Pressure-induced flow

The degradation ribbon at the merger of the flows occurs because of the crosschannel flow of material from the region between the solid bed and the screw root to the melt pool. As shown by Fig. 6.35, this flow is relatively large. As previously stated, the flow occurs because of pressure-induced flow and the dragging of fresh material under the solid bed by the backwards motion of the screw root. This process is consistent with the physics presented for screw rotation. The flow fields developed for a barrel rotation system would not create the low-flow region such as shown in Fig. 6.37. 

Once A is determined using Eq. 7.56, the corrected rotational flow rate is calculated using Eq. 7.57. This corrected rotational flow rate should be very close to the actual rotational flow for the channel geometry. The calculation of the pressure gradient using Eq. 7.58 is now much more accurate since the pressure-induced flow rate is now more accurate that is, 0 p = Q d Qm-... 

The viscous dissipation that occurs due to pressure-induced flow is caicuiated in the same manner and it has two components due to the motion at the core and at the flights. For pressure-induced flow in the z direction and the motion of the screw... 

The absolute value of the pressure gradient is used for all dissipation calculations. Although the pressure-induced flow is in different directions for a positive- and negative-valued pressure gradient operation, the dissipation level, however, is identical. Viscous energy dissipation is always positive definite. 

For pressure-induced flow in the z direction, the motion of the screw flights is as follows ... 

Eq. A7.165 is a negative stress. Since dissipation is always positive, the sign in Eq. A7.165 will be changed. The rate of work due to pressure-induced flow in the z direction at the core is as follows ... 

Removal of the melt, also discussed in Section 5.1, is made possible, in principle, by two mechanisms drag-induced flow and pressure-induced flow (Fig. 5.4). In both cases, the molten layer must be sheared, leading to viscous dissipation. The latter provides an additional, important source of thermal energy for melting, the rate of which can be controlled externally either by the velocity of the moving boundary in drag-induced melt removal or the external force applied to squeeze the solid onto the hot surface, in pressure-induced melt removal. 

Figure 33-8 Flow profiles for liquids under (a) electroosmotic flow and (b) pressure-induced flow.

The total throughput of such a medium (Fig. 12) exhibiting pure pressure-induced flow reads ... 

Labs-on-chips that are developed for drug screening need access to microtiter plates, in which the compounds of interest are stored. These chips contain sipper capillaries through which samples are introduced into the microchannels by vacuum. Continuous analysis of samples is possible if the chip contains a sample channel of larger dimensions (approximately in the millimeter scale), through which sample solutions are pumped hydrodynami-cally and which is connected to the separation or reaction channel. The principle is illustrated in Figure 3. To increase the impedance against pressure-induced flows the separation chaimel may be filled with a gel. 

The pressure/vacuum, the EOF, hydrodynamic pressure (leveling of the inlet and outlet vials), etc. may be used as a driving force for counter-mobilities in this technique [15]. An interesting remark regarding the potential of a technique similar to the above mentioned was made by Dovici et al. as early as 1990 If pressure-induced flow balances the average velocity of two closely spaced analytes, then the resolution of the analytes becomes infinite [32]. 

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