Effect of Reynolds number on the coherent structure and dynamics of vortex in double-stage counter-rotating swirling flows

In this paper, the isothermal swirling flow in a combustion chamber equipped with a double-stage swirler is studied by a combination of experiments and numerical simulations at the Reynolds number (Re) ranging from 2712 to 43 396. The swirl numbers of the inside and outside entrances of the swirler are 0.81 and 0.89, respectively. The effect of Re on the mean flow field, the oscillation and evolution characteristics of instantaneous vortex structures such as vortex breakdown and precessing vortex core (PVC) are systematically analyzed. It is found that there is a significant difference between low and high Re numbers, with the critical Re number occurring at a value of 16 273. After the critical Re, the mean axial and tangential velocity indicates the presence of Reynolds self-similarity. The precession intensity of the PVC increases significantly, while the helical diameter of the PVC decreases with the increase in Re number. The characteristic frequency of the PVC in the swirling field increases linearly with the Re number, and the second characteristic frequency appears at Re = 16 273, which is confirmed to be related to the combined effects of the PVC and shear layer (SL) thermoacoustic instability, which is caused by a regular high-frequency vortex shedding along the SL.