6.3 Results and discussion
6.3.1 Flow of bidisperse suspension in a straight channel
For a thorough understanding of the effect of particle size ratio and the individual species concentration on the shear-induced migration and margination phenomenon, simulations were carried out for suspension flow in a straight channel without bifurcation. The geometrical details of the straight channel were same as that of the inlet branch. For the comparison purpose, simulations were also carried out for the monodisperse suspension for different particle sizes. Fig. 6.2(a) depicts the fully-developed concentration profiles for monodisperse suspension in the straight channel for two different particle radius (a
= 10, 30 µm). The average inlet particle concentration (φ) was 0.25 and the velocity magnitude was 0.0063 m/s. The suspension flow in the channel leads to the migration of the particles from high shear-rate wall regions to the low shear-rate center of the channel and forms an inhomogeneity in the particle distribution. Except at the channel center, both sizes of the particles show similar behavior. It is observed that the center-line peak in the case of monodisperse suspension witha= 10µm was slightly high when compared with that ofa= 30µm. The non-local shear-rate term which we have introduced in the simulations is a function of the square of the particle size and is responsible for the slight decrease in the peak fora= 30µm. Fig. 6.2(b) depicts the evolution of the concentration profiles along the channel length at various positions in the gradient direction. Since
6. Shear-induced particle migration and size segregation in bidisperse suspension
the particle concentration was uniform at the channel entrance, the evolution at various positions starts from a common point. As the suspension flows through the downstream locations of the channel, the shear-induced migration phenomenon causes the gradual increase in the particle concentration at the channel center (x/H = 0) and reached the steady-state once the profiles become fully-developed. Whereas, the concentration at the channel walls (x/H = 1) and at the location between the channel wall and center (x/H = 0.5) decrease gradually. Since the migration flux (Nt) scales linearly with the square of the particle size, it is observed that for a given concentration, the suspension with larger particles reaches steady-state relatively quickly in comparison to the smaller particles.
Figure 6.2: (a) Fully-developed concentration profiles and (b) evolution of profiles of monodisperse suspension along the length of the channel in the gradient direction for particle sizesa= 10µm and 30µm.
Fig. 6.3(a) depicts the fully-developed concentration contour planes of both smaller (φS) and larger particles (φL) in a bidisperse suspension. The average inlet particle volume fraction was 0.25 for both the species, withaS = 20µm andaL= 30µm having the size ratio,aL/aS = 1.5. The corresponding contour planes for another mixture with aS = 10 µm and aL = 30 µm (aL/aS = 3) are shown in Fig. 6.3(b). The quantitative comparison of the fully-developed concentration profiles of bidisperse suspension in the
6.3. Results and discussion
Figure 6.3: Fully-developed concentration contour planes φi of bidisperse suspension in the straight channel for aL/aS = (a) 1.5 and (b) 3. Other parameters were φS = 0.25 and φL= 0.25.
Figure 6.4: Fully-developed concentration profiles φi of bidisperse suspension in the straight channel for aL/aS = (a) 1.5 and (b) 3. Other parameters were φS = 0.25 and φL= 0.25.
straight channel foraL/aS = 1.5 and 3 are shown in Fig. 6.4(a) and6.4(b) respectively.
The profiles clearly show that the particles in the bidisperse suspension behave differ- ently when compared with the monodisperse suspension having the same concentration and size. Irrespective of the particle size, the particle concentration increased at the
6. Shear-induced particle migration and size segregation in bidisperse suspension
Figure 6.5: Evolution of concentration profilesφi of bidisperse suspension in the straight channel foraL/aS = (a) 1.5 and (b) 3. Other parameters wereφS = 0.25 andφL= 0.25.
channel center in the monodisperse case. On the other hand, size segregation of the particles occurs favoring the larger particles at the channel center in the bidisperse case composed of an equal concentration of smaller and larger particles (φS = φL). The particle size ratio clearly affects the size segregation of the smaller particles in a bidis- perse suspension. For a size ratio ofaL/aS = 1.5, the concentration peak for the smaller particles is located between the channel center and wall. Whereas, for aL/aS = 3, the peak is located near the channel walls. Previous experimental studies ofHusband et al.
