Turbulence structure of bubbly upward pipe flow (high spatial and temporal resolution measurements using high speed time series PTV)

Abstract Turbulence modification by dispersed bubbles in an upward pipe flow was investigated by particle tracking velocimetry and bubble shape projection imaging. Time duration of CMOS camera frames was much smaller than the Kolmogorov time scale, so that measurments with a high time resolution were achieved to resolve the dissipation process of turbulence in the presence of bubbles. Two different bubble diameters at the void fraction up to 1.5% were examined. The profiles of mean streamwise velocity of water were flattened in the pipe middle region, because the bubbles accumulating near the pipe wall accelerated the fluid. The flattened mean flow profiles suppressed the shearinduced turbulence intensities. The dissipative eddies around small bubbles, whose size is approximately five times the mean diameter of small bubbles, increased the dissipation rate of turbulence kinetic energy. On the other hand, the turbulence intensities were augmented by the interactions between the eddies generated by the large bubble and the shear-induced turbulence amongst large bubbles, which resulted in the significant enhancement of dissipation process of turbulence kinetic energy.