Global-local decoupling of instability modes and phase reversal in premixed jet flames

This study examines the coexistence of global buoyancy oscillations and local shear-induced deformation of the premixed flame front in methane/air premixed jet flames. High-speed schlieren imaging, chemiluminescence imaging, and photomultiplier tube (PMT) measurements were combined with frequency-domain and time-frequency analyses, mode decomposition, phase-locked averaging, and cross-power spectral analysis to characterize the stability limits, modal energy distribution, global-local response, and phase relation. The results show that the critical blowout velocity exhibits an approximately D 1/2 scaling with nozzle diameter. With increasing flow velocity, the flame transitions from a steady regime to a low-frequency buoyancy-dominated regime, a transition regime, and a high-frequency shear-dominated regime. Spatial diagnostics show that the peripheral flow field retains large-scale vortex shedding induced by buoyancy, whereas the premixed flame front develops increasingly localized high-frequency deformation. Time-frequency analysis further reveals intermittent broadband oscillations of approximately 350–450 Hz in the flame-tip response near the blowout limit, while the PMT signal remains dominated by low-frequency components because of the spatial integration of chemiluminescence. Phase analysis shows that the low-frequency phase difference between the global chemiluminescence response and flame-tip height changes from positive to negative with increasing velocity. Using a modified Strouhal number based on mean flame height, the phase-difference data under fuel-rich conditions collapse onto a common linear relation with a zero-phase crossing near St* ≈ 0.15, providing a dimensionless link between the global-local phase reversal and the transition from buoyancy-dominated global oscillation to shear-dominated local flame dynamics.