Investigation of Precessing-Vortex-Core–Flame Interaction Based on Tomographic Reconstruction Techniques

Unsteady helical flow structures, such as the precessing vortex core (PVC), are often observed in swirling flows with vortex breakdown. Although this type of flow is of high relevance for industrial combustors, the role of these flow instabilities in reacting systems, in particular their effect on flame stabilization and combustion instabilities, remains poorly understood. The three-dimensional structure of the interaction between the helical mode and the flame is difficult to assess with common measurement techniques, such as chemiluminescence imaging, due to the non-axisymmetry of the oscillation pattern. In the present work, a novel method is proposed to determine the full field of the heat release rate perturbation associated with the helical mode. This method requires only line-of-sight integrated information from a single camera. Tomographic reconstruction techniques are used, exploiting the fact that the helical mode is a rotating structure. Reconstruction algorithms are presented that are tailored to the specific spatio-temporal structure of the oscillation pattern, and it is shown that these techniques outperform standard methods. The proposed methodology is applied in a turbulent swirl-stabilized model combustor with significant PVC oscillations. Images from an intensified high-speed camera are used for the reconstruction. The analysis shows that the helical mode perturbs the flame in the inner and the outer shear layers of the annular jet and thereby creates helical traveling waves. The perturbation in the outer shear layer grows significantly in downstream direction and causes strong heat release rate fluctuations when impinging on the combustor wall.