Noise generation from methane/air premixed flames

Abstract

Recent demands for high combustion loads and stringent emission requirements frequently induce combustioninstabilities. Since these are undesirable sources of noise and sometimes cause disastrous destruction ofcombustion equipment, the active control of combustion instabilities becomes one of the key interests incombustion research. In order to develop effective control technologies, it is required to understand themechanisms and the influencing factors governing combustion instabilities. Premixed methane/air flames in thelaminar thin annular jet burner have been studied experimentally to investigate the characteristics of self-excitednoise generation. The flame in the annular jets has various flame shapes, including ring shape, near conical shape,crown shape, and oscillating crown shape depending on equivalence ratio, mixture velocity, and air velocity. Theregimes of the existence of these flames were identified. Self-excited noise with about 70 dB was generated forthe unsteady crown shape flame. The frequency of sound pressure depended on the equivalence ratio and mixturevelocity. Sound pressure and CH* chemiluminescence were measured by using a microphone and a photomultipliertube. The frequency of generated noise was measured as functions of equivalence ratio and pre-mixturevelocity. The frequency doubling behaviour was also observed. The variation of sound pressure with time showedsimilar behaviour as that of the CH* chemiluminescence. The calculated sound pressure from the CH*chemiluminescence signal was in satisfactory agreement with the measured one, demonstrating the source ofsound generation is from the flame surface fluctuation near the downstream part of the crown shape flames. Thepresent study demonstrated that significant noise can be generated when a rich fuel/air mixture propagate into air.The flame stability regime was influenced sensitively to the supplying air through the inner tube. Further study isneeded in the future to identify the mechanism of flame surface corrugation, the noise source location, theinteraction with the diffusion flame surrounding it, and the mechanism of frequency doubling.</p