The nonlinear response of inverted “V” flames to equivalence ratio nonuniformities

Recent studies indicate that heat release fluctuations generated by equivalence ratio perturbations may constitute sources of instability with effects similar to those induced by acoustic perturbations. The present article addresses this issue by considering the dynamics of an inverted laminar dihedral (“V”) flame spreading in an open geometry when this flame is submitted to equivalence ratio modulations. The problem is investigated with numerical simulations by first establishing a steady state flame which then evolves in a uniform flow transporting a fixed level of equivalence ratio perturbations. The flame features wrinkles of increasing amplitude locking on the convected composition perturbations. The wrinkling amplitude grows with distance from the injector. For sufficiently large wrinkle amplitudes, the flame interacts with the fresh mixture outer boundary, giving rise to sudden disruptions of the flame sheet. The rapid burning of fresh mixture pockets generates a nonlinear heat release signal with abrupt changes in the waveform. It is found that high levels of modulation induce axial velocity perturbations, which in turn interact with the flame and modify the response. Calculations described in this article may serve to guide analytical modeling of the response of combustion to equivalence ratio inhomogeneities. A simple model is devised on this basis to distinguish regimes corresponding to weak and strong interactions.