Bi-global stability and forced response analysis of reacting, swirling jets.

Swirling jets are canonical flow fields used to stabilize flames in combustion systems. In this study, a bi-global hydrodynamic stability analysis is used to model the unsteady structures and vortical hydrodynamic modes of a harmonically forced, reacting swirling flow. The base state of this study is an axisymmetric, swirling and reacting mean flow computed with LES, based upon a commercial nozzle. First, the unforced, natural stability eigenmodes are analyzed for the linearized Navier Stokes equations around the base state using finite elements in COMSOL. Then, an empirical velocity transfer function is calculated by determining the flow response to a harmonically varying 150-1500 Hz velocity disturbance imposed normal to the inflow boundary. A key result of this study shows that 1050 Hz inlet-forced disturbance is dominant in amplification of axial velocity disturbances. Excitations around this St = 0.375 mode lead to axial velocity disturbances with amplification factors varying between 20 and 50 times at axial locations 0.24 < z/d_Sw < 1.5 and over 60 times at downstream axial locations z/d_Sw > 1.5. Lastly, this study also considers comparison of stability solutions in a fully or tri-global framework for a 3-D, cartesian base flow as a future direction.