Distribution of velocities within the hydrocyclone

In this section, the tangential and axial velocity distributions at the axial positions Z=350 and 100 mm of the hydrocyclones are studied. The velocities are measured for the hydrocyclone with a conical length (cl) 240 mm, Do=15 mm and for the particle dimension of 20 micrometers.

• Tangential velocity

The tangential velocities for the viscosities of 1 cP (centipoise) and 15.1 cP at the position Z=350 mm of the hydrocyclones illustrated in Fig. 7.14. The figure shows the tangential velocity increases with the reduction of the radius from the wall to the centre of the hydrocyclone. It reaches its maximum near the wall of the vortex finder, and then reduces with a reduction of radius until it obtains zero in the centre of the cyclone. Additionally, it depicts that the tangential velocity for 1 cP viscosity is greater than one for 15.1 cP viscosity. As shown in section 2.10, the centrifugal force which is important to the separation in a hydrocyclone, is derived from the tangential velocity. Moreover the tangential velocity is bigger in value than the axial and radial velocities. According to the equilibrium orbit model, Eq.

2.20, the viscosity of liquid impacts the flow pattern inside the hydrocyclone. This

particularly influences the vortex intensity and as a consequence, the tangential velocity reduces. This results in the cut size increasing, according to the equation [2]. From Eq. 2.14, the higher tangential velocity results in greater centrifugal force which leads to a higher separation efficiency. As a result, the grade-efficiency is greater for the tangential velocity with 1 cP in viscosity as is shown in the figure.

One can conclude that the liquid viscosity has a big influence on the value of the tangential velocity. The variation of the liquid viscosity influences the tangential velocity such that the tangential velocity decreases with the increasing of viscosity.

-1 0 1 2 3 4 5 6 7

-0.02 -0.01 0 0.01 0.02

vtan[m/s]

r [m]

1 cP 15.1 cP

Figure 7.14: Tangential velocity profiles for 1 and 15.1 [cP] in viscosity at axial position Z=350 [mm] and the particle dimensions of Dp=20 [µm] within the hydrocyclones.

15.1 cp 1 cP

Tangential velocity 5.0182

1.4546

-2.1090

-5.6726

-9.2361

-12.800

Figure 7.15: Tangential velocity distributions of hydrocyclones in the position of Z=350 mm and for particle dimensions of D_p

Fig. 7.16 shows the tangential velocities for the viscosities 1 and 15.1 cP in the position Z=100 mm of the hydrocyclones. Here, the distribution curves are prac-tically identical with the curves in Fig. 7.14, but the distribution of tangential velocities are different in character due to the effect of the feeding. As it can be seen, the maximum value of the tangential velocities at the axial position Z=100 mm is reduced in comparison to the tangential velocities in the position Z=350 mm. This means that the maximum values of the tangential velocities are decreased with decreasing axial distance from the bottom. Additionally, the maximum tangential velocity of the hydrocyclone with 1 cP is significantly higher than that of the hydrocyclone with 15.1 cP.

Nemeth and Verdes [38] studied the distribution of the tangential velocities for pure water and mixture of water and glycerine. The results from their work indicated that an increase in viscosity had no influence onto the tangential velocities, and the distribution curves were practically identical.

As it can be seen from the figure, the minimum tangential velocity for water is reduced to -2 (m/s). This is due to the vortex centre precessing around the centre of the cyclone (and therefore the coordinate system used for the velocities), which leads the tangential velocity to go in the opposite direction in comparison with the previous situation, as is shown in Fig. 7.17. The maximum and minimum tangential velocities are depending on the cross sectional position, the shape of

the tangential velocity as well as the position of tangential velocity through the cyclone.

-3 -2 -1 0 1 2 3 4

-0.015 -0.005 0.005 0.015

vtan[m/s]

r [m]

1 cP 15 cP

Figure 7.16: Tangential liquid velocity profiles for 1 and 15.1 [cP] in viscosity along radius position at Z=100 [mm] and the particle dimensions of D_p=20 [µm] within the hydrocyclones.

Vθ

- Vθ

Figure 7.17: The circulation tangential velocity on horizontal cross section in the positionZ=100 [mm] of the cone part of the hydrocyclone.

• Axial velocity

The axial velocities for the liquid viscosities of 1 cP and 15.1 cP in the position of Z=350 and 100 mm of the hydrocyclone are investigated. Fig. 7.18 shows the axial velocity for the liquid viscosities 1 and 15.1 cP in the position Z=350 mm of the hydrocyclone. The figure illustrates a symmetric distribution that is changing from negative to positive in the direction along the wall of the hydrocyclone towards the centre. It also indicates that the liquid along the wall goes to the underflow outlet, and the liquid around the centre moves to the overflow outlet. The downward velocity increases with the increasing of the radius distance from the centre of the hydrocyclone, and the upward velocity rises with the radial distance form the wall.

For the hydrocyclone with 1 cP viscosity, both the downward and the upward axial velocity is higher than for the one with 15.1 cP viscosity, as shown in Fig. 7.18 and Fig. 7.19. This means that the liquid viscosity has a certain impact on the axial velocity.

Figure 7.18: Axial velocity profiles for 1 and 15.1 [cP] in viscosity at axial position Z=350 [mm] and the particle dimensions of D_p=20 [µm] within the hydrocyclones.

Figure 7.19: Axial velocity distributions for 1 and 15.1 [cP] in viscosity at axial position Z=350 [mm] and the particle dimensions of D_p=20 [µm]within the hydrocyclones.

The axial velocity profiles for 1 and 15.1 cP at axial position Z=100 mm are presented by Fig. 7.20. As it can be observed, the maximum axial velocities are reduced and the shape of curves are changed in comparison to the axial velocity profiles in the position Z=350 mm. This is due to the effect of feeding and the position of axial velocity within the hydrocyclone. As can be seen, the values of the maximum axial velocities are decreased with distance from the bottom, i.e.

the maximum axial velocity is higher at the axial position Z=350 mm than for Z=100 mm.

Figure 7.20: Axial velocity profiles for 1 and 15.1 [cP] in viscosity at axial position Z=100 [mm] and the particle dimensions of D_p=20 [µm] within the hydrocyclones.