(1994), Lyon and Leal(1998b), and Semwogerere and Weeks(2008) explained that the enrichment of the larger particles at the channel center is due to the particle size scaling
6.3. Results and discussion
Figure 6.6: Fully-developed concentration profilesφS of bidisperse suspension for various φ¯L. The mean concentration of the smaller particles was kept constant ( ¯φS = 0.25). (a) aL/aS = 1.5, (b)aL/aS = 2, (c)aL/aS = 2.5 and (d)aL/aS= 3.
of shear-induced migration flux in both Brownian and non-Brownian suspension. Ac- cording to their study, since the migration flux scales linearly with the square of the particle size, the size segregation of the particles is a result of the faster migration of the larger particles to the channel center than that of the smaller particles in the bidisperse suspension having an equal concentration of the individual species. In order to obtain a complete picture of the fully-developed concentration profiles, the evolution of the in- dividual species concentration profiles along the channel length at various positions in the gradient direction is shown in Fig. 6.5. As we have discussed in section 2.2.4, the shear-induced migration and collective diffusion will decide the position of the individual
6. Shear-induced particle migration and size segregation in bidisperse suspension
Figure 6.7: Fully-developed concentration profilesφL of bidisperse suspension for various φ¯S. The mean concentration of smaller particles was kept constant ( ¯φL= 0.25). (a)aL/aS= 1.5, (b)aL/aS = 2, (c)aL/aS = 2.5 and (d) aL/aS = 3.
species in a bidisperse suspension. Moreover, these fluxes depend upon the shear-rate, particle size ratio and individual concentration of the particles. Since the concentration of the individual species was uniform at the channel entrance, the flux due to diffusional currents can be neglected across the channel cross-section. Thereby, only the flux due to shear-induced migration dominates. As a result, both species migrate towards the low shear-rate channel center. Since the shear-induced particle migration scales with the square of the particle size, faster migration is observed in the case of larger parti- cles. This is in good agreement with the previous experimental findings. As soon as the inhomogeneity in the particle concentration exists, the flux due to diffusion of the
6.3. Results and discussion
particles comes into play. Though the particles considered in the simulations were in the non-Brownian limit, the thermal collective diffusion dominates close to the channel center over shear-induced migration. Since the larger particles reach high enough local volume fraction at the channel center, they effectively push the smaller particles away from the channel center. Thereby, de-mixing of the particles takes place and the smaller particles will settle at a position where the two aforementioned fluxes balance each other.
For aL/aS = 1.5, the smaller particle concentration at the channel walls and location between the channel wall and center increases gradually until the steady-state is reached.
Since the size ratio is less for aL/aS = 1.5, the concentration peak for the smaller par- ticles is located in between the channel walls and center. The qualitative nature of the evolution of the larger particles was observed to be almost similar for particle size ratio aL/aS = 3. However, the evolution of the smaller particles is observed to be different in the case of aL/aS = 3. From the Fig. 6.5(b) it is observed that unlike the previous case, the smaller particles concentration monotonically decreases at the channel cen- ter. For higher particle size ratios, the collective diffusion dominates over shear-induced migration. Thereby, smaller particles are pushed towards the walls regions.
The effect of the individual particle size ratio and species concentration on the segre- gation phenomenon in bidisperse suspension was studied by carrying out simulations for a range ofaL/aS and φS/φL. Fig. 6.6depicts the fully-developed concentration profiles of smaller particles (φS) in the bidisperse suspension for various φS/φL and aL/aS. In all cases, the mean particle concentration of the smaller particles (φS) was 0.25 and the size of the larger particles was 30 µm. It is observed that for the low concentration of the larger particles, the smaller particles migrate to the center of the channel and this is in good agreement with the previous findings for Brownian suspension as well as for non-Brownian suspension (Lyon and Leal,1998b;Semwogerere and Weeks,2008). The particle size ratio,aL/aS, also have an influence on the size segregation. From the Fig.
6.6, it is observed that the size segregation of the particles enhances with particle size
6. Shear-induced particle migration and size segregation in bidisperse suspension
ratio (aL/aS) for a givenφS/φL. The corresponding comparison of the fully-developed concentration profiles for the larger particles is shown in Fig. 6.7. It was observed that in all the cases, the larger particles always migrate away from the walls, and thereby, the concentration peak is always at the channel center.