Dynamics of laminar diffusion flames at sub-atmospheric pressure under axial acoustic forcing

Acoustic forcing substantially alters the dynamics of laminar diffusion flames. Previous studies at atmospheric pressure have shown that acoustically forced laminar diffusion flames exhibit amplitude-dependent transitions from forcing-locked oscillations to nonlinear flame responses. However, the dynamics of laminar diffusion flames under axial acoustic forcing at sub-atmospheric pressures remain unclear, particularly the effects of reduced pressure on transition pathways between flame behaviors. This experimental study investigated the combustion dynamics of coaxial methane–air laminar diffusion flames at atmospheric and sub-atmospheric pressures under axial acoustic forcing. The flame-acoustic coupling processes were examined at ambient pressures (100, 40, and 30 kPa) over forcing amplitudes (A=û/u¯=0.05–0.5) and forcing frequencies (30, 60, and 90 Hz). The observed flame responses can be classified into several flame-acoustic coupling behaviors, including sustained oscillatory combustion (SOC), periodic liftoff and reattachment (PLOR), periodic pinch-off (PPO), and permanent flame liftoff (PFLO). Proper orthogonal decomposition (POD) of high-speed flame images and OH* chemiluminescence analysis were used to characterize the dynamical signatures of different flame behaviors and identify their pressure-dependent transition pathways. The transitions were identified from changes in POD modal energy distribution, variations in phase portrait smearing, and the appearance of additional peaks in the spectrograms. The results show that reduced pressure weakens flame-base anchoring, thereby promoting liftoff-related responses, including PLOR and PFLO. These findings provide insight into the dynamics of flame-acoustic coupling and flame stability in acoustically forced laminar diffusion flames at sub-atmospheric pressures.