RTD in Screw Extruders

Pinto and Tadmor [295] developed expressions for the RTD in single screw extruders. The cumulative RTD function can be written as  

The dimensionless time 0 (time divided by mean residence time) can be expressed as a function of the dimensionless normal coordinate 

The single screw RTD is narrower than the flat plate RTD because of the re-circulation of the fluid in the screw channel. Fluid spends more time in the lower portion of the channel = 0 to 2/3 than in the upper portion of the channel = 2/3 to 1. 

In order to properly determine the RTD of an extruder we have to consider not only down- and cross-channel velocity components but also normal velocity components that occur at the flight flanks. This will require a numerical analysis either FDA, FEA, or BEA. Even though the depth of the channel is usually quite small compared to the channel width, the residence time at the flight flanks is substantial because 

The Pinto RTD for a single screw extruder is narrower than the RTD for pressure flow between a flat plate for a Newtonian fluid. The effect of shear thinning is to further narrow the RTD as discussed earlier. These two effects explain why hackmixing is such a critical issue in screw extruders. 

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