Flame structure analysis and composition space modeling of thermodiffusively unstable premixed hydrogen flames — Part I: Atmospheric pressure

Abstract This work focuses on flame structure analysis and composition space modeling of a multidimensional premixed hydrogen flame transitioning from a laminar stable condition to a thermodiffusively unstable state. Specifically, budget and a priori analyses are conducted based on a detailed chemistry simulation of a 2D expanding, thermodiffusively unstable hydrogen flame, and the recurring issues for modeling differential diffusion, the strain rate and curvature in the thermodiffusively unstable flame are addressed in a single newly proposed flamelet tabulation method. The model is based on recently developed self-contained strained-curved premixed flamelet equations in composition space (Scholtissek et al., 2019, CNF), which inherently incorporate the interactions among differential diffusion, the strain rate and curvature. The validity of the newly proposed flamelet tabulation method is evaluated based on the representative strongly strained-curved flamelets extracted from the reference simulation, featuring wide ranges of strain rates and curvatures. The advance realized in the proposed flamelet model is confirmed by comparing it with a conventional flamelet tabulation method. Through the budget analysis, the effects of curvature on the diffusion along the flame front (i.e., tangential diffusion) are quantified. Through the a priori analysis, the suitability of the proposed flamelet tabulation method in predicting differential diffusion is confirmed. For the prediction of the strain rate and curvature, it is found that introducing the strain rate and curvature themselves as the trajectory variables does not necessarily improve the prediction accuracy in the reaction zone, compared to the flamelet model based on the 1D freely-propagating premixed flame with differential diffusion. To remedy this, the trajectory variables that can characterize the flame structure’s internal response to the strain rate and curvature are identified. The results show that the thermo-chemical variables in the thermodiffusively unstable flame at atmospheric pressure can be accurately predicted by the newly introduced trajectory variables based on the 1D strained-curved flamelet equations.