Flow characteristics and entrained bubble dynamics of underwater plunging jet measured by time-resolved three-dimensional Lagrangian particle tracking velocimetry

Plunging water jet onto a water pool is widely utilized in various industry field to promote heat and mass transfer. However, studies to measure flow field at air entrainment condition have rarely reported. In this study, we investigate gas-liquid multiphase flow field generated by plunging jet and air entrainment using time-resolved three-dimensional particle tracking velocimetry (4-D PTV) measurement technique. 4 high-speed cameras were installed to measure bubbly flow in a water tank with 16 angles and alternative illumination of blue and white LEDs is adopted to get each particle and bubble images. 3-D Lagrangian particle tracking with Shake-the-Box (STB) algorithm and vortex-in-cell sharp (VIC#) reconstruction were used to obtain flow field from measured particle images and bubble properties are calculated by binarized bubble images. 3-D Lagrangian particle tracking shows well-tracked result without any trajectory cut-off. Eulerian reconstructed flow field shows axisymmetric, and axial mean velocity profiles follows a Gaussian function. Axial turbulent intensity at the center is a unity at the center and maximum turbulent intensity of radial, azimuthal direction is one-fifth of axial turbulent intensity. Bubble equivalent diameter has large standard deviation because of existence of both large bubbles from air entrainment and small bubbles from bubble collapse. Despite of the existence of bubbles, impinging jet is similar to typical free shear flow.