Particle tracking using micro bubbles in bubbly flows

Micro bubbles do not contaminate multiphase flows like conventional tracer particles so that the velocity in such can be determined completely non-intrusive by tracking them. Micro bubbles, however, are often larger as conventional tracer particles and have a significantly different density than the surrounding fluid. The ability of micro bubbles to follow the flow is investigated in this work. For this purpose, the particle tracking velocimetry results that are obtained with naturally occurring micro bubbles are compared to particle image velocimetry measurements using PMMA tracer particles. In combination with the used volume illumination, a simple, robust and reliable measuring technique is presented, which is deployable for complex problems from biological to oceanic engineering.
Averaged liquid velocities as well as basic turbulence parameters are determined in a rectangular bubble column for different gas volume flow rates. High flow rats are good manageable due to the volume illumination whereas the PIV measurements using a light sheet are approaching their limits. The general sampling bias in multiphase flows found recently for PIV measurements is also present for particle tracking methods; a hold processor that waits a time depending on the distribution of the particle information over the measuring area gives reasonable results.