Characterization of Freely Propagating Hydrogen Flames

We explore the detailed structure of heat release and the energy budget for a freely-propagating lean, premixed hydrogen flame. At lean conditions, the flame is thermodiffusively unstable and spontaneously develops the cellular burning structures, characteristic of this type of flame. The instabilities in the system saturate quickly, resulting in robust, slowly evolving cellular burning features. In the simulations, we employ a feedback control strategy to stabilize the mean location of the evolving flame so that in the computational domain the flame is statisically stationary, allowing us to collect data for analysis. To analyze the local flame structure we identify the flame surface with an isotherm and construct a local coordinate system around the flame by following integral curves of the temperature gradient. We extract local heat release along these integral curves as we trace along the flame surface. We compare these local heat release profiles with heat release profiles from flat laminar flames over a range of equivalence ratios and show that the heat release profiles in the two-dimensional flame are well matched by the laminar profiles. We also examine the energy budget along integral curves through the flame and compare to the flat laminar flame. From a flat, laminar flame simulation we also identify the dominant reactions contributing to the heat release and analyze their behavior in local flame coordinates. Finally, we explore how the features of the heat release profiles correlate with flame curvature.