Extended Phase-Doppler Anemometry for Measurements in Three-Phase Flows

With an Extended PDA system (based on a commercial instrument from Aerometrics), details of the behaviour of the disperse phase in a bubble column were investigated for varying air concentrations (hold-ups) in water. The evolution of mean bubble velocities, sizes and relative bubble velocity in the flow was quantified. It is shown that, for the small air flow rates between 0,31n/min and 0,41n/min in the bubble column, corresponding to a maximum superficial gas velocity of 0,043 cm/s, the effect of bubble coalescence on the flow is small. During the motion of the bubbles through the flow field, an acceleration is observed in the lower part of the column, which is followed by a deceleration after a maximum velocity was reached around 250mm above the aerator. Mean bubble sizes around 300µm are found throughout the measurements. The influence of the air flow rate on the bubble size was found to be small. For bubbles larger than 150µm, a positive correlation between bubble size and velocity is found. An interesting correlation between the relative velocity of pairs of bubbles and the ratio of their sizes was found. The bubbles tend to approach each other if a small bubble moves in front of a larger one. The contrary is true for the inverse arrangement of the bubbles. The velocity of the relative motion increases with increasing deviation of the size ratio from the value one.