Experimental observation of the nonlinear coupling of flame flow and acoustic wave

• The experimental system is developed to observe the nonlinear phenomenon.
• The coupling between buoyancy and acoustic excitation create complex nonlinear flame oscillation.
• The flame response could be coupled with the sub-harmonic excitation frequency (fe/5) and the flame oscillating frequency (ff).
• The nonlinear frequency modes of flame oscillation can be explained by nonlinear theory.

This is an experimental investigation to study the nonlinear coupling characteristics of a propane/air flame with acoustic standing waves, using chemiluminescence emission and phase-locked PIV measurements. A variety of coupling modes are observed for the excitation source with combustion instability oscillations and its harmonics and sub-harmonics. The frequency analysis shows that flame/acoustic coupling behaviour results in complex nonlinear coupling. The coupling behaviour is weak at lower excitation intensities (0.3 V). At a voltage amplitude of 2 V, the results show that the excitation frequency (fe) is only coupled with the sub-harmonic frequency (fe/5) for the premixed flame. However, for the diffusion flame, more complex frequency components are observed, which exhibit relationships of fe±ff and fe±fe/5. At a voltage amplitude of 13.7 V, the sub-harmonic frequencies (2fe/5 and 3fe/5) and the premixed flame buoyancy oscillations (ff) are increased. PIV measurements provide detailed flow velocity vector fields in an acoustically excited tube for different phase angles and the effect on the flames at different equivalence ratios, which increases the understanding of flame oscillation behaviour. It is found that all of the nonlinear phenomena that are observed occur because of the coupling between buoyant and acoustic excitation and create complex nonlinear frequency couplings.