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.