Influences of heat release, blockage ratio and swirl on the recirculation zone behind a bluff body

Abstract

The recirculation zone created through vortex breakdown mechanisms in swirling flows plays a vital role for aerodynamic stabilisation of turbulent flames in practical combustion systems. This zone interacts with the central recirculation zone (CRZ) of an upstream bluff body and this leads to a complex flow behaviour that depends on the blockage ratio and swirl number. It has been previously observed that the vortex breakdown bubble (VBB) merges with the CRZ at large swirl number or blockage ratio. In this study, the influences of heat release on this flow structure and their physical mechanisms are explored through a series of large eddy simula- tions and experiments of bluff body stabilised premixed flames with swirling flows. Comparisons of simulation results with measurements are good. It is observed that in isothermal flows, as the swirl number or blockage ratio is increased, the vortex breakdown bubble moves upstream and its mean structure changes. The effect of heat release leads to considerable differences in the flow characteristics as the vortex breakdown bubble is pushed downstream due to dilatation. The critical swirl num- ber, at which the VBB and CRZ merge, is observed to be higher in reacting flows for the same blockage ratio.