Experimental investigation on lean blowout dynamics of spray flame in a multi-swirl staged combustor

Lean combustion has been widely applied in gas turbine combustors due to its superiority in reducing NOx emissions. However, it poses a significant risk of blowout for aero-engines especially when operating under low power conditions. In this paper, the LBO (lean blowout) process of spray flame was experimentally investigated and the flame was visualized by high-speed imaging of CH* chemiluminescence. A stratified partially premixed injector was applied and fueled with aviation kerosene at simulated flight-idle conditions. During the experiments, LBO was induced through linear reduction of pilot fuel flow rate. It was found that the LBO performances deteriorated as the altitude increases and the relationship between LBO FAR (fuel to air ratio) and altitude conformed to an exponential function. Evolution of flame macrostructures during LBO process showed that the flame changed from a double V-shape to a single V-shape with the decrease of FAR. FFT (fast Fourier transform) and POD (proper orthogonal decomposition) methods were introduced to post-process the high-speed imaging data in different periods before flame extinction. At the last second before LBO, POD modal distributions presented typical axial oscillation, circumferential rotation and high-order radial oscillations successively. The spectral analysis showed that the peak frequencies in the spectra of global CH* chemiluminiscence intensity and the spectra of POD modal time coefficients both changed prominently within 3 similar to 7 s before flame complete extinction, which was of good repeatability during repeated tests and was considered to be a precursor for blowout event.