Effect of impeller geometry on bubble breakage and the contributions of different breakage mechanisms in a stirred tank

Abstract

The effect of impeller geometry on bubble breakage in a stirred tank was investigated for a range of impeller Reynolds number (Re) using a high speed imaging method. The bubble dynamic behavior and breakage mechanism were investigated for four different impeller geometries namely, two-flat blades impeller, four-flat blades impeller, four-twisted blades impeller, and two-pinned blades impeller. The performance of each geometry was investigated by determining the breakage probability and number of fragments (daughter bubbles) produced. The contributions of dominant breakage mechanisms for each geometry were specified and discussed by identifying the breakage locations relative to the impeller. Three main breakage mechanisms were observed, namely: bubble collision with the blade, bubble breakage by blade shear, and breakage by turbulent fluctuation away from the blade. The number of fragments by each breakage mechanism was specified for the entire range of Re. The four-flat blades impeller exhibited the highest breakage probability and produced the highest number of fragments. The Pinned blades gave a high performance compared to the smooth blades, especially at higher Re. This is considered to be due to the high turbulence level provided by this type of impeller. The twisted blades impeller showed low bubble breakage probability compared with the other geometries. Breakages by collision with the blades and by shearing effect resulted in a higher number of fragments compared to the breakages caused by turbulent fluctuations. The number of fragments produced by ‘collision with blade’ exhibited a higher dependence on Re than by ‘blade shear’ or by ‘turbulent fluctuations’.

Experimental study on the differences between bubble and drop breakages in agitated turbulent flows

“…As shown in Figure 8, when N increases, the percent of binary breakage (PBB) of bubble or drop decreases while the percent of multiple breakage (PMB, including ternary, quaternary and higher‐order breakages) increases. The results are similarity with some existing experimental studies 48–50 . Note that the mean sizes of bubble and drop are not identical but very close (the average deviation is no more than 0.10 mm, similarly hereinafter) at each given N .…”

“…Figure 11 shows the effects of N and d 0 on breakage probabilities of bubble and drop. It can be seen that the breakage probability of bubble or drop quickly increases with increasing N or d 0 , which is consistent with the experimental finding in the literature 30,31,48–51 . This is because the larger d 0 may cause the weaker stability stress to resist the external destructive stress inflicted by turbulent eddy.…”

“…The results are similarity with some existing experimental studies. [48][49][50] Note that the mean sizes of bubble and drop are not identical but very close (the average deviation is no more than 0.10 mm, similarly hereinafter) at each given N. The larger N may mean the higher eddy kinetic energy, thus the eddy may have enough residual energy to cause the further breakage of daughter fragments. It can be seen from Figure 8 that the PBB of bubble is always higher than that of drop at the same N, which is consistent with the observation of Andersson and…”

“…It can be seen that the breakage probability of bubble or drop quickly increases with increasing N or d 0 , which is consistent with the experimental finding in the literature. 30,31,[48][49][50][51] This is because the larger d 0 may cause the weaker stability stress to resist the external destructive stress inflicted by turbulent eddy. Besides, the size range of turbulent eddy contributing to fluid particle breakage and the probability of collision between fluid particle and the eddy may increase with d 0 .…”

Effect of Electrode Surface Enhancement on the Performance of Microbial Fuel Cell Under Flow Conditions

“…As a result, the potential gap widens, causing the generated current to rise. Because yeast is anaerobic bacterium, the current reduces at steady-state because increased O2 transfer from the cathode chamber to the anode chamber induces a reduction in bacterial activity [28].…”