Investigation of Vortex Breakdown and Scalar Mixing in Swirling Turbulent Jet Flows using Unified LES-RANS Models

Abstract

The understanding of the vortex breakdown phenomenon in turbulent swirling flows is essential for many practical applications. Hence, a lot of research has been carried out to explain this phenomenon. However, the exact mechanism responsible for vortex breakdown phenomenon still remains unclear. Consequently, there is a clear need for further investigation of this phenomenon. Large-eddy simulation (LES) has proven to be an accurate and numerically feasible approach for swirling turbulent jet flow simulations. However, the inflow conditions for turbulent swirling jet flows are often determined by a nozzle flow, which has a significant influence on the jet flow development and on the prediction of the onset of vortex breakdown. LES simulations of the nozzle flow are often infeasible: the computational cost of simulation of near-wall fluid motions are comparable with those of direct numerical simulation. Hence, nozzle flow simulations are usually performed using Reynolds-averaged Navier-Stokes (RANS) methods. The numerical predictions of RANS models are unreliable when no experimental or benchmark data is available for comparison. Hence, there is a need for the development of models, which can simulate both the nozzle flow and the jet flow region accurately. This study investigates the combined numerical simulation of a nozzle flow and swirling turbulent jet flow by a unified turbulence model. The unified turbulence model combines RANS methods in the near-wall region with LES methods away from the wall. The numerical simulations performed using the unified model have a much lesser computational cost than the combined nozzle and jet flow simulations using LES. The accuracy of the numerical predictions is validated against experimental data for non-swirling and swirling turbulent jet flows. The application of the validated model to study the effect of swirl clearly shows the onset of vortex breakdown at swirl number comparable to experiments. The vortex breakdown phenomenon was found to enhance the mixing efficiency.