Noise generation by open inverse diffusion flames

Abstract

The acoustic spectra of noise radiation from a turbulent non-premixed inverse diffusion flame were measured and the effect of the point of observation, air jet Reynolds number and overall equivalence ratio on the spectra were examined. The tests were conducted in a wide range of air and fuel flow rates and both the non-reacting and reacting cases were considered and discussed. For the non-reacting case, the noise emitted from the cold flow is mainly generated by the central air jet, with only a small role played by the fuel jets. The dominance of the noise produced by the air jet is confirmed by the observation that the cold flow noise is a strong function of the air jet flow rate or air jet Reynolds number. The spectral features of the noise from the combusting flames differ significantly from those of the cold flow noise due to the chemical reactions. Upon combustion, the noise radiated from the flames significantly overwhelms the corresponding cold flow noise in the range of frequency under consideration (80–3000 Hz). The distance of the point of observation only affects the magnitude of the sound pressure level while both the shape and magnitude of the sound pressure level are influenced by the azimuth of the point observation. The total sound pressure level increases with Re, while the effect of Ф on the total sound radiation level indicates that the highest level of noise occurs as complete combustion is approached.