Flow and mixing characteristics of double-concentric jets pulsed at annular flow

Abstract The flow and mixing characteristics of double concentric jets pulsed at the annular flow were experimentally studied. The evolution process of the flow was observed and analyzed by means of the laser-light sheet assisted flow visualization method using the instantaneous streak pictures. The jet spread width was obtained using the binary boundary detection method based on the long-exposure pictures of the flow. The average velocities and turbulence intensities in both the centerline and radial directions were measured using the hot-wire anemometry. The mixing properties were determined using the tracer gas concentration detection technique. Three characteristic flow modes were identified in the domain of the central jet and the annular flow Reynolds numbers, namely the annular flow dominated mode, central jet dominated mode-low shear, and central jet dominated mode-high shear. In the annular flow dominated mode, the annular flow broke up the central jet in the centerline, where the axial velocity decayed quickly and the turbulence intensity increased to the maximum value. The larger jet spread in the radial direction enhanced the mixing characteristics in the recirculation region. In the central jet dominated mode-low shear, the central jet penetrated the recirculation region, resulting in significant entrainment of the annular flow. The axial velocity was modest and the turbulence intensity was insignificant; therefore, low mixing was observed in the recirculation region. In the central jet dominated mode-high shear, the recirculation bubble shrunk rapidly toward the jet exit, while the jet spread was controlled by the strong central jet entrainment. Although the annular flow interrupted the central jet and the turbulence intensity reached a peak after the combined jet, the enhancement of the mixing was quite seen to be lower than in the case of the annular flow dominated mode.