Large eddy simulation of isothermal turbulent swirling jets

Swirl stabilized flames are common in many engineering applications and modelling of such flames are particularly difficult due to their recirculation and vortex characteristics. Most classical approaches such as Reynolds averaged Navier-Stokes (RANS) models, which work very well in other situations, fail to perform well in high recirculation swirling flows. Large Eddy Simulation (LES) offers the possibility of improving calculations of such flows. This paper is concerned with the application of the LES technique to turbulent isothermal swirling flows. The aim is to improve our understanding of the flow physics and turbulence structure of unconfined swirling flows and examine the capability of LES to predict the formation of vortex breakdown in recirculation zones. In this study, a recently developed LES code has been applied to the prediction of isothermal swirling flows experimentally studied by Al- Abdeli and Masri (2003). The filtered Navier-Stokes equations are closed using the Smagorinsky eddy viscosity model with localized dynamic procedure of Piomelli and Liu (1995). Advanced numerical schemes with finite volume formulation on non-uniform Cartesian grids are employed for discretization of the conservation equations. Three different test cases have been investigated here covering a range of swirl numbers and stream wise annular velocities. The cases considered have swirl numbers ranging from 0 to 1.59 and Reynolds numbers from 32400 to 59000. With suitable inflow, outflow boundary conditions and sufficient grid resolutions the LES calculations found to be in good agreement with experimental data. It has been observed that the onset of downstream recirculation and vortex breakdown does not depend on the attainment of high swirl number alone. It appears that the bubble type vortex breakdown is achieved in the flow with a lower rather than higher swirl number. The axial momentum of the swirling annulus plays an important role in the onset of vortex breakdown. The combination of lower swirl number and higher axial velocity of the primary annulus leads to establish the downstream central recirculation zone (VB). These features have been successfully reproduced by LES calculations. For all the cases considered here LES calculations were successful in predicting observed recirculation zones and generally showed reasonably good agreement with experimentally measured mean velocities, their rms fluctuations and Reynolds shear stresses.