Abstract

Self-excited noise generation from laminar flames in thin annular jets of premixed methane/air has been investigated experimentally. Various flame shapes were observed in this flow configuration, including conical shaped flames, ring shaped flames, steady crown shaped flames, and oscillating crown shaped flames. Self-excited noise with a total sound pressure level of about 70dB was generated from the oscillating crown shaped flames for equivalence ratios larger than 0.95. Sound pressure and CH* chemiluminescence were measured by using a microphone and a photomultiplier tube. The frequency of generated noise was measured as a function of equivalence ratio and premixture velocity. A frequency doubling phenomenon has also been observed. The measured CH* chemiluminescence data were analysed and which the corresponding sound pressure has been calculated. By comparing the measured and calculated sound pressures, the noise source can be attributed to the flame front fluctuation near the edge of the oscillating crown shaped flames. The flame stability regime was influenced strongly by the mass flow rate of air through the inner tube.