Variation of Residence Time with Channel Position

2 Variation of Residence Time with Channel Position 

Referring to the mixing process mechanisms described in Sections 8.2 and 8.3, the variables of screw geometry and flow restriction effect down channel velocity profile but not those of the cross channel. 

Whilst the layers nearest the screw and barrel are spiralling around the top and bottom of the channel to give good lamina shear mixing, the area centred on the stationary (with respect to cross channel flow) two-thirds depth layer travels down channel at a faster rate. Furthermore a spiralling particle spends less of its time above the 2/3 h level than below it. 

The overall effect is that the 2/3 h region and a significant area on either side have a relatively short residence time, but the relative residence times near the top (barrel surface) and particularly in the bottom of the charmel are much greater. However, the theory, (verified by Wolf and White [8]) shows that only about 5% of the output exceeds double the mean residence time. 

The flow patterns which produce this effect in a single screw give a spread of laminar shear strain such that additives in central regions of the screw may have thicker striations than those that circulate around the regions nearer the screw and barrel surfaces. 

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