Tangential velocity, hydrocyclones

In the hydrocyclone, or hydraulic cyclone, which is discussed extensively in the literature(29 35), separation is effected in the centrifugal field generated as a result of introducing the feed at a high tangential velocity into the separator. The hydrocyclone may be used for ... 

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 tangential velocity inside the hydrocyclone is very inqrortant, it is the means by which a suspended particle following the liquid flow path because of drag will experience a centrifugal force. The tangential velocity of the solid will be similar to the liquid on entry into the hydrocyclone and it is assumed that this is also the case at any instant at radii less than that of the entry. 

Equation (8.28) provides a value for the tangential velocity at the outer radius of the hydrocyClone. Tang tial velocities at radii less than that of the hydrocyclone can be estimated by means of the princ le of conservation of angular momentum under flictionless conditions ... 

Many standard sizes of hydrocyclone are available with cylinder diameters of 1-30 cm and cone angles of 25°-50°. The particle cut size, which is the size equally likely to find its way into the underflow or overflow, is limited to about 5 pm and dependent on several factors including the size and geometry of the hydrocyclone, the inlet flow rate and the pressure drop across the unit. Separation is often more effective (in terms of a lower cut size) with a series of smaller diameter hydrocyclones as higher tangential velocities can be achieved. 

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. 

As the radius is further increased, the tangential velocity decreases and is proportional to r, this relationship holds until the cylindrical air column (which normally forms in a hydrocyclone discharging at atmospheric... 

This relationship was then used to calculate the tangential velocities from static pressure measurements in different places within hydrocyclones run with clean liquids. Driessen and many others following him thus deduced the general expression for tangential velocity profiles in the outer vortex given previously in equation 6.1, where n is an empirical exponent, usually from 0.6 to 0.9. Note that for a free vortex in inviscid flow n =, while in a forced vortex (solid body rotation) = 1. 

The effect of turbulence on the separation in hydrocyclones has been of concern to researchers ever since the early work of Driessen. One aspect of interest is how turbulence modifies the tangential velocity profiles, i.e. its effect on the exponent n in the exponential equation for the tangential velocity in equation 2.2. Rietema " made a detailed investigation into this problem and estimated the turbulent viscosity with the aid of the tangential velocity profiles measured by KelsaU. This was done using a dimensionless parameter A ... 

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. 

The tangential velocity of the fluid in the hydrocyclones increases as the radius decreases, which is expressed by the empirical relationship... 

Hydrocylone efficiency generally increases with Increased centrifugal acceleration, obtained by high tangential velocity and the small diameter of the hydrocyclone. The velocity is dependent on the pressure differential between inlet and accept. 

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)...

A different situation arises for hydrocyclones, in which the density of the disperse phase is only moderately greater than the density of the continuous phase. Therefore, for hydrocyclones, < 1, and their CE (Eig. 19.4, b) is small and essentially depends on the profile of the tangential flow velocity. From the three laws of swirling considered above, the greatest efficiency is provided by the law of quasi-solid rotation. 

The reverse-flow hydrocyclone shown in Figure 1.9 is a relatively cheap, compact and versatile device. The basic unit has no moving parts and comprises an inverted conical bottom section attached to a cylinder containing a tangential inlet port. Feed is injected through the port at a mean velocity between 10 and 30 m s whence geometry-induced motion causes the (usually denser) suspended particles to experience centrifugal forces of between... 

Due to the vortex flow in the hydrocyclone, the static pressure in the flow increases radially outward. This centrifugal static head is primarily determined by the distribution of both, the tangential fluid velocities and the suspension densities, within the flow and it constitutes the major contribution to the total pressure loss across an operating hydrocyclone. 

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