Experimental study and analytical reconstruction of precessing vortex in a tangential swirler

Abstract

We report on experimental and analytical studies of strongly swirling air flow in the near-field of a generic axisymmetric tangential swirler used often as swirl generator in combustors, phase separators or mixing devices. The focus was on the characterization of the precessing vortex core (PVC) for a range of swirl numbers S=1.4–2.4 and Reynolds numbers of 14,460–38,300. The velocity field and the spectra measured with a Laser–Doppler anemometer confirmed in all cases the occurrence of PVC and the central recirculation zone (CRZ) that for higher swirl numbers penetrates back into the swirler chamber. The instantaneous pressure, its frequency characteristics and the phase-average values have been measured with a sensitive microphone equipped with a special probe tip. We show then that the PVC frequency, helix diameter and the core displacement computed from the analytical model of a confined helical vortex (Alekseenko et al., 1999, 2007) using only the time-averaged velocity field agree well with the measurements, whereas the analytically reconstructed phase-averaged velocity and pressure distributions show good qualitative agreement with the measured pressure fields. The exception is the discrepancy in the pressure amplitude, which for the highest S=2.4 reaches almost 50%. Despite the latter imperfection, it has been confirmed that the analytical model can be used to identify the existence of a PVC and to compute its structure parameters solely from the measured (or otherwise available) time-mean axial and tangential velocities.