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

TL;DR

This study investigates how turbulence influences the structure and propagation speed of lean hydrogen-air premixed flames using DNS, revealing effects of differential diffusion, curvature, and flame interactions on flame speed and structure.

Contribution

It provides new insights into the variation of flame displacement speeds and structure in turbulent hydrogen flames, especially near zero-curvature regions, through detailed DNS analysis.

Findings

Enhanced local flame speed due to differential diffusion in low Le cases.

Reversal of flame speed gradient caused by flame-flame interactions.

Statistical analysis of flame structure variations at zero-curvature regions.

Abstract

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 differential diffusion in positive curvatures downstream of the zero-curvature locations. Furthermore, locally, the flame structure is broadened for all cases due to a reversal in the direction of the flame speed gradient. This reversal is caused by cylindrical flame-flame interactions upstream of the zero-curvature regions, resulting in localized scalar mixing within the flame structure. These non-local effects, in combination, define the mean flame structure and the associated variation in local flame speed in turbulent premixed flames.