Axial velocity, hydrocyclones

Figure 1.39. Typical velocity distributions in a hydrocyclone. (a) axial (h) radial (c) tangential (broken line LZVV is the locus of zero axial velocity)...

Figure 15 Axial velocity contours in hydrocyclone (Cullivan et al, 2004) (see Plate 14 in Color Plate Section at the end of this book).

FIGURE 4M (a) Left Measured and predicted tangential velocities in a 75-nun hydro-( lone right measured and predicted axial velocities in a 75-nun hydrocydone. (b) Predicted fluid streamlines and particle tirgectories in a 75-nim hydrocyclone. 

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. 

Figure 6.3 Vertical (axial) velocity distribution in a hydrocyclone. LZVV, the locus of zero vertical velocity...

Reitema went on to speculate that for an efBcient separation the left-hand side of Equation (8.36) should be as small as possible, and that the terms on the right-hand side are effectively constant (the ratio of axial to inlet velocity being a constant), thus there should be a hydrocyclone design which provides the optimum design, i.e. the lowest value in Equation (8.36). The right side of Equation (8.36) is known as the characteristic cyclone number and experimental work yielded a minirmim value of 3.5 for the designs that Reitema tested. These have become a set of standard or optimum hydrocyclone designs. 

The velocity of the fluid flow in a hydrocyclone can be resolved into three components tangential, axial, and radial. The most useful and significant of these three components is the tangential velocity. 

A few streamhnes of the secondary flow in a meridian plane of the hydrocyclone 0 = constant) are sketched in Figure 17.7. The secondary flow is determined by the radial and vertical components of the velocity. Under the hypothesis that the rotational component is dominant, the flow has a two-dimensional structirre (equations [17.4]). Only the axial component of the velocity varies with z. The variation with z is linear, resulting from the conditions of incompressibihty. 

The separation takes place in the centrifugal field of soheavy particles are forced to the outer wall (HW cleaners) whereas the fight ones are driven to the center (LW cleaners). The flow streams where the heavy or fight particles are accumulated are separated from the cleaned stock stream. The flow in a hydrocyclone is a three-dimensional two-phase flow. The circumferential component generates the centrifugal force, the axial component moves the solid particles towards the cleaner outlet and the radial component of the suspension flow proceeds from the outside towards the center and vice versa. 

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