Effects of Pulsation Intensity on Flow and Mixing of Swirling Double-Concentric Jets

Abstract

The effects of pulsation intensity on the flow and mixing characteristics of acoustically excited swirling double-concentric jets were studied experimentally. The central jet was excited with various pulsation intensities. The flow evolution process, jet spread width, velocity instabilities, and jet dispersion characteristics were examined by the flow visualization, edge-detection technique, hot-wire anemometer, and tracer-gas concentration detection, respectively. As the pulsation intensity was increased over 0.16 one vortex ring was induced during each excitation cycle. This induced vortex ring evolved downstream and suddenly broke up into turbulent eddies at a distance downstream from the jet exit. The dispersion and mixing were simultaneously dominated by two mechanisms: 1) the breakup of the induced vortex ring into turbulent eddies, and 2) the near-field entrainment of the induced vortex rings. The former was more important at pulsation intensities smaller than about 0.5, whereas the latter was more prominent at pulsation intensities larger than approximately 0.5. The results of flow visualization as well as the measurements of velocity, fluctuation intensity, turbulence time and length scales, and tracer-gas-concentration distributions showed close correlations in interpreting the flow and mixing characteristics. Exciting the swirling double-concentric jets at a pulsation intensity of 1.0 might increase the mixing index by 90% over the unexcited jets.