The effects of flame orientation and mean shear on flame stretch in highly turbulent premixed flames using DNS

Abstract

In the present work, direct numerical simulation data of a laboratory-scale high Karlovitz number (Ka) jet flame with a strong mean shear and a high Ka freely propagating planar flame without a mean shear were analyzed to explore the influence of flame orientation and mean shear on flame stretch in highly turbulent flames. Three flame fronts, i.e. the leading, in-plane and trailing front, were identified for the jet flame. The leading and trailing front is defined as the flame normal being facing the downstream and upstream regions, respectively, while the flame normal of the in-plane front is near the cross-stream plane. It was found that the flame stretch is positive in the leading front, and is negative in the trailing front of the jet flame. As for the planar flame, the statistics of the flame stretch are similar in various flame fronts. The components of the flame stretch are analyzed. The normal vector of the leading front is aligned with the most compressive strain rate in the jet flame, resulting in a large tangential strain rate, while the tangential strain rate of the trailing front is significantly lower. In contrast, the alignment characteristics and the strain rates of the planar flame are not sensitive to the flame orientation. The value of the stretch due to curved propagating flame front is large and negative in the trailing front of the jet flame due to the negative correlation between the displacement velocity and curvature, which is weak in the leading and in-plane front. The correlation between the displacement velocity and curvature is, however, similar in various fronts of the planar flame. Overall, it is suggested that the effects of flame orientation and mean shear should be accounted for in understanding turbulence-flame interactions in practical combustion devices.