Analysis of Combustion Noise Using 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.