Local drag flow mechanisms

Extruders that rely solely on the pressure developed by the rotating screws employ hydrostatic pressure as the transport mechanism and are generally high pressure extruders. Those extruders that utilize dragging or rolling motion (for the latter see Section 4.2.2.6.6) feature a localized drag flow transport mechanism and, consequently, the rate of work performed and internal pressure developed are lower. 

The mechanism of heat flow in forced convection outside tubes differs from that of flow inside tubes, because of differences in the fluid-flow mechanism. As has been shown on pages 59 and 106 no form drag exists inside tubes except perhaps for a short distance at the entrance end, and all friction is wall friction. Because of the lack of form friction, there is no variation in the local heat transfer at different points in a given circumference, and a close analogy exists between friction and heat transfer. An increase in heat transfer is obtainable at the expense of added friction simply by increasing the fluid velocity. Also, a sharp distinction exists between laminar and turbulent flow, which calls for different treatment of heat-transfer relations for the two flow regimes. 

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