MODELING PULSATING COMBUSTION DUE TO FLOW-FLAME INTERACTIONS IN VORTEX-STABILIZED PRE-MIXED FLAMES

Large amplitude oscillations have been observed in pre-mixed combustio n systems, where the flame is stabilized in the recirculation zone beh ind an obstacle or a sudden expansion (a dump), and have been related to coupling between the combustion dynamics and the acoustics of the s ystem. We have constructed a model, which includes an intake duct conn ected to a large upstream reservoir at a fixed pressure, a compact com bustor with a sudden expansion followed by a sudden contraction (a cav ity), and a long exhaust duct connecting the combustor to the atmosphe re, to describe a system typical of a pre-mixed dump combustor. The up stream and downstream components are modeled as 1D non-reacting flow s ystems, while the combustor is modeled using a vortex simulation of th e 2D Navier-Stokes equations with a low-Mach number, thin flame combus tion model. Results indicate that the system dynamics is comprised of the combustor wake-mode, fD/U=O(0.1), and several other modes characte ristic of the acoustics of the overall system, with the former being o ne of the lowest subharmonic of the latter. This subharmonic selection mechanism, manifesting the coupling between the convective dynamics i n the combustor and the acoustics of the system, is consistent with th e fact that although several experiments and numerical simulations exh ibit the same tow frequency instability, the high frequency contents a re different since they are dependent on the system configuration. The origin of the low frequency is the coalescence of a group of eddies, which are shed from the separating shear layer, as they move into the recirculation zone.