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ABSTRACT
Direct numerical simulations are carried out to study the effects of the Karlovitz number (Ka) on the surface wrinkling of lean methane-air flames of equivalence ratio 0.5 and non-dimensional turbulence intensities (urms/SL) of 2–25. The mixture pressure is 20 bar and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. The Karlovitz number varies from 1.1–49.4, and the Damköhler (Da) number from 0.26–3.2, corresponding to turbulent premixed combustion in the thin reaction zone regime. With increasing Ka, there is increased probability of forming highly curved surfaces locally. The distribution of the shape factor of the local surface does not vary with Ka. The surface-averaged tangential strain rate is proportional to the Ka for flames in the thin reaction zone regime. By examining the flame displacement speed on the flame surface, it is found that on a highly curved surface, irrespective of whether it is convex or concave, flame stretch becomes negative due to the negative propagation term. The functional dependence of the conditional mean flame displacement speed on curvature is only weakly dependent on Ka and the shape factor. The physical insights derived from the analysis are applied to the modeling of the flame surface density equation.
Acknowledgments
This work used the Extreme Science and Engineering Discovery Environment (XSEDE), which is supported by National Science Foundation grant number ACI-1053575, and eResearch South Australia facilities for computational resources. Discussions with Dr. E. Motheau on the use of HOLOMAC and with Professor V. Magi on the interpretation of the numerical results are gratefully acknowledged.
Funding
The authors wish to thank Caterpillar, Inc. for the financial support for this work.