In the previous seminar, an understanding of gas entrainment mechanisms in a vertical shaft rotating at
high speeds was covered. This seminar is focused on mitigation strategies for gas entrainment using
computational fluid dynamics (CFD). Based on the previous findings, it was determined that the primary
source of gas entrainment at higher shaft speeds is the breakage of primary vortices (Regime 4).
In order to mitigate this issue, various mitigation techniques were used to eliminate or minimize the
impact of vortex breakage, as well as reduce the shear waves observed in Regime 2. Various
arrangements of baffle plates were simulated, including horizontal/vertical plates at an angle placed at
different heights on the shaft or outer vessel. The porosity of the plates and the influence of contact
angles and slip were also examined during the study.
The simulation results revealed that specific plate arrangements on the shaft effectively eliminated gas
entrainment resulting from vortex breakage. However, they inadvertently increased gas entrainment
through shear waves. The inclusion of baffle plates on the shaft led to greater instability in the free
surface flow, consequently amplifying gas entrainment. On the other hand, a plate placed 50mm below
the surface on the outer vessel demonstrated a reduction in gas entrainment, although it failed to
eliminate the vortex break near the shaft. Porous plates placed horizontally at the outer vessel reduced
the gas entrainment; however, vertical porous plates gave more promising results, based on encouraging
results from the porous plate study, a model of the plate with holes was also developed to achieve more
realistic results. Simulation results suggest reduced gas entrainment.
To investigate the impact of shaft surface characteristics, simulations were conducted for the contact
angles of 90o, 150o, and 30o. Similarly, to simulate a hydrophobic shaft surface, simulations
incorporating slips on a surface with a 10 mm slip length were modeled. And the study showed reduced
gas entrainment.