Hydrocyclone flow patterns

Figure 4.22 Schematic diagram of hydrocyclones, a) Flow pattern, (h) Alternate a.spect ratios adapted for service after Wallas, 1988)...

Centrifugal force can also be used to separate solid particles from fluids by inducing the fluid to undergo a rotating or spiraling flow pattern in a stationary vessel (e.g., a cyclone) that has no moving parts. Cyclones are widely used to remove small particles from gas streams ( aerocyclones ) and suspended solids from liquid streams ( hydrocyclones ). 

As flow patterns are influenced only slightly by gravitational forces, hydrocyclones may be operated with their axes inclined at any angle, including the horizontal, although the removal of the underflow is facilitated, with the axis vertical. 

The flow patterns in the hydrocyclone are complex, and much development work has been necessary to determine the most effective geometry, as theoretical considerations alone will not allow the accurate prediction of the size cut which will be obtained. A mathematical model has been proposed by Rhodes et alP6), and predictions of streamlines from their work are shown in Figure 1.38. Salcudean and Gartshore137 have also carried out numerical simulations of the three-dimensional flow in a hydrocyclone and have used the results to predict cut sizes. Good agreement has been obtained with experimental measurements. 

There have been very few studies of the effects of non-Newtonian properties on flow patterns in hydrocyclones, although Dyakowski et al.,AU have carried out numerical simulations for power-law fluids, and these have been validated by experimental measurements in which velocity profiles were obtained by laser-doppler anemometry. 

Fig. 2. Perspective view of a hydrocyclone showing its internal flow pattern...

Another novel concept is the Air-Sparged Hydrocyclone developed at the University of Utah. In this device, the slurry fed tangentially through the cyclone header into the porous cylinder to develop a swirl flow pattern intersects with air sparged through the jacketed porous cylinder. The froth product is discharged through the overflow stream. 

Figure 1.9 Cross-section through a reverse-flow hydrocyclone showing the typical flow patterns. The inset photograph (Axsia-Mozley) shows a bank of six cyclones connected to a common feed manifold system.

As with all separation principles involving particle dynamics, a knowledge of the flow pattern in the hydrocyclone is essential for understanding its function and subsequentiy for the optimum design and evaluation of the particle trajectories, which in turn allow prediction of the separation efficiency. A short account of the flow pattern within typical hydrocyclones and the known or probable behaviour of solid particles in the flow is given in the following section. This is done for the case of low viscosity liquids under conditions in which the particles cause little or no interference to the flow patterns (i.e. for low solid concentrations). 

The flow pattern in a hydrocyclone has circular symmetry, with the exception of the region in and just around the tangential inlet duct. The velocity of flow at any point within the cyclone can be resolved into three components the tangential velocity Vt, the radial velocity Vr and the vertical or axial velocity Va, and these can be investigated separately. 

It should be pointed out here that this short account of velocity profiles in a hydrocyclone is only qualitative the flow patterns are highly complex even for water with a low specific gravity and viscosity, and it may be incorrect to assume that precisely similar profiles occur in cyclones with a considerably different geometry or with liquids of high viscosity. 

The analytical mathematical models of the flow patterns inside the hydrocyclone and of the particle trajectories, including the boundary layer flow, the short circuit flow and the internal eddies but at low feed concentrations only. 

At low solids concentrations of below 1 or 2% by volume, the flow pattern in hydrocyclones is unaffected by the presence of particles, and particle-particle interaction is negligible. The volume of the particles that separate into the underflow is small and the underflow-to-throughput ratio, R, is usually assumed to have no effect on the cut size, xso, except for the effect of flow splitting which can be easily accounted for by using the reduced efficiency concept (section 3.4.1). Dimensional analysis coupled with the conclusion of most of the simple theories of separation in hydrocyclones... 

Figure 23 Hydrocyclone (a) schematic diagram, (b) flow pattern. (From Rushton, 2000.)...

A full understanding of the hydrocyclone requires a detailed analysis of the flow pattern within its body. A number of reviews on this subject may be found in the literature (Bradley and Pulling, 1959 Fontein, 1951 Kelsall, 1952). Only a brief qualitative description will be presented in this section. 

In addition to the main flow pattern there exists a secondary flow pattern, short circuit flow, at the top of the hydrocyclone. The short circuit flow is a flow pattern that moves across the cover of the cylindrical section to the base of the vortex finder. It flows along the outer wall of the vortex finder until it combines with the fluid in the overflow created by the main flow pattern. This short circuit flow pattern is due to the wall effect of the cyclone top cover and the outer wall of the vortex finder. The quantity of the short circuit flow can be as much as 15% of the total feed flow. 

The central air core is another important flow pattern in the hydrocyclone. The rotation of the fluid in the hydrocyclone creates a low-pressure axial area that results in the formation of a rotating free liquid surface. The central air core is cylindrical in shape and filled with air the whole way through the length of the hydrocyclone, the central air core tends to stabilize the vortex flow pattern within the hydrocyclone. 

As discussed earlier, there are two spiral flow patterns existing in the hydrocyclone. Only particles existing in the outer spiral flow will be separated by the centrifugal force. Any particles in the inner spiral flow will pass upward to the overflow outlet. It should be noted that there are two important stages in the process of particle separation. One is the separation of the solids from the main body of the flow into the boundary layer adjacent to the inner wall of the hydrocyclone by centrifugal forces. The other is the removal of the separated solids from the boundary layer by downward fluid flow (not by gravity) to the apex of the cone and out of the hydrocyclone. 

The second important operating variable is the feed solid concentration. With increasing feed solid concentration, the separation efficiency falls off rapidly owing to its effect on liquid flow pattern and the interaction among sohd particles. Therefore hydrocyclones are usually operated with dilute feed solids concentrations ( < 2% by volume). 

A reliable scale-up and performance prediction of the hydrocyclone is limited to the low solids concentration (<2% by volume). Under dilute conditions the flow pattern in hydrocyclones is unaffected by the presence... 

The pattern of fluid flow within the hydrocyclone body is best described as a spiral within a spiral with circular symmetry. A schematic view of the spiral flow inside a hydrocyclone is shown in Fig. 23b. The entering fluid flows down the outer regions of the hydrocyclone body. This combined with the rotational motion creates the outer spiral. At the same time, because of the wall effect, some of the downward moving fluid begins to feed across toward the center. The amount of inward motion of fluid increases as the fluid approaches the cone apex, and fluid that flows in this inward stream ultimately reverses its direction and flows upward to the cyclone overflow outlet via the vortex finder. This reversal applies only to the vertical component of velocity, and the spirals still rotate in the same circular direction. In the meantime, the downward flow near the wall carries solid particles to the apex opening (bottom outlet). 

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