Numerical study on a bluff-body-stabilized lean premixed turbulent flame

Abstract

A bluff body has been adopted as flameholders in various combustion systems. While the bluff body has an advantage of improving stability, a bluff-body-stabilized turbulent flame can reach the blow-off at the fuel lean condition. To study the dynam- ics at near blow-off condition, an appropriate numerical method is required. Numer- ical simulations were conducted for the bluff-body-stabilized lean premixed turbulent flame which recently has been used as a validation target of numerical methods in the turbulent premixed flame community. For the present study, an OpenFOAM-based solver was used for large eddy simulation (LES) with the eddy dissipation concept (EDC) as a turbulent combustion model. For an optimal simulation strategy, effects of computational domain sizes and global reaction mechanisms were investigated. To assess effects of the spanwise computational domain, the size in the spanwise direc- tion where a periodic boundary condition is applied was varied between 4H, 2H, and 1H with an x-y domain of 25H x 3H where H is the flameholder height. The results for the two reduced sizes (4H and 2H) are reasonably consistent with the experimen- tal results of 6H spanwise size, which implies that 2H periodic domain is sufficient to accommodate the spanwise variation of turbulence. The results using two-step global reaction mechanisms of Ghani and Westbrook & Dryer (WD2) were compared where both reaction mechanisms are performed well at the equivalence ratio of the valida- tion case. To resolve the dynamics at near blow-off condition in a bluff-body-stabilized premixed turbulent flame, by using both reaction mechanisms, the flame character- istics were observed while the equivalence ratio of inflow mixture was changed. The flame became unstable showing the local extinction as close to the blow-off in both results of reaction mechanisms. However, blow-off conditions were different for each mechanism used. At the near blow-off conditions of reaction mechanisms, a time scale analysis was conducted. Through the time scale analysis, it was found that the lo- cal extinction is strongly related with ignition delay. This means that the Damköhler number which is based on the ignition delay controls the local extinction events.