Flow patterns in hoppers

The first and most important single feature of a flow pattern in a container is whether slip takes place on all contact surfaces between the contents and the container walls during a fiilly developed discharge condition. If it does, it is termed mass flow by virtue of the movement of the entire mass (see Fig. 5.1). If it does not mass flow, it is often termed fimnel flow after the characteristic shape this type of flow channel takes in some cases (see Fig. 5.2), or cote flow . The first two definitions were laid down by Jenike in 1960, in his fundamental work on the gravity flow of bulk solids. Arnold Redler, in his UK and USA patents of 1920 relating to chain-type extractors, had previously defined the latter flow mode, and his term core flow is common parlance in the UK. This form is sometimes referred to as internal flow . 

However, within these expressions lie the seeds of simple misunderstandings. In some cases of what is called funnel flow , the flow channel looks nothing like a funnel. Its shape may vary from an unpredictable route drawing down from a flat surface in an obscure and unstable manner, as within a fluidized product. Fig. 5.3, to the other extreme where the flow channel has broadened to intersect the container walls underneath the surface of the stored contents. Fig. 5.4. The upper part, shown as region 4, moves in a bed flow maimer, while the lower section, region 2, is distinctly core flow. This latter mode of behaviour may be externally assessed as a form of mass flow, but it most certainly is not. 

The term core flow is similarly misapplied to the overall flow pattern, whereas the core section is usually only part of the material movement, as shown in region 2, Fig. 5.2. The top region of a core flow channel is normally an unconfmed boundary activify, shown as region 1, where material drains on a repose surface into the confined flow section, but in some cases the upper section similarly expands to embrace the total cross-section of the stored product. 

The term mass flow as defined above is clear-cut, but it does not distinguish between converging channels, for which the expression was 

In practice, flow is either mass flow, or it is not. Describing the basic flow pattern as either mass flow or non-mass flow removes all grounds for confusion. It is proposed that these should be preferred expressions in the description of pattern of flow behaviour in storage containers. 

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Types of flow patterns in hoppers, (a) Mass-flow, (b) Funnel-flow. D silo diameter, H average depth of material in cylindrical part, H critical depth to initiate flow in mass flow, H dynamic flow depth in funnel flow, 0 hopper slope, B bottom aperture dimension. 

To be consistent with a mass flow pattern in the bin above it, a feeder must be designed to maintain uniform flow across the entire cross-sectional area of the hopper outlet. In addition, the loads appHed to a feeder by the bulk soHd must be minimised. Accuracy and control over discharge rate ate critical as well. Knowledge of the bulk soHd s flow properties is essential. 

A large one-sixth-scale model of the unloader hopper was selected so that flow patterns in the enclosure could be evaluated.Smoke was used to simulate the behavior of the lime dust in the enclosure. The lime drop from the clamshell was simulated by releasing coarse sand, thus modeling the flow patterns caused by the volume displacement and the air entrainment. The effects of local wind speed and direction on the enclosure were also simulated. 

Normal machine vibration and flow patterns in the hopper bring about some separation of the color pellets during residence time in the hopper. Again, the result is inconsistent coloring of parts. 

Most extruders are of the plasticating type in which solid pellets are fed to the extruder where they are converted to melt and pressurized. The extruder is fed by solids that enter the extruder from a hopper (which is a metallic cylinder with a converging section as shown in Fig. 8.9) or are metered in. The flow patterns in the hopper are complex and are still the subject of research. Our intentions here are to estimate the pressure at the base of the hopper as this value is needed to calculate the pressure rise in the exttuder. 

Because mass flow bins have stable flow patterns that mimic the shape of the bin, permeabihty values can be used to calculate critical, steady-state discharge rates from mass flow hoppers. Permeabihty values can also be used to calculate the time required for fine powders to settle in bins and silos. In general, permeabihty is affected by particle size and shape, ie, permeabihty decreases as particle size decreases and the better the fit between individual particles, the lower the permeabihty moisture content, ie, as moisture content increases, many materials tend to agglomerate which increases permeabihty and temperature, ie, because the permeabihty factor, K, is inversely proportional to the viscosity of the air or gas in the void spaces, heating causes the gas to become more viscous, making the sohd less permeable. 

A combination of tapered shaft diameter and increasing pitch is shown in Figure 10a. This allows a length-to-diameter ratio of about 6 1 instead of 3 1. A half pitch screw is used over the tapered diameter. This approach results in an exceUent mass flow pattern provided that the hopper to which it attaches is also designed for mass flow. 

An alternative to traditional mass flow bin design is to use a patented BINSERT, which consists of a hopper-within-a-hopper below which is a single-hopper section (Fig. 15). The velocity pattern in such a unit is controded by the position of the bottom hopper. A completely uniform velocity profile can be achieved which results in an absolute minimum level of segregation. Alternatively, by changing the geometry at the bottom of the hopper, a velocity profile can be developed in which the center section moves faster than the outside, thus providing in-bin blending of the materials (7). 

The key to solving these problems is to design the vessel for a mass flow pattern. This involves consideration of both the hopper angle and surface finish, the effect of inserts used to introduce gas and control the soHds flow pattern, and sizing the outlet valve to avoid arching and discharge rate limitations. In addition, the gas or Hquid must be injected such that the soHd particles ate uniformly exposed to it, and flow instabiHties such as fluidization in localized regions are avoided. 

One of the most important factors in determining whether powder will discharge reliably from bins or hoppers is establishing the flow pattern that will develop as powder is discharged. The flow pattern is also critical in understanding segregation behavior. 

Two flow patterns can develop in a bin or hopper funnel flow and mass flow. In funnel flow (Fig. 1), an active flow channel forms above the outlet, which is surrounded by stagnant material. This is a flrst-in, last-out flow sequence. As the level of powder decreases, stagnant powder may slough into the flow channel if the material is sufficiently free flowing. If the powder is cohesive, a stable rathole may remain. 

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