Insights into the intrinsic asymmetry of bluff-body stabilised swirling and non-swirling annular flows

Abstract

Bluff-body stabilised annular jets possess simple, well-defined, axisymmetric geometry, yet exhibit highly complex flow fields, are susceptible to asymmetry at larger blockage ratios, and embody both complex zones and shear layers. Practical annular combustors feature swirl and a central jet which further complicates the flows. Despite the importance of asymmetry in practical combustor design, a systematic investigation for the role of flow regime (transitional, turbulent), Reynolds number, swirl number, or central jets on asymmetry remains lacking. In this study, two-dimensional time-averaged planar particle image velocimetry measurements are conducted to resolve the intrinsic asymmetry associated in transitional (Res = 2700) and turbulent (Res = 17,800 and 35,500) bluff-body stabilised confined annular jets. Experiments are conducted on non-swirling (S = 0) and swirling annular jets (with S = 0.3), both operating independently and with the addition of a turbulent central jet (Res = 10,400). Constant temperature anemometry measurements resolve boundary conditions at the exit of annulus and an Asymmetry Index is presented.Results show that swirl significantly mitigates asymmetric vortex shedding and restores annular jet symmetry. Quantification (at x/D = 1, r/D = 0.5) reveals that turbulent swirling annular jets demonstrates a 42 % (for Res = 17,800) and 78.6 % (for Res = 35,500) reduction in asymmetry compared to non-swirling annular jets. Whereas, swirling annular jets under transitional flow regime show a negligible impact on the flow asymmetry. In addition to this, increasing the Reynolds number from Res = 2,700 to 17,800 results in a 36.2 % and 58.7 % decrease in the asymmetry for non-swirling and swirling annular jets, respectively. A further increase to Res = 35,500 brings an additional 68.6 % decrease in the asymmetry for swirling annular jets, showing that swirl in combination with high turbulence restores and maintains symmetry in bluff-body stabilised annular jets. Moreover, at medium turbulence (Res = 17,800), the non-swirling annular jet with a superimposed turbulent jet exhibits a nearly axisymmetric flow behaviour compared to stand-alone non-swirling annular jet.