Statistics of local and global flame speed and structure for highly turbulent H[formula omitted]/air premixed flames

Abstract

A statistical analysis is conducted for turbulent hydrogen-air premixed flames at a range of Karlovitz numbers up to 1,126 by direct numerical simulations (DNS) with detailed chemistry. The local and global burning velocities are evaluated and the deviation from the laminar flame speed is assessed. It is found that the global turbulent flame speed is largely determined by the integral length scale than the turbulent Karlovitz number, due to the flame surface area enhancement. The turbulent flame speed in all examined cases correlates well with the flame surface area, according to Damköhler’s first hypothesis; even at Karlovitz number well above 1,000, reaction zones stay intact and only the preheat zone is broadened by the strong turbulence level. The statistical analysis with the probability density function (PDF) for the displacement speed shows that the highest probability of the local flame speed coincides with the one-dimensional unstretched flame speed. Despite some deviations, the mean flame structures and reaction rate of hydrogen of the higher Ka cases are found to resemble those of the laminar flame, and this further confirms that the turbulent flame brush topology is mainly determined by the large scale turbulence behavior. The results also suggest that the engineering modeling based on the flamelet concept may be valid for a wider range of Ka conditions.