COMPARISON OF EQUIVALENCE RATIO FLUCTUATIONS IN A LEAN PREMIXED COMBUSTOR FOR KEROSENE AND NATURAL GAS

Equivalence ratio fluctuations have been identified as a major driving mechanism for thermoacoustic instabilities in lean premixed combustion. In the present study, equivalence ratio fluctuations are determined experimentally from flame chemiluminescence in a lean premixed combustor at atmospheric conditions for kerosene and natural gas combustion. The phase-resolved CH* and CO₂* chemiluminescence signals of acoustically excited flames are acquired with an intensified camera equipped with an image doubler and bandpass filters. A calibration chart derived from stationary operation provides the correlation between the global CH*/CO₂* ratio and the equivalence ratio for both fuels. Equivalence ratio fluctuations are found to be particularly dominant as long as the convective wavelength of the equivalence ratio wave exceeds the distance between fuel injection and flame. The fluctuations decrease significantly with increasing frequencies and finally become almost negligible. The more complex mixture preparation for liquid fuels compared to gaseous fuels is shown to significantly influence the generation and propagation of equivalence ratio fluctuations. The applied measurement method is validated by investigating externally premixed natural gas combustion, where no equivalence ratio fluctuations are detected. A one-dimensional transport model is employed to analyze the generation and propagation of the equivalence ratio wave. A comparison with the experimental results reveals that the propagation of the equivalence ratio wave can be approximated quite accurately by the model. However, the model fails to predict important features attributed to an interaction of the equivalence ratio wave with swirl fluctuations.