Application of linear hydrodynamic stability analysis to reacting swirling combustor flows

Abstract

This paper presents recent research activities on coherent structures in combustor flows employing linear hydrodynamic stability theory. Large-scale coherent structures play an important role in swirling combustor flows. On the one hand isothermal swirling jets undergoing vortex breakdown are susceptible to self-excited flow oscillations. They manifest in a precessing vortex core and synchronized growth of large-scale helical vortical structures. On the other hand, thermoacoustic oscillations are often related to axisymmetric flow structures that are driven by the acoustic field. Despite the qualitative difference between the self-excited helical instabilities and the acoustically forced axisymmetric instabilities, the linear analysis is capable to describe both phenomena with an astonishing accuracy. The proposed theoretical framework allows for a systematic analysis of the dominant flow dynamics in the turbulent combustor flow and their interplay with the flame. This is demonstrated by considering two examples:a swirl-stabilized flame featuring a precessing vortex core that becomes suppressed when changing from steam-diluted to dry conditions, and a swirl-stabilized flame subjected to strong axial forcing mimicking thermo-acoustic oscillations.

Keywords

• linear hydrodynamic stability
• reacting swirling flows