Spatial Distribution of Combustion Noise Sources

Abstract

The acoustic radiation characteristic of turbulent flames, such as their power spectra, spatial coherence, and directivity, is controlled by the amplitude and spatial distribution of heat release fluctuations. Hence, an understanding of the unsteady heat release distribution is critical in understanding the characteristics of noise radiation from turbulent flames. This paper continues our prior studies on acoustic emissions from a turbulent premixed jet flame and describes combined analysis of high speed images of the line-of-sight integrated flame chemiluminescence and two point microphone measurements. Analysis of these images shows that the heat release fluctuations convect axially along the flame, likely due to instability shear waves that disturb the flame, and are correlated over some spatial region that decreases in size with increases in frequency. The dominant noise source location can be directly correlated to the region of largest light intensity fluctuations, as would be expected. However, the finite correlation size of these heat release fluctuations cause the flame noise to be spatially correlated only over the frequency range where the flame is acoustically compact; i.e., the acoustic measurements at 45 and 90 degrees have a coherence near unity for low frequencies but falls rapidly for frequencies whose wavelengths are shorter than the flame source region. Considering the spectra of fluctuations, if the heat release fluctuations have constant power in the frequency domain and are perfectly correlated in space, then the noise spectra should scale as f . Measurements indicate this to be nearly the case at low frequencies, but also show that the sound power peaks at a frequency fpeak and then rolls off as f -α at higher frequencies. Our results suggest that this peak frequency and roll off exponent, α, are controlled by two factors: the decreasing correlation length of the heat release fluctuations with increases in frequency, and the convective character of the heat release fluctuations.