How 'mixing' affects propagation and structure of intensely turbulent, lean, hydrogen-air premixed flames

Understanding how intrinsically fast hydrogen-air premixed flames can be rendered much faster in turbulence is crucial for systematically developing hydrogen-based gas turbines and spark ignition engines. Here, we present fundamental insights into the variation of flame displacement speeds by investigating how the disrupted flame structure affects speed and vice-versa. Three DNS cases of lean hydrogen-air mixtures with $Le$ from 0.5 to 1 and $Ka$ from 100 to 1000 are analyzed. Suitable comparisons are made with the closest canonical laminar flame configurations at same mixture conditions and their suitability and limitations in expounding turbulent flame properties are elucidated. Since near zero-curvature surface locations are most probable and representative of the average flame geometry in such large $Ka$ flames, this study focuses on the statistical variation of flame displacement speed and the concomitant change in flame structure at those locations. Relevant flame properties are averaged normal to the zero-curvature isotherm regions to obtain the conditional mean flame structures. In the smallest $Le$ case, downstream of the most probable zero-curvature regions, the temperature exceeds that of the standard laminar flame, leading to enhanced local thermal gradient and flame speed. This is due to increased heat-release rate contribution by preferential diffusion in positive curvatures downstream of the zero-curvature locations. In addition, for all cases, the local flame structure is also broadened due to reversal in the direction of the flame speed gradient underpinned by ubiquitous, non-local, cylindrical self-interaction upstream of the zero-curvature regions, leading to localized scalar mixing within the flame structure. These non-local effects combine to define the most probable flame structure and the associated variation in local flame speed in turbulent premixed flames.