magnitudes than in the case of flame displacement speed. The alignments between the normal vector on the enstrophy isosurface and local principal strain rates have been found to be qualitatively similar to the corresponding alignments between flame normal and local principal strain rates, and the same holds true for the distributions of curvature shape factor of reaction progress variable and enstrophy isosurfaces. These findings indicate that the isosurface topologies and the alignments of normal vectors with local principal strain rates for enstrophy and reaction progress variable exhibit qualitatively similar behaviours. Consequently, turbulence and combustion modelling strategies cannot be considered in isolation in premixed turbulent flames.

Citation
Chakraborty, N., Klein, M., & Im, H. G. (2020). A comparison of entrainment velocity and displacement speed statistics in different regimes of turbulent premixed combustion. Proceedings of the Combustion Institute. doi:10.1016/j.proci.2020.06.241

Acknowledgements
NC is grateful to EPSRC (EP/K025163/1, EP/R029369/1) and ARCHER for computational support. HGI is grateful to KAUST for research funding and computational support respectively.

Publisher
Elsevier BV

Journal
Proceedings of the Combustion Institute

DOI
10.1016/j.proci.2020.06.241

Additional Links
https://linkinghub.elsevier.com/retrieve/pii/S1540748920303333

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