Mechanism Of Fluid Flow

Flow inside a extruder is complex. Levine (1988) describes three types of extruder flow; drag flow, pressure flow and leakage flow. These types of flow can be found in any screw design (Levine, 1988). The drag flow refers to velocity of the screw in relation to the barrel. The velocity of the product being conveyed down the barrel by the screw is essentially 0 at the surface of the screw and gradually increases as you move away to the barrel wall. Figure 4 demonstrates the drag flow. Drag flow is like a liquid between two plates. Movement of one of the plates causes the liquid at the surface of the moving plate to move at the same speed the plate is moving The further the liquid is from the moving plate and the closer to the stationary plate, the velocity of the liquid decreases as the force exerted to that area decreases (Levine, 1988). Drag flow results in a forward moving of material (Rossen and Miller, 1973).

Pressure flow refers to the pressure gradient between the feed zone of the extruder and the metering zone where the product is being discharged. The extent of this flow can be controlled by the size of the die orifice and the screw configuration. During normal operation, this flow is from the die to the feed section as shown in Figure 4 (Rossen and Miller, 1973). The leakage flow is similar to the pressure flow in that it is driven by a pressure gradient and results in flow going from the die plate to the feed zone (Levine, 1988). For this example, leakage flow would be the gap between the barrel and the screw. In a twin screw extruder, leakage flow can also exist when screws do not fully intermesh. When calculating the net

Flow Component

Velocity Profile

Pressure Flow

Drag Flow Qd

Net Flow Qnet

Velocity Profile

Down Channel

Barrel

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