Large Eddy Simulation of Isothermal Confined Swirling Flow with Recirculation

ABSTRACT The present paper reports progress in applying Large Eddy Simulation (LES) techniques to the analysis of a flow problem relevant to Lean-Premixed-Prevapourised (LPP) combustor designs currently under investigation. Such flows combine high levels of swirl with flow confinement geometries that induce a central recirculation zone (CRZ). The turbulent mixing processes in these flows are difficult to capture accurately using RANS statistical turbulence models. In addition, the required precise control of fuel/air ratio means that detailed knowledge of the unsteady fluctuating behaviour is required, hence the interest in the ability of LES methods for flow simulation. Details are presented of the particular LES algorithm adopted, together with two benchmark problems (one including swirl) that have been used for code validation purposes. The multi-block structured mesh LES method is then applied to the isothermal flow in a typical LPP pre-mixer geometry. The instantaneous behaviour predicted when the calculations have reached a statistically stationary state is analysed by examination of time histories at selected points. These show evidence of extremely large-scale oscillations of the CRZ not obvious in the time-averaged flow pattern. Some indications of swirl precession are also observed. Time-mean data evaluated by averaging the LES predictions show good agreement with measurements and an improved level of fidelity compared to RANS eddy viscosity predictions. This encourages further use of the LES approach for this type of flow, for example by extension to scalar mixing predictions.