Experimental Demonstration of Mixing

The stretching of the dye for operation in extrusion mode is shown in Fig. 8.3. As shown in Fig 8.3(a), the dyed drop stretched in a nearly constant and helical trajectory down the extruder. Closer to the exit the drop continued to be stretched as indicated by the smaller cross section of the tinted fluid filament, as shown in Fig. 8.3(b). The typical mixing in this mode of operation results in a dye monofilament that remains contiguous. Although not shown here, the trajectory and stretching depends on the starting position of the dye drop. 

A similar experiment and numerical analysis was performed by positioning three dye drops in the same position and then rotating the helix and keeping the core stationary. The measured RTD and numerical simulation are shown in Fig. 8.f0. As shown by this figure, the three drops were better mixed than in extrusion mode, but the mixing was still poor as indicated by the magnitude of the first concentration peak. The long concentration tail was caused by the dye near the walls of the screw. 

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. 

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