This paper describes an experimental investigation of acoustically forced turbulent bluff-body stabilised flames with swirl and with three different degrees of premizing: fully premixed, non-premixed with radial fuel injection and non-premixed with axial fuel injection flames. The flame was imaged using OH~* chemiluminescence at 5 kHz. The heat release response of these flames to acoustic forcing at 160 Hz, which was the frequency that gave the maximum oscillation amplitude, was studied quantitatively with the calculation of the Nonlinear Flame Transfer Function (NFTF) for various forcing amplitudes, equivalence ratios, and air velocities. The post-processing analysis also consisted of phase-averaged OH~* chemiluminescence images. It was found that non-premixed flames with radial fuel injection exhibited a much greater response to acoustic forcing, followed by premixed and non-premixed flames with axial fuel injection. In the premixed system, the magnitude of the heat release response was greater for higher air velocities, however the effect of equivalence ratio was more complex. Both non-premixed systems showed a reduced sensitivity to input air velocity and global equivalence ratio. Also, it was found that for the conditions studied, all three systems showed a nonlinear response. The qualitative analysis of the flame dynamics showed that in the premixed system the flame roll-up at the bluff body edge plays an important role in the flame response. In the non-premixed system with radial fuel injection, apart from the flame roll-up, the temporal variation in equivalence ratio constitutes an important phenomenon.
展开▼
