Fluidization & Granular Matter
Positron Emission particle Tracking
|Generally speaking 2 flow regimes exist in bubble columns: (i) at low superficial gas velocities we find the homogeneous bubbly flow and (ii) at higher supreficial gas velocities the heterogeneous regime. The former is characterized by a narrow bubble size distrbution with all bubbles rising vertically upwards with more or less the same velocity. No large scale liquid velocity is induced. In the latter, the gas fraction distribution in a cross-section of the bubble column is non-uniform. Time averaged, the gas fraction is higher in the center than close to the wall. The liquid flows upwards in the center and downwards in the wall region, creating a broad residence time distribution of the bubbles and backmixing in the liquid phase.||
Fluidization at relatively low gas velocities or with coarser particles, creates a state in which the powder mass is very mobile,
but still densely 'packed'. Bubbles, i.e. voids, me rise upwards through this powder mass. These bubbles cause mixing and flow at a
macro-scale. The gas that flows through these bubbles has a different contact time than the interstitial gas in the powder mass.
Experimental study of fluidized beds is difficult, as the powder mass in opaque. Therefore, we use X-ray Tomographic techniques to study the internal solids distribution and measure the motion and size of the bubbles.
Multi Phase Flow