Entry and Exit Flows

A complete understanding of flow phenomena occurring in entiy and exit regions of complex geometries is still elusive at the present time for viscoelastic fluids. Understanding entry and exit flows of non-Newtonian fluids like polymer melts is of importance in polymer processing operations, such as extrusion and injection moulding. 

An illustrative example of the previous considerations may be given for polyethylene melts. It is admitted that low density polyethylene (LDPE) melts develop rapid vortex growth in an abrupt contraction, and that high density polyethylene (HDPE) and linear low density polyethylene (LLDPE) melts do not. However, in exit flows, all these polyethylene melts can swell notably, and, for many years, there has been no clear understanding about differences in entry and exit flows of these polymer melts. 

In the purpose of comparisons between theory emd experiments, this short review points out the need for significant experimental studies related to flow patterns, pressure drops and extrudate swell ratios for polymer melts characterized in shear and elongation. Streamlines, velocity components and stress fields related to flow birefringence studies should be used to validate numerical simulations of entry and exit flows. Progress in flow computations in domains involving the upstream and downstream channels together with the exit region are also necessary. 

The multimode Wagner-type and PTT models fit the experimental data rather well, that justifies the use of these constitutive equations to simulate the entry and exit flow of LLDPE and LDPE. The GOB model is not represented because of the prediction of infinite extensional viscosities in the long time range. Nevertherless, the parameters of equations 11 and 12 have been determined firom the shear experiments shown in Pigs. 1 and 2. 

Dispersion of solute bands in cells of different dimensions have been experimentally measured with concentric inlet and outlet connections (13), but this data although pertinent to some detectors (e.g. conductivity detectors) is not generally useful for optical detectors where inlet and outlet tubes normally have to be radially oriented. Radial entry and exit flow from the detector cell introduce significant radial mixing and thus reduces the extent of the dispersion that would be expected to result from low... 

Figure 26.3 shows the relationship between port diameter and fluid velocity at 4 and 7 m/s and highlights the nominal maximum velocities for various plates. As the flow through the machine increases, the entry and exit pressure losses also increase. The nominal maximum flow rate for a plate heat exchanger limits these losses to an acceptable proportion of the total pressure losses, and is therefore a function not only of the port diameter but... 

In the thin-layer cavity cell technique, a cell is constructed to give a thin cavity on one wall of which the metal-plate working electrode is mounted. This wall is separated by a Teflon sheet in which a central aperture has been cut out, from the opposite wall of the cavity this wall contains entry and exit tubes for the test solution which is caused to flow past the working electrode provision is made for connections to the other electrodes. If the Teflon sheet is thin enough (about 0.05 mm), the distance between the two walls of the cavity is less than the normal thickness of the diffusion layer of the electrolyte when undergoing electrolysis, and so electrolysis within the cavity is rapid.26... 

Stations should be separated physically to prevent cross-contamination and should be arranged in order of decreasing contamination, preferably in a straight line. Separate flow patterns and stations should be provided to isolate workers from different contamination zones containing incompatible wastes. Entry and exit points should be conspicuously marked, and the entry to the CRZ from the exclusion zone should be separate from the entry to the exclusion zone from the CRZ. Dressing stations for entry to the CRZ should be separate from redressing areas for exit from the CRZ. Personnel who wish to enter clean areas of the decontamination facility, such as locker rooms, should be completely decontaminated. 

Alternatively, reactant and product gases can be distributed to and removed from individual cells through internal pipes in a design analogous to that of filter presses. Care must be exercised to assure an even flow distribution between the entry and exit cells. The seals in internally manifolded stacks are generally not subject to electrical, thermal, and mechanical stresses, but are more numerous than in externally manifolded stacks. 

The UOP rotary valve has been used in hundreds of Sorbex units across a variety of applications. The purpose of the rotary valve is to simply move the inlet and outlet ports of the net streams (feed, desorbent, raffinate, extract) around the 24 beds in stepwise fashion, creating a semi-continuous countercurrent flow of adsorbent relative to the entry and exit points of the net streams to and from the adsorbent chambers [25]. The rotary valve consists of a rotor plate pressed against a stator plate inside a pressure vessel that indexes the net desorbent, feed, extract and raffinate streams around the adsorbent chambers [26]. An alternative method of moving the inlet and outlet streams around the adsorbent chambers is used in other technologies where multiple automatic on-off valves at each distributor grid inlet are employed [5]. 

The flow of materials and people must be carefully managed. Following GMP principles, materials should pass through an airlock that does not allow both doors to be open simultaneously. Used material and products should be removed by an independent exit. Personnel movement occurs through separate rooms with separate doors for entry and exit. 

In a shell and tube type condenser, water flows through the tubes which are 10 m long and 40 mm diameter. The pressure drop across the tubes is 5.6 kN/m2 and the effects of entry and exit losses may be neglected. The tube walls are smooth and flow may be taken... 

Figure 7 Flow model for pressure drop calculations, (a) Entry and exit losses (b) pressure drop through clean channel (c) pressure drop through cell wall (d) pressure drop through sooted channel. (Courtesy of Society of the Automotive Engineers.)...

Air flow used on entry and exit of lance from powder bed... 

It is clear that a much more complicated analysis would be required for considering the entry and exit regions near the ends of the tube, because the axial velocity component uz in that region will be dependent on the distance from the ends of the tube in violation of the basic unidirectional flow assumptions (see Section A). 

In this analysis, we neglect entry and exit effects and concentrate on the fully developed flow regime where the motion (u, v, w) is independent of 9. In a straight circular tube, we have already seen that the velocity field takes the form [(0, 0, w(r)]. However, in the case of a coiled tube, the tube geometry is no longer unidirectional, and there is no reason to suppose that the velocity field will be so simple. The equations of motion and continuity, specified in dimensional form, for a fully developed, 3D flow are... 

Equation (4.23) applies to a theoretical pipe, but for a real pipeline additional allowances need to be made for pipe entry and exit losses, bends and fittings. Consider a fluid flowing in a typical process plant pipeline as shown in Figure 4.2. 

The pressure drop through a pipe is the sum of the hydrostatic pressure due to the increase in level and the frictional pressure drop due to the flow through the pipe, to the flow past fittings such as bends and valves and to the entry and exit losses ... 

---