Analysis of Combustion Noise uings locally Resolved Density Fluctuations and a Microphone Array

For turbulent swirl-stabilized flames combustion noise can
be directly calculated, if density fluctuations as a function of
time and space are known. It is however not easily possible to
assess the density fluctuations directly. Therefore, in the past,
combustion noise has been expressed as a function of
chemiluminescence, an approach bringing in more
assumptions. Now, by using interferometry, density fluctuations
in the flame can be measured quantitatively. The advantage of
this technique is that it measures the time derivative of density
fluctuations directly. In this work laser interferometric
vibrometry (LIV) was used to scan a two dimensional field in
the flame in order to calculate the sound power emitted by the
flame. Sound intensity was measured in a half-hemisphere by
pressure-pressure-probes in order to record the total sound
power of the direct combustion noise emitted by the unconfined
flame. The goal of this study was to compare the measured
sound power exhibited by the flame with the sound power
predicted due to fluctuations of density within the flame. By
using a siren to generate linear excitation, it was possible to
qualitatively predict combustion noise with good agreement in
trend. A quantitative comparison between both measurement
techniques showed a deviation of a factor of six.