Large eddy simulation of swirling flows in a non-reacting trapped-vortex combustor

Abstract

Numerical investigation of swirling flows in a trapped-vortex combustor (TVC) is carried out using large eddy simulation (LES). A multi-block flow solver is employed to solve three-dimensional filtered compressible Navier-Stokes equations in generalized coordinate system through the use of high-order compact differencing schemes. The effect of swirl strength on the behavior of swirling flow features including vortex-breakdown, precessing-vortex core (PVC) and shear layer is practiced for a TVC with circumferential cavity and annular inlet mainstream. The axial curved-vane swirlers are designed to generate flows with swirl numbers of 0 and 0.34 (low-swirl), 0.6 and 0.75 (medium-swirl), and 1.03 and 1.34 (high-swirl). Evaluation of the mean flow field reveals that in the medium-swirl range, a central recirculation zone (CRZ) is established due to the vortex-breakdown mechanism apart from the bluff-body recirculation zone, while no CRZ is observed for low and high swirl ranges. The case with S = 0.6 in the medium-swirl range demonstrates a better performance by generating a larger central recirculation zone. The PVC is located in periphery of central recirculation zone and its precession frequency linearly increases with swirl number. The strain rate and overall pressure drop are invoked for further analysis of the flow field. The strain rate is sharp and intense in the low-swirl range and becomes diluted as the swirl number increases with its maximum point moving towards the cavity. Moreover, an overall pressure drop of under 5% is achieved for the low and medium-swirl ranges. Finally, turbulent kinetic energy (TKE) and local turbulence intensity as potential indicators of higher mixing in combustion systems are investigated. High levels of TKE are found to occur in all cases. For medium-swirl range, the high-TKE region is located at the proximity of shear layer between mainstream jet and cavity recirculation zone. Furthermore, for medium-swirl numbers the local turbulence intensity near the central recirculation zone boundary is between 50 and 100%.