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Fig. 9 shows the comparison between the TIME profile at impel— ler plane for 0$, TO and 7$ solids volume fraction. Within the impeller radius, the TKE increases due to the increase in turbu-
Fig. 6. Tangential velocity at axial plane raft — 0.5 for 0.04 solid volume fraction ..d lence. Initially, rate of increase is low as the impeller disc offers
1000 rpm. I Guha et aI. (2007), - - Wen and Yu model, Gidaspow Model, _
Brucato drag model, — Modified Brucato drag model.
_ resistance. After the disc, the TIME increases steeply and i eaches
a maximum slightly beyond impeller radius. This behaviour is
D. Wadnei kar ct aI. / Advanced P0wder Teclin0f0gy 23 (2012) 445-453
(a) Single Phase Flow (b) Solid- Liquid System (19r v/v)
Fig. 8. Turbulent I‹inetic energy contours in stirred tanks at 1000 rpm.
Radial Location [ in ]
Fig. 9. Turbulent kinetic energy profiles at impeller plane at 4 000 rpm.
attributed the vortices leaving the impeller blade that result in high magnitude fluctuating velocities. After this point, the TKE gradually decreases along the radius due to decrease in velocities. As the velocity jet hits the vessel wall, it creates eddies resulting in fluctuating velocities. As a result a small peals in the TKE is ob- served near the wall.
The l‹inetic energy in the liquid is imparted to solids resulting in the solids following the jet. It is also the reason of maximum energy dissipation in this zone. The comparison shows 50 and
65 decrease in the l‹inetic energy observed for 4 é and 7$ vol- ume fraction of solids, respectively in the impeller plane. The l‹inetic energy of the liquid is dissipated in the suspension and dispersion of solid particles. This results in the decrease in the level of turbulence and is visible as lower levels of TIME. Nouri and Whitelaw [21 ] measured and analysed liquid and solid phase velocities in stirred vessels with solid concentration up to 0.02a. The effect of presence of particles, particle concentration and density is studied on the slip velocities and turbulence and the turbulence was found to decrease by up to 25a. Specifically, they found the dampening in the turbulence in impeller zone. In the impeller zone, both the TUE and particle concentration are maxi— mum. As a result, the dissipation of energy is the maximum in this region and leads to the maximum decrease in turbulence as particle concentration increases. The dampening of turbulence found by Derl‹sen et al. [9] was around ISO. This value is far low- er than as observed in this paper. Similar observations were made by Michelleti et al. [29] that presented the turbulence dampening
values between 50 and 70$. The decrease in turbulence with increase in solid concentration was also observed by Barresi and Baldi J30], Micheletti et al. [29,31 ] and Ayazi Shamlou and Koutsal‹os [32]. Micheletti et al. [29] conducted experiments to study velocity characteristics in stirred solid liquid suspension. The flow field measurement in the pi-esence of solids revealed significant influence of their presence. The maximum difference was observed in the impeller plane that diminished with increas- ing radial distance. These points support the findings in this paper where the turbulence is the maximum in the single phase flow and corresponding lower values of turbulence is observed for higher solid concentration. The difference in turbulence also decreases with the increase in the radial distance. In practical conditions, due to the increase in solids concentration, the dissipation of energy will be higher due to the high frequency of particle—particle, particle-wall and particle blades collision. The turbulence dampens in the presence of solids and the magni- tude of vortices leaving the impeller decreases. For- the same reason, a shift in the peak of TUE is observed with increase in solid concentration.