Numerical investigation of shear layer effect on sound generation in jet diffusion flame.

Shear layer effect was pointed out to be associated with the vortex-flame interaction and leads to the difference in combustion noise. In this work, numerical simulation combining with an acoustic analogy equation was performed to investigate the shear layer effect on the sound generation by jet diffusion flames. To easily identify the cause and effect, the approach that varies the thickness of shear layer by changing the Prandtl number is adopted, which provides deeper physical insight into the characteristics of the acoustic near field. The formation and evolution mechanisms of sound sources in jet diffusion flames are interpreted. It is observed that the sound sources are concentrated in vortex centers and appear as either the sources or the drains for the sound waves. The spectral analysis shows that the low frequency sound is emitted from the flame base, while the high frequency sound is radiated from the downstream region. Moreover, the theoretical analysis based on vortical dynamics shows that the low frequency sound is primarily affected by the combustion-induced buoyancy effect, whereas the high frequency sound is determined by the baroclinic torque.