Characterisation of external acoustic excitation on diffusion flames using digital colour image processing

Abstract

The experimental investigation of the dynamic flame flickering properties of acoustically excited combustion process has been conducted. In particular, the variations of flame oscillation frequency response in diffusion-like sooty and premixed-like chemiluminescence flame colour entities under external acoustic perturbation were extracted, analysed, and compared using digital colour image processing technique. The technique employed makes use of both the RGB and HSV colour modelling principles to identify and tag digital image data that conforms to the different flame colour distribution regimes. It was found that the bluff-body stabilised diffusion flame considered in this experiment has a natural low-oscillation frequency range of below 20Hz; the typical diffusion flame frequency response without external acoustic excitation. External acoustic excitations of 15–100Hz were applied to the burner. The processed frequency spectrum from the diffusion-like digital colour signals presented in the captured high-speed video data showed that external acoustic excitation did little to shift the dominant frequency location away from the well-known low-frequency flame flicker, and only minor peak distributions can be found to match the exact point of external frequency in the spectrum. On the other hand, the frequency spectrum processed using digitally isolated premixed-like flame colour signals showed that the dominant frequency in the processed FFT spectrum is located at the exact frequency of acoustic excitation from 15 to 35Hz. Between 40 and 100Hz, the presence of external acoustic excitation corresponds to the location of the dominant distribution in the secondary group of peaks identified from the frequency spectrum. Within this range of forced acoustic excitation, the dominant frequency returned to the natural convective response of the flame at below 20Hz.