Interaction of a turbulent flame with the very-large-scale structures in a channel flow

Combustion processes taking place in large-scale burners may need to consider the impact of the recently identified very-large-scale motions (VLSM) appearing in wall-bounded turbulent flows. While VLSM in non-reacting, cold flows have been found to be important and noticeably impact turbulence statistics, the possible influence of VLSM on turbulent premixed flames has not been considered up to now to the best of our knowledge. The present study investigates the interaction of a turbulent flame with VLSM in a channel flow using direct numerical simulations (DNS). Detailed chemical kinetics have been used to compute a H2 flame. The turbulent channel flow corresponds to Reτ= 280, while the premixed flame is associated to a Damköhler number Da = 0.167. First, VLSM have been identified for the non-reacting flow by proper orthogonal decomposition (POD). Interestingly, the propagation of the turbulent flame is found to have a negligible effect on the VLSM, highlighting their resilience. At the same time, the VLSM do not modulate strongly the flame structure, while it is noticeably affected by small-scale vortices. This decoupling might be explained by the very different length scales of both processes, the VLSM covering the full channel length, while the turbulent flame is characterized by small-scale structures. In the same manner, the characteristic time scales also differ widely, the flame time scale being much faster than the characteristic time of the VLSM. The flame curvature is positively correlated with the fluctuating streamwise velocity when the flame is still far from the wall. When approaching the wall, flame quenching takes place in a complex process. Overall, it appears for the present conditions that VLSM do not impact directly the turbulent flame propagation, but there is still an indirect impact through the modifications of the turbulence statistics.