Numerical study on the response of axisymmetric bluff-body stabilized flames near blow-off conditions forced by external velocity disturbances

This paper investigates the response of near blow-off (NBO) flames in an unconfined axisymmetric bluff-body stabilized burner to external velocity disturbances at 100–400 Hz via numerical simulations using the large eddy simulation-flamelet generated manifold (LES-FGM) method. Near blow-off conditions were achieved by increasing the amplitude of the external velocity fluctuation at a constant frequency. Results show that the blow-off amplitude first falls, then rises, with increasing fluctuation frequency, and that near blow-off flame response modes differ distinctly with frequency. Up to 300 Hz, an axial-dominant mode emerges: dominant vortices lie at Y > 15 mm, flame structures switch between arch and M-shapes cyclically, and cold gas entrainment by induced vortices is critical to blow-off transients. At 400 Hz, a radial-dominant mode occurs: dominant vortices are located at Y < 30 mm, flames form a compact, stably twisted M-shape, and flame stretch-induced local extinction dominates blow-off. Axial-dominant mode instability stems from cold, unburnt gas penetrating the hot recirculation zone and local low equivalence ratio caused by fluctuations, while radial-dominant mode adjacent–vortex interaction generates a momentum counteraction mechanism that mitigates these effects, yielding a higher blow-off amplitude.