THE DYNAMIC RESPONSE OF TURBULENT DIHEDRAL V FLAMES: AN AMPLIFICATION MECHANISM OF SWIRLING FLAMES

Abstract

The authors analyze the dynamic response of swirl-stabilized flames submitted to upstream acoustic perturbation. Extensive measurements were performed in an optically accessible single-nozzle gas turbine combustor operating on natural gas-air at inlet temperatures of 200 and 300 degrees C over a range of equivalence ratios from 0.55 to 0.70, a range of inlet velocities from 60 to 100 m/s, and swirl angles of 30 degrees and 45 degrees. Temporal oscillations of inlet velocity and heat release rate in the whole flame were measured using the 2-microphone method and global OH*, CH*,and CO2* chemiluminescence emission intensities, respectively, whereas spatially resolved measurement of heat release rate was made using time-averaged CH* chemiluminescence flame images. For the dihedral V flames, amplification characteristic of the flame transfer function was observed. This effect is, unlike the amplification mechanism of a small laminar flame, controlled by the relative ratio of the two length scales, disturbance convective wavelength and flame length. The measured transfer functions show resonance-like behavior when a nondimensional number, the ratio of half the convective wavelength to flame length, approaches unity. It was found that the flame geometric properties, specifically flame angle, also play a crucial role in the flame transfer function. The frequency-dependent behavior of swirl-stabilized flames is closely related to eigenfrequency selection processes at limit cycle pressure oscillations.