Gravity Driven Bubbly Flows: The Role of Vortical Structures

Gravity driven bubbles are found in many industrial applications. Two typical reactors are the bubble column, in which the liquid is stagnant and the air lift reactor in which the liquid circulates, under the action of gravity, through the reactor. These reactors are attractive for a number of reasons: they have no moving parts and are thus low in maintenance; the size can be enormous (diameters of several meters, heights of tens of meters) allowing large volume flows to be processed; good mixing and heat transfer characteristics, etc. Our knowledge about the structure of the flow induced is rather limited. This makes design, fine tuning of operation and scale up still difficult. The two-phase flow in a bubble reactor is complicated. In the bubble columns, the liquid exhibits a large-scale circulation in a time averaged sense, with upward flow in the center and downward flow in the wall region. The first reliable data on this large-scale circulation date back to the work of Hills (174). In 1984, Franz et al. reported on the motion of what was later called vortical structures, eddy like structures (with sizes on the order of the column diameter) that move through the bubbly mixture. These vortical structures have been research more extensive during the last ten year by e.g. Fan and coworkers, Dudukovic and coworkers and Mudde & Van den Akker. The structures are found for a wide range of gas fractions, ranging from a few percent to well above 20%. The vortical structures seem to be a universal feature of the gravity driven bubble flows as they were also found in air lift reactors. For this reactor it has been reported that the liquid flow behaves more or less like the superposition of a net liquid flow and the complicated flow features found in the bubble column. The similarities will be high lighted. The vortical structures have important consequences for e.g. the (pseudo-)turbulence and the mixing in the bubbly flow. In 2-dimensional equipment they appear very regular and a separation between the low frequency fluctuations and the high frequency ‘turbulence’ is easily made. However, in 3-dimensional columns the situation is more complicated. LDA data show that the vortical structures are still responsible for a the occurrence of low frequency oscillations (on the order of 0.1Hz), but they are no longer appearing regularly and a separation of frequencies is no longer possible. Finally, the newest experiment seem to show that the vortical structures can be suppressed up to gas fraction of (at least) 10%. These new experiments suggest that the gravity driven bubbly flow is not inherently unstable, but rather sensitive to the conditions at the gas inlet.