Moffat eddies

In a final RTD experiment, a sheet of dye was frozen as before and positioned in the feed channel perpendicular to the flight tip. The sheet positioned the dye evenly across the entire cross section. After the dye thawed, the extruder was operated at five rpm in extrusion mode. The experimental and numerical RTDs for this experiment are shown in Fig. 8.12, and they show the characteristic residence-time distribution for a single-screw extruder. The long peak indicates that most of the dye exits at one time. The shallow decay function indicates wall effects pulling the fluid back up the channel of the extruder, while the extended tail describes dye trapped in the Moffat eddies that greatly impede the down-channel movement of the dye at the flight corners. Moffat eddies will be discussed more next. Due to the physical limitations of the process, sampling was stopped before the tail had completely decreased to zero concentration. 

Figure 8.13 Two-dimensional flows in a screw channel with a 6/14 = 1 and operating in extrusion mode. The arrows show the recirculation flows. The shaded area in the lower right corner is expanded in Fig. 8.14 to show the Moffat eddy...

Moffat, H. K., Viscous and Resistive Eddies Near a Sharp Corner, J. EluldMech. 18, 1 (1964)... 

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