Large eddy simulation of unconfined turbulent swirling flow

Abstract

Large eddy simulations (LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of experimental data show that the LES results are capable of predicting mean and root-mean-square velocity profiles. The LES results show that the annular swirling flow has a minor impact on the formation of the bluff-body recirculation zone. The vortex structures near the shear layers, visualized by the iso-surface of Q-criterion, display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit. Spectral analysis was employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core (PVC). In order to extract accurately the unsteady large-scale structures in swirling flow, a three-dimensional proper orthogonal decomposition (POD) method was developed to reconstruct turbulent fluctuating velocity fields. POD analysis reveals that flow fields contain co-existing helical and toroidal shaped coherent structures. The helical structure associated with the PVC is the most energetic dynamic flow structure. The latter toroidal structure associated with vortex shedding has lower energy content which indicates that it is a secondary structure. Cambridge swirl burner, large eddy simulation, proper orthogonal decomposition, vortex shedding and breakdown, precessing vortex core Citation: Zhang H D, Han C, Ye T H, et al. Large eddy simulation of unconfined turbulent swirling flow.

Single- and double-helix vortex breakdown as two dominant global modes in turbulent swirling jet flow

“…The addition of swirl has a significant impact on the flow structures in the initial merging region of an annular jet. If a very small amount of swirl is added, the Kelvin-Helmholtz instability in the inner and outer shear layers becomes helical (Garcia-Villalba, Frohlich & Rodi 2006;Jones, Lyra & Navarro-Martinex 2012;Zhang et al 2015) and the meandering of the wake at zero swirl transforms into a precession (Vanierschot & Van den Bulck 2011;Vanierschot et al 2014). As the level of swirl is further increased, these helical instabilities become stronger.…”

Large Eddy Simulation of Premixed Stratified Swirling Flame Using the Finite Rate Chemistry Approach

“…In the present work, the 2-step global mechanism for methane oxidation is selected to handle the combustion and the chemical reactions are formulated as equations (15) and (16). Two sets of Arrhenius parameters are adopted to describe the chemical kinetics, one for the 2sCM2 scheme [37] (referred to as Mech I) and the other for the Arrhenius parameter-optimized 2-step scheme (referred to as Mech II) reduced based on the GRI 3.0.…”

“…Velocity and major species profiles showed good agreement with experimental data, whilst local temperature profiles had slight discrepancies in the central regions for swirling cases. Zhang et al [16] simulated both reacting and nonreacting SwB cases to assess different LES SGS models. Dynamic thickened flame combined with the FGM tabulation approach (DTF-FGM) and presumed PDF combined with the FGM approach (PPDF-FGM) were used to model premixed flame and stratified flame, where flame features and unsteady large-scale structure behaviors were investigated.…”

Determination of single and double helical structures in a swirling jet by spectral proper orthogonal decomposition

“…Jones et al 37 simulated turbulent confined swirling annular flows using LES to study the behavior of the large-scale coherent structures formed by the high levels of swirl. Zhang et al 38 found that the helical structure associated with the PVC is the most energetic dynamic flow structure in the non-reacting flow fields of a Cambridge swirl burner by performing LES. In addition, LES was utilized to asses the capability of Reynolds-averaged Navier-Stokes simulations on predicting helical instabilities in swirling flows [39][40][41] .…”