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Turbulence-Flame Interactions in Lean Premixed Hydrogen Flames


Introduction

Based on series of experiments of highly-stretched methane Bunsen flames by Mansour and Chen, Peters (2000) concludes that turbulent flames are quenched when turbulence levels become sufficiently high, see also the vortex-flame interactions studied experimentally by Roberts et al. and numerically by Poinsot et al.

However, in a recent study by Aspden et al. (2008) in supernova flames, global extinction was not observed, even at a Karlovitz number of 230. In fact, the turbulent flame speed was found to be around 5 or 6 times the laminar flame speed. Turbulent mixing was able to disrupt the flame structure and a distributed flame was observed.

In this study, we consider lean premixed hydrogen flames at a range of Karlovitz numbers from 10 to 1560.

Simulations

Three-dimensional simulations were run arranged with fuel below products as shown in the figure on the left.

Periodic lateral boundary conditions were used, with a solid base and outflow at the top, resulting in a downward propagating flame.

A high aspect ratio domain was used to allow the flame sufficient room to propagate.

A background turbulent field was maintained using a time-dependent zero-mean forcing term consisting of a superposition of low wavenumber Fourier modes.

Karlovitz numbers of 10, 100, 260 and 1560 and equivalence ratios of 0.31, 0.34, 0.37 and 0.40 were used.

The figure below shows vertical slices through four simulations at an equivalence ratio of 0.40. The left four panels are density, the middle four are burning rate, and the right-hand four are temperature. In each case, the four panels correspond to the four different Karlovitz numbers, increasing left-to-right. The domain size in these cases is five laminar flame widths across (40 high).

At low Karlovitz numbers, the cellular burning structure typical of low Lewis number hydrogen flames is observed. As the Karlovitz number is increase, the turbulence continues to disrupt the flame structure. The flame brush is broader but there is a decrease in individual structure size.

At the highest Karlovitz numbers, a different burning behavior is found. There is no longer a sharp interface between fuel and products, and the flame brush resembles a turbulent mixing zone. The local burning rate is reduced and occurs at the high temperature end of the flame brush. This kind of flame structure is similar to the distributed flame observed in the supernova study.

An interesting feature of the high Karlovitz number flame, is that there are no hot spots - the peak temperature is approximately 1420K, compared with approximately 1700K in the low Karlovitz cases. This has potential significance for the reduction of thermal NOx emissions.

The figure on the right shows the joint probability density function of temperature and equivalence ratio from the high Karlovitz number case. Note that temperature can be thought of as a progress variable in this case. The solid lines denote the laminar flame distributions (red-to-black corresponds to equivalence ratios 0.31 to 0.40).

Not only is the turbulent distribution very different to the laminar flames, but there is little change in the local equivalence ratio - only at high temperatures is the appreciable variation from 0.40. This is indicative that turbulent mixing is playing a significant role, possibly dominating thermal and species diffusion.

More details will appear in an upcoming paper, including many more simulations and a detailed examination of the diffusive processes at work.

Any questions should be directed to Andy Aspden.

References

A. J. Aspden, M. S. Day, and J. B. Bell, "Turbulence-Flame Interactions in Lean Premixed Hydrogen Flames", submitted for publication. [pdf]

A. J. Aspden, M. S. Day, and J. B. Bell, "Characterization of Low Lewis Number Flames", accepted for publication in the Proceedings of the Combustion Institute. [pdf]

A. J. Aspden, M. S. Day, and J. B. Bell, "Lewis Number Effects in Distibuted Flames", accepted for publication in the Proceedings of the Combustion Institute. [pdf]

A. J. Aspden, J. B. Bell, M. S. Day, S. E. Woosley, M. Zingale, "Turbulence-Flame Interactions in Type Ia Supernovae", Astrophysical Journal, 689, pp.1173-1185, December 20, 2008. [ApJ] [pdf] [pdf2]

N. Peters, Turbulent Combustion, Cambridge University Press, 2000.

T. Poinsot, D. Veynante, and S. Candel, Quenching processes and premixed turbulent combustion diagrams, Journal of Fluid Mechanics, 228, 1991.

W. L. Roberts, J. F. Driscoll, M. C. Drake, and L. P. Goss, Images of the quenching of a flame by a vortex: to quantify regimes of turbulent combustion, Combustion and Flame, 94, 1993.