Relation between Pressure Fluctuations and Flow Characteristics Concerned with Combustion Oscillation Onset in Turbulent Swirling Lean Premixed Combustion

Abstract

Change of velocity distributions, flame structures and pressure fluctuations corresponding to combustion oscillation onset in a methane/air turbulent swirling lean premixed combustion were investigated by simultaneous high-speed measurements of stereoscopic particle image velocimetry (SPIV), OH chemiluminescence and pressure fluctuation. Acoustic forcing was applied to make the coupling systems by means of acoustic interference clear. The transition process towards combustion oscillation was defined based on the root-mean-square (rms) values of pressure fluctuation p'_rms. Experimental conditions on flow rate, equivalence ratio, length of the combustion chamber and a degree of the swirler vane were set so as to be feasible to occur the transition process. Phase-based analyses of physical properties' fluctuation show that the transition process in all cases holds Rayleigh criteria. In the early stages of the transition process, dynamic instabilities were observed in sheer layers. Involved unburnt regions by vorticity growth were found in the latter half stage. Dynamic mode decomposition (DMD) for the velocity distributions around the beginning of the transition process revealed high-frequency asymmetry features in inner recirculation zones and low-frequency asymmetry features in outer recirculation zones.