Scale-wise analysis of upward turbulent bubbly flows: An experimental study

Abstract

In this work, we investigate the upward turbulent bubbly pipe flow according to relevant length scales, to understand the bubble-induced turbulence modulation of the liquid-phase flow. Using two-phase particle image velocimetry, the bubbly flow fields in a vertical pipe (diameter of 40 mm) were obtained for Reynolds numbers (ReD) of 5300 and 44 000, while increasing the volume void fraction (bubble size of 2.5–4.5 mm) to 1.8% and 1.5%, respectively. The turbulent bubbly flow is visualized and further quantified from the decomposed flow fields (according to the length scales) using the discrete wavelet transform. At ReD = 5300, flow structures of the energy-containing scales (larger than the Taylor microscale) are energized by the disturbance from bubble surface deformation and near wake (whose scales are located close to the integral scale), resulting in the turbulence enhancement across the entire pipe. The interaction between wake vortices, sized between the integral scale and Taylor microscale, apart from the near wake also contributes to the turbulence enhancement. At higher ReD, despite the similar bubble size and void fraction, the scales of flow agitation owing to the bubble surface and the shed wake vortices become smaller, having a negligible effect on turbulence modification. However, the scale of the near wake is still large enough to enhance the turbulence at the pipe center (although it is significantly reduced toward the wall). With the highest void fraction among tested, on the other hand, the mean liquid velocity gradient becomes milder near the pipe wall, which causes a turbulence suppression locally near the wall.