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duced in the flow with the ability to mimic the motion in the phase of interest. Considering the limitation of CARPT technique which has a spatial resolution of the data is 7 mm [26], the CFD results are reported on ensemble average basis in a 7 mm zone around the centreline of the measurement point.
The radial velocity of the solid particles at impeller plane is shown in Fig. 4. On x axis, r is radial position, Pi is impeller radius and fi is stirred tanl‹ radius. Out of the four drag models wide dispai ity with experimental data was observed when using the Wen and Yu and the Gidaspow model. These two models predicted the highest radial velocities. Guha et al. [27] observed similar overprediction of radial velocities while using the Schiller—Nauman drag model. The Brucato drag model slightly overpredicted the radial velocity, whereas the predictions from the modified Brucato drag were in reasonable agreement with experimental data. The solid velocities are higher at the impeller tip. As the solids approach towards wall, the velocity gradually decreases. Due to no slip condition on the wall, the velocity should gradually reach zero value at wall. But, quantitatively, there is an over—prediction of the velocities in simulations in near wall region. The disparity can be attributed to lesser number of data points available for averaging in experiments. The experiments clearly show a zero ensemble averaged value even at (r — fiiJ/(fi — ii) = 0.8, which is not reasonable.
At low solid concentrations, Gidaspow drag model acts lil‹e Wen and Yu model and at higher concentrations it takes the form of the Ergun equation. Therefore, both Wen and Yu model and Gidaspow models predict the same result. The modified Brucato drag model accounts for the effect of solid phase on the turbulence. At higher impeller speed, the role of turbulence in calculation of drag is vital factor, hence, the modified Brucato drag model predicts better re- sults as compared to the other drag models.
Fig. 5 shows the comparison between the simulations results and experimental data for radial velocity at axial plane raft = —0.5. A po- sitive radial velocity is expected in the upper zone of the impeller. As compared to the negative velocity in the region higher than the impeller zone where the magnitude of negative velocity is merely 2$ of the maximum velocity attained by the solids, the negative ra— dial velocity in the bottom of the tank is 10$ of the maximum veloc- ity. This corresponds to a relatively stronger flow towards the centre in the bottom of the tanl‹. It indicates the presence of stronger clock- wise currents. All the drag models could qualitatively capture this flow behaviour. For the experimental data, the highest tangential velocity is observed at zJT= 0.36 i 0.04. This compares well the sim- ulation result of zJT - 0.34. In the lower region of the stirred tank,
0.40
0.00
S.3.2. Velocity components
The simulations were run using different drag models and the results were then compared with the experimental data. Guha et al. [8] used CARPT technique, in which a single particle is intro—
020 040 0.60 0.80 1.00
Fig. 4. Radial velocity at impeller plane for 0.04 solid volume fraction and 1000 rpm. 0 Guha et al. (2007), - - Wen and Yu model, Gidaspow Model,
— — Brucato drag model, — Modified Brucato drag model.
D. VVad ilerkar ct aJ. Advanced Powder Technology 23 (20 12) 445—453