Classification of Different Types of Flow
Figure 14.1 shows that the void fraction </> comes close to zero as soon as the ratio C/ v approaches zero, and, the smaller the mixture ratio (u,, the greater the void fraction </>. In some cases, the void fraction <p alone may be a good characteristic number for classification of the type of flow in pneumatic conveying. But generally speaking, the void fraction 4> is not the only criterion that determines the behavior of the flow.
4> = |
Besides the void fraction <f>, a very important parameter is the size of the particles. Using a simple cubic model for packing spherical balls, which represent the particles, we get the following expression for the void fraction
D + s
Where D is the diameter of the balls and s is the empty distance between the balls, i. e., the distance between the centers of the balls is D + s. Solving for s in the equation above, we get
(14.18)
From which we see that for the same void fraction <f>, the distance between particles is proportional to the size of the particle D.
Obviously, the closer the particles are to each other, the more likely it is that they will stick together and form larger clumps, which usually means that the flow is not uniform. This view combined with Eq. (14.18) is a greatly simplified explanation of why the mass flow ratio Fi for dry wood chips can rise to five or even higher and still the flow of the mixture of air and large chip particles can be handled as a uniform suspension, a uniform dilute-phase flow, although it is not actually dilute. The mass flow ratio for fine coal powder, however, has to be much less than five in order for the flow to be handled as a uniform dilute flow.
Even the void fraction together with particle size distribution does not provide all of the necessary information on the kind of flow. The mutual forces between distinct particles depend not only on the distance between the particles but also on the surface properties of the particles. The strength of the attractive forces between particles depends on conditions. For instance, the moisture content of the solid is essential for determining the attractive forces between particles, especially for hydroscopic materials such as wood. Airflow between particles usually tends to separate particles, whereas the surface forces, adhesion forces, tend to bring them together.
One widely-used picture for illustrating the different types of flow in pneumatic conveying is the so-called state diagram,1’2 in which the pressure drop is related to the air velocity.
As shown in Fig. 14.2, the material conveying region is bounded by the air — onlv curve and the stationary-plug curve, where the air merely percolates through or flows above a packed bed of stationary particles. Dense-phase conveying occurs when V, the velocity of air, is below the so-called saltation velocity.
The dense-phase regime can be further subdivided into three distinct regions,3’4 which are shown in Fig. 14.3. In continuous dense-phase flow the material moves by saltation over a stable creeping bed, in discontinuous dense — phase flow particles move as groups, and in the solid dense-phase the solids are extruded through the pipe as a continuous slag.
P |
Dense-phase conveying offers some clear advantages over dilute-phase conveying. It has a lower power consumption, wear is reduced, and the gas/ solids separators and the size of pipe required are smaller. The disadvantage is the greater pressure loss per unit length of the pipe, which limits the use of dense-phase conveying to shorter transport distances. The other factor is that many materials, mainly granular materials and materials with large particle size, simply do not flow as dense-phase flow. Granular materials (plastic pellets or seeds) that are more or less consistent in size, flow in stable plugs because the void fraction between grains is high enough. Only granular materials with “fines” have to be stabilized artificially.
Besides the conventional classification of flow shown in Fig. 14.3, there are also other possibilities, see, e. g., Leung.5 Independent of the method of classification, the essential point is that there is no general method and there are no general simple parameters that reliably predict the behavior of the flow in a new application. For each case the type of flow has to be examined experimentally.
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