Flame self-interaction during turbulent boundary layer flashback of hydrogen-rich premixed combustion

Abstract

A three-dimensional direct numerical simulation database of turbulent boundary layer flashback of a hydrogen-rich premixed flame with an equivalence ratio of 1.5 has been analyzed to investigate flame self-interaction (FSI) events. The nonreacting turbulence characteristics of the channel flow are representative of the friction-velocity-based Reynolds number, Reτ=120. A skeletal chemical mechanism with nine species and twenty reactions is employed for the representation of hydrogen-air combustion. Three definitions of the reaction progress variable, c, based on the mass fractions of H2, O2, and H2O, have been considered to define the progress variable. It is found that the FSI events predominantly occur close to the burned gas side for all definitions of c at all the wall normal distances. No FSI events adjacent to the wall have been identified for the c definition based on O2 and H2O mass fractions, whereas FSI events occur for c based on H2 in the near-wall region. In the regions further away from the wall, all c definitions show that tunnel formation and tunnel closure type FSI events remain predominant, which is consistent with the earlier findings by Griffiths et al. [Proc. Combust. Inst. 35, 1341 (2015)1540-748910.1016/j.proci.2014.08.003] involving hydrogen-air premixed flame under shear flow conditions. In this work for c based on H2 mass fraction, unburned gas pockets have also been identified at all wall normal distances and are a consequence of the hydrogen-rich nature of the flame. The reason for the variations in topologies with the change in the definition of c based on different species and wall normal distance is a consequence of several factors, including the changes in the level of turbulence within the turbulent boundary layer, heat loss to the isothermal wall in the near-wall region, and the differential diffusion induced by the nonunity Lewis number. The results from the current analysis show that the turbulent boundary layer and heat loss at the wall play important roles in determining the FSI topologies. The differences in the qualitative nature and distributions of the FSI events between different definitions of c have important implications on the possible extension of flame-surface-based modeling methodology for hydrogen-rich flames within turbulent boundary layers.