Numerical Studies on the Influence of Periodical Flow Forcing on Mixing Quality and Flow Structure of a Swirl Burner

Helical coherent structures and strong axial mass flow fluctuations related to acoustic instabilities are common features in swirl-stabilized gas turbine combustors. The investigation of these dynamical phenomena, in particular their interaction, is therefore important. In the present work, the periodical excitation of the flow field of a swirl burner and its impact on coherent flow structures and mixing properties is examined. The effect of axial mass flow excitation with the dominant frequency of the coherent structure is studied. The investigations are conducted with unsteady RANS (URANS) and Large Eddy simulations (LES) for isothermal conditions to examine their capability of capturing the impact of forcing on the mixing and flow characteristics. Mean and turbulent flow fields as well as the concentration fields at the burner outlet are studied. The mixing quality is characterized by the degree of unmixedness, and the flow structures of the unforced and forced cases are analyzed by proper orthogonal decomposition (POD). With external forcing of the mean flow, the concentration field at the burner outlet becomes more homogeneous, i.e. the mixing quality increases. The unforced flow field exhibits a helical structure, which changes its characteristics when forcing is applied. The simulations (both URANS and LES) are able to reproduce the main features. URANS, however, strongly underpredicts the turbulence intensity in the recirculation zone and shear layers, and as a result, the kinetic energy distribution captured by the POD modes is not in agreement with experimental the findings. The results from LES are qualitatively and also quantitatively in good agreement with the experimental data.