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To better understand the balance between drag on particles due to mean flow, and engulfment of particles from the surface by turbulent eddies, CFD simulations were performed for four different fully baffled tank configurations: the PBTD and the PBTU at S = T/2 and S = T/4, all at [N.sub.jd].
For the PBTD, the direction and magnitude of the velocity arrows at the surface indicates that the particles are carried from the walls and then immersed around the shaft for both submergences.
The results for the PBTU shown in Figures 7c and d are quite different from those for the PBTD. The impeller discharge flow reaches the wall before it reaches the surface, splitting into two branches and forming a weak secondary circulation loop at the surface.
The axial velocity at the surface (Figure 8a) is very large at large submergences, and very small at small submergences for both the PBTU and PBTD. Figure 8b shows that the TKE is small at large submergences, so mean drag dominates at large submergences.
The A340 and the PBTD both form a single circulation loop between the impeller and the surface while the PBTU forms a secondary circulation loop at the surface for S/T > 0.375, mirroring the behaviour of the PBTD which forms a secondary circulation loop in the bottom of the tank once the off bottom clearance (C) exceeds 0.35 T (Kresta and Wood, 1993).
Figure 9a shows the [N.sub.jd] results for the PBTD for all three baffle configurations.
The trend for the unbaffled configuration is similar to the PBTD unbaffled case, differing only in the maximum [N.sub.jd] required at large submergences (350 rpm vs.
For the fully baffled case, the A340 will require less power than the PBTD at more typical submergences (for S = 0.25 T, [N.sub.jd] = 300, 225 and 350 rpm for the PBTD, PBTU, and A340, respectively, with P = 4.2, 1.7 and 1.7 W and CD = 0.21, 0.24 and 0.16 m).
The energy that the PBTD uses in the recirculation loop to draw down the solids is stable and strong over the range of submergences tested.
When comparing the three impellers in terms of the surface vortex, the A340 surface vortex is smaller in diameter and depth than the ones created by the PBTD and PBTU for both the unbaffled and the one baffle configurations.
Turbulence is the main mechanism at small submergences for both the PBTD and PBTU.
Four full baffles and a PBTD is the mixing geometry recommended for large submergences, while for smaller submergences the results suggest that the up-pumping PBT will be much more efficient.