Single screw residence time distribution

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. 

A consequent dimensional-analytical treatment of the homogenization process, residence time distribution and heat transfer behavior in single-screw machines was performed by Pawlowski and was published in a sequence of scientific papers. A summary of this work is printed as a monograph [65],... 

Residence time distribution measurements have been presented by Sakai [127], who contrasted the behavior ofthe various types of twin-screw extruders (Figure 6.26). Sakai found that intermeshing co-rotating twin screw machines produce a narrower distribution of residence time than a single-screw extruder. He argues that this is... 

Knowledge of the residence time distribution (RTD) of an extruder provides valuable information about the details of the conveying process in the machine. The RTD is directly determined by the velocity profiles in the machine. Thus, if the velocity profiles are known, the RTD can be calculated. Various workers have made theoretical calculations of the RTD in singie screw extruders [48-50]. Obviously, theoretical calculations of the RTD require knowledge of the velocity profiles in the machine. Thus, the predicted RTD is only as accurate as the velocity profiles that form the basis of the calculations. In single screw extruders, the velocity profiles can be determined reasonably well, although usually a substantial number of simplifying assumptions are made. In other screw extruders, e.g., twin screw extruders, calculation of velocity profiles is rather complex and thus prediction of the RTD more difficult. 

In addition to mass temperature, residence time also plays a decisive role. Fig. 4.6. Figure 4.7 shows the residence time distribution in various extruder configurations. A single screw channel provides poor axial mixing. In addition, there is a stagnant layer on the screw surface with a long residence time. In a twin-screw... 

Thanks to their residence time distributions, co-rotating, closely intermeshing twin-screw extruders have assumed a dominant market position. Other machines, such as single-screw and counter-rotating extruders are used only for special tasks. Fig. 4.7 [35]. 

As discussed in Chapter 6, the quality of mixing is related to the increase in interfacial area, which is proportional to strain. The calculation of the average strain, y, requires the residence time distribution function,/(f). We present the analysis for isothermal Newtonian flow in a single-screw extruder only, which is due to Pinto and Tadmor (1970). We give only a descriptive analysis for twin-screw extruders. 

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