An analysis of unsteady highly turbulent swirling flow in a model vortex combustor

This paper reports an experimental investigation of a non-reacting turbulent swirling flow in a practical vortex combustor. The flow was examined for the conditions characteristic of the presence of a breakdown zone and a strong flow instability appearing at swirl numbers S>0.5. Flow visualization techniques, LDA measurements and acoustic probes were employed to study the unsteady flow characteristics. Based on the experimental results a positive first helical mode of instability was identified with a wavelength and frequency depending on swirl. The wavelength was confirmed to grow monotonically with S, while the dominant frequency of the flow pulsations was found to have an unusual parabolic evolution with swirl, with a minimum at S _min=0.88. This finding was interpreted using a proposed kinematic model based on the contribution of two mechanisms: rotation and axial motion of the helical vortex. It was concluded that for S<S _min the instability frequency is essentially dominated by the axial translation of the spiral vortex being inversely proportional to S and therefore giving a decreasing trend. For S>S _min the frequency of the flow precession is more dependent on the angular transportation of the vortex core, which resulted in the expected growing dependence on S.