Pressure flow term

It should also be noted that in some cases correction factors, Fj, and Fp are applied to the drag and pressure flow terms. They are to allow for edge effects and are solely dependent on the channel width, T, and channel depth, h, in the metering zone. Typical values are illustrated in Fig. 4.11. 

The volumetric pressure flow term, Op, and the mass flow pressure flow term, 0,... 

We note that if the pressure gradient is positive (pressure rises in the direction of flow), the pressure-flow term is negative and Q < Qd, whereas if pressure drops in the direction of flow, <2 > Qd- The ratio of pressure to drag flow is obtained from Eq. 6.3-19... 

Without too much difficulty, we can extend the model to any screw design consisting of constant depth channels, and moderate tapers by using the taper correction factors of Eq. 6.4-4 separately for each section, and adding up the pressure drops (rises) according to Eq. 9.2.2. Thus, for a tapered channel, the drag and pressure flow terms are multiplied by the expressions 2/(1 + 0) and 2/ 0(l + CoX respectively, where 0 = Hq/H, with Hq... 

We now take the drag and pressure flow terms in Eq. 9.2-5 and substitute the relevant numerical values. We assume a square pitched screw, neglecting the difference between mean and barrel surface helix angle, and neglecting shape factors and flight clearance. We further assume that flight width is 10% of the barrel diameter. We can make these simplifying assumptions because, at this point, we only wish to select the barrel diameter and the screw speed. The channel width can be expressed in terms of the screw diameter as follows ... 

Engineering Estimates of the Drag- and Pressure-Flow Terms of the Screw Characteristic Lines of Co-TSE Conveying Screws and Kneading Elements... 

Drag flow term kPa r[NnMAB, Pressure flow term 56.8 0.0 -30.0... 

The first term on the right-hand side of the equalation is the drag flow term. The second term is the pressure flow term. The ratio of pressure flow to drag flow is termed the throttle ratio, r (see Section 7.4.1.3) ... 

The second term of Eq. 7.198 is the pressure flow term. It represents the flow rate in pure pressure flow, i. e., without relative motion between the screw and barrel (zero... 

As with single-screw extruders [eqs. (16-4) to (16-6)], equations can be developed for drag and pressure flow terms for TSEs (Janssen, 1978 Todd, 1991). However, because of the great variety of screw and kneading paddle arrays, it is necessary to determine the relevant equations for each configuration in series. 

The results are summarized in Figure 1. A small value, e, was used in the experiment as a criterion for turning the pressure flow terms on and off. Whenever and wherever the film thickness (or gap) is smaller than e, the reduced Reynolds equation without the pressure flow terms is used. Otherwise the full Reynolds equation is used. It can be seen that, as long as e is sufficiently small (below 56.75 nm in this case), the pressure flow terms appear to be negligible and changing edoes not have a notable effect on the final solution. 

The expression in Eq. 8.90 is referred to as the screw characteristic and consists of drag flow and pressure flow terms. Finally, the cross-flow term, Vx, is obtained by integrating Eq. 8.44 using the velocity field in Eqs. 8.87 and 8.88 and the following boundary conditions ... 

This is the screw characteristic for the Newtonian case, and as shown in the simple one-dimensional flat plate model described at the start of this section, it consists of drag and pressure flow terms. The ratio of pressure to drag flow rates (this is sometimes called the throttle ratio) is... 

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