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

This paper is a continuation of the work we carried out in Part I (Wen et al., 2021), studying the flame structure analysis and composition space modeling of an outwardly expanding thermodiffusively unstable premixed hydrogen flame at atmospheric pressure. In this work, we focus on the case of elevated pressure (5 atm), which is particularly challenging for the model proposed in Part I due to wider ranges of curvatures and strain rates induced by promoted intrinsic instabilities in the flame studied. A detailed chemistry simulation of the thermodiffusively unstable premixed hydrogen flame at 5 atm is first conducted, and the flame structure is comprehensively analyzed by comparing the simulation results with the asymptotic theory proposed by Matalon and co-workers (PCI, 2002; JFM, 2003). Then, budget terms of the generalized premixed flamelet equations are calculated to identify the dominating processes of the cells with different length scales. Finally, the performance of the proposed tabulation method based on premixed flamelet equations in composition space presented in Part I is evaluated through an a priori analysis by comparing tabulated values with the reference results. The critical flame radius and average cell size calculated with the asymptotic theory are found to agree well with the reference results. The calculated critical Péclet number at the elevated pressure is smaller than that under the condition of atmospheric pressure, suggesting a smaller critical flame radius as the pressure increases. The budget analysis shows that for either small- or large-scale cells, tangential diffusion is only important for flamelets with negative curvature. The a priori analysis shows that the proposed tabulation method performs well in predicting the intermediate species in regions with negative curvature, although the peak values of some species mass fractions are under-predicted. The reasons are explained and guidelines for future research to improve the model are outlined