Experimental study on flow and turbulence characteristics of bubbly jet with low void fraction

Abstract

Abstract An experimental study was conducted on the flow characteristics and turbulence suppression in vertically issued bubbly jets with low void fraction (less than 1%, Re\xa0=\xa03,172). The effect of bubble distributions on the flow characteristics of the bubbly jet was investigated using planar Laser-Induced Fluorescence (PLIF) in the same measurement plane as Particle Image Velocimetry (PIV) for the phase separation. To ensure uniform size of the small bubbles, a bubble generator consisting of small needles was designed and manufactured. Due to the low void fraction, the velocity fields of each phase were successfully obtained. The slip velocity was constant in the far-field of the flow as 2.1 times of the inlet velocity in this study. In the early stage of the jet, there is an acceleration region that is attributed to the buoyancy of bubbles. As the potential core of the bubbly jets collapses, the distribution of bubbles seems to have an annular truncated cone shape, which evolves with the liquid flow structure. Most bubbles are populated the region around the velocity radius of the bubbly jet. Reynolds shear stresses and normal stresses of the dilute bubbly jet generally show smaller magnitudes than those of pure jets, buoyant jets, bubbly jets, and bubbly plumes. The case with higher void fraction (1%, Exp.330) shows similar magnitude of Reynolds axial stresses to jets, but magnitudes of other stresses are similar to the case with lower void fraction (0.66, Exp.320). The results are probably attributed to suppression of turbulence in low-void-fraction condition.