Analysis of coherent structures in partly confined swirling flows

Abstract

The precessing vortex core (PVC) and symmetric coherent structures in partly confined swirling flows of gas turbine combustors are investigated numerically. The computational combustor features a sidewall in the circumferential direction, and the ratio between the sidewall length (L) and dome height (H) is specified as 0, 0.27, 0.68, and 1 to construct various confinements. Then a large eddy simulation is performed under the isothermal condition to investigate the effect of different L/H values on dominant coherent structures in combustors. The time-averaged flowfields exhibit different shapes of the inner recirculation zone (IRZ) with increasing L/H. Proper orthogonal decomposition is used to extract coherent structures from the turbulence; the single helical structure referred to as the PVC, and the symmetric vortices that shed and are amplified in the shear layer, are found to dominate the evolution of the flow in all cases. Relevant phase-averaged velocity fields show that the PVC synchronizes with the IRZ rotating around the centerline of the combustor while the symmetric coherent structure causes the deformation of the IRZ. By examining the azimuthal wave related to the PVC, it is shown that the PVC is suppressed in the partly confined swirling flow, and its strength is enhanced as L/H increases. Changes in shear layer thickness are suggested as being responsible for the suppression of the PVC under the partly confined condition.