Unsteady temporal fluctuations of the equivalence ratio in lean premixed gas turbine combustors are one of the most important driving mechanisms for thermoacoustic instabilities. In this work, high-amplitude equivalence ratio fluctuations in the mixing section of a swirl-stabilized burner are assessed for the first time. The applied non-intrusive sensor is based on fixed-wavelength modulation spectroscopy of methane at 1653 nm using a near-infrared tunable diode laser. The measurements are performed at isothermal operating conditions without the presence of a flame at 25°C and at atmospheric pressure. The equivalence ratio fluctuations are generated by acoustic forcing of the air flow while the fuel injection flow rate is kept constant. Acoustic forcing amplitudes up to 220% of the mean flow velocity are assessed. Measurements are conducted at different axial distances from the fuel injection point to study the spatio-temporal evolution of the equivalence ratio fluctuations. The results show a frequency-dependent saturation of temporal equivalence ratio fluctuations with increasing forcing amplitude, which can not be described through the available model. These results are in good agreement with preceding studies and indicate the saturation of the flame response due to a saturation of equivalence ratio fluctuations. Furthermore, a decreased attenuation of temporal mixture inhomogeneities for small forcing amplitudes is found.

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