Changes in Flow Direction

In order to understand the possible mal-distribution, it is essential to make an accurate prediction of flow in the upper region of the reactor, where severe changes in flow directions occur. Typical values of throughput for the RFR under consideration indicate that the flow is turbulent (for the specific case modeled here, feed velocity at the inlet was 40 m s ). The selection of an appropriate turbulence model is, therefore, crucial. Anticipating recirculating flow in the upper region of the reactor with spatial variation of velocity and length scales of turbulence, it will be necessary to use at least a two-equation turbulence model. The standard k-s model of turbulence, which has been tested and found to be useful for a variety of applications, may be used in absence of more specific information. 

When solid particles are carried in a gas stream and not in a guided liquid stream, any changes in flow direction are very important because due to the large difference in density, the solid particles are pressed with high force onto the tube surface that guides the flow. The velocities of the transported gas may be very high, and then, as a result of intensive sliding jet wear at localized points, lead to short-term, but usually accelerated, loss of wall thickness and to rupture of the tube bends. 

Figure 4.2). These die forms create a more tortuous flow path which avoids localised orientation by introducing changes in flow direction until the pipe extrudate achieves a uniform, isotropic, microstructure in the pipe wall. PVC and other more amorphous polymers such as ABS, PMMA and PC can still be extruded with the more free-flowing spider mounted dies without creating lines of weakness. 

The mechanism of flow is not clearly defined. At least two types of losses are superposed—skin friction and the loss due to change in flow direction, and... 

Knit lines occur in parts made from processes in which the plastic fills out a mold (cold press, compression, injection, reaction injection, resin transfer, structural foam, transfer molding), except when the shape is very simple and there is an unobstructed flow path. Cores, variations in wall thickness, depressions, changes in flow directions, etc., cause the melt to divide as it moves through the mold. Where the melt flows rejoin, knit lines are created. 

Inclined tube evaporators sometimes perform well in foaming services because of the sharp change in flow direction at the vapor head. It sometimes offers advantages when treating heat sensitive materials. Inclined tube evaporators require low headroom and permit ready accessibility to the tubes. 

The solution must then be obtained wiA this change in flow direction [initial condition P2( ) = 0 J. 

A sound design of fluid flow systems reduces the risk of flow accelerated corrosion (Chapter 10). Most important is to avoid sudden changes in flow direction at sharp bends. As a rule of thumb, the radius of a pipe bend carrying an aqueous fluid should be greater or equal to three times the pipe diameter (Figure 12.3). 

The conditions of a change in flow direction after cycling time t is (z, (l - z. ), where and T denote right hand side limit and left hand... 

Rotational flow in the forced vortex within the cyclone body gives rise to a radial pressure gradient. This pressure gradient, combined with the frictional pressure losses at the gas inlet and outlet and losses due to changes in flow direction, make up the total pressure drop. This pressure drop, measured between the inlet and the gas outlet, is usually proportional to the square of gas flow rate through the cyclone. A resistance coefficient, the Euler number Eu, relates the cyclone pressure drop Ap to a characteristic velocity v ... 

Cavitation is not a stable regime and can be avoided by increasing the diameter of the pipes that form the circuit, appropriate dimensioning of accessories such as pumps, and avoiding sudden changes in flow direction. Prevention of cavitation in aluminium equipment is based on the same problems and the same solution as for other metals. 

In this section, we present the rheological behaviors of TLCPs namely, (1) transient shear flow, (2) steady-state shear flow, (3) intermittent shear flow, (4) shear flow after the change in flow direction (reversal flow), and (5) variation of dynamic moduli after cessation of shear flow. We will show that some rheological behaviors of TLCPs are different from those observed for lyotropic LCPs. Experimental results will be interpreted, where appropriate, using the currently held theories presented in this chapter. 

One of the most interesting aspects of the flow shown in Fig. 12.1.3b, and another potiential cause of vortex tube erosion, is the very abrupt change in flow direction at the location where gas that has already entered the cyclone circles around and encounters the incoming gas stream. In the detailed plan view shown in Fig. 12.1.4, this is seen to occur at about the 2 30 clock position. It is here that the gas that is abont to complete its first revolntion within the... 

The machine relies upon a forced flow of contaminated air to pick up a quantity of water that then washes the air in an inertial separator section. Air flowing through a stationary impeller at high velocity picks up the water in a turbulent sheet. The centrifugal force exerted by rapid changes in flow direction causes the dust particles to penetrate the water film and become permanently trapped. The water is continually re-used and since the water curtain is produced by the air flow, no pumps or nozzles are required. 

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