Experimental Investigation of Flame Dynamics Based on High-Speed Images in Swirl Combustion Systems

The interaction mechanism of internally-staged-swirling stratified flame is complex, and the pilot flame has a manifest influence on flame stability. To study this, a series of experimental investigations for the pilot flame has been carried out in a model swirl combustor by only supplying the pilot fuel. The CH* chemiluminescence images of the pilot flame are acquired by a high-speed camera with a CH* bandpass filter, whose dynamic characteristics are identified by image statistical analysis and proper orthogonal decomposition (POD) analysis. And the fast algorithm based on matrix theory proposed in this paper increases the operation efficiency and operability of POD. With the pilot equivalence ratio phi increase, the pilot flame gradually shows an unstable state, whose POD energy distribution is significantly different. In the unstable state, the flame dynamics include three modes-spiral motion mode, flame shedding mode, and axial oscillation mode, whose formation reasons have also been further analyzed in combination with the experimental characteristics. And the fast Fourier transform (FFT) analysis of the time coefficients for the first four POD modes indicates all the dominant frequency is 280 Hz, which means the model combustor is in resonance. In addition, a sensitivity analysis based on the different image resolutions further reveals the robustness of the POD method and its optimization direction. These results emphasize the important influence of the pilot fuel flow rate on the stability of the pilot flame.