Semicircular channel flow

Figure 5.5 shows the principle of semicircular impinging streams in which a primary gas stream is split equally into two secondary streams flowing inside two ducts bent to form semicircular channels. In this case, the flow of the drying material (and, to a smaller extent, the flow of the carrying gas) is affected by centrifugal forces, which might reduce the thickness of the boundary layer at the solid surface. 

Because of the relatively low gate temperature, good conditions were created for the injection of thermally-stable amorphous plastics with an injection temperature of less than 300 °C, and for injection of slow-setting crystalline plastics (like PE or PP). This type of nozzle is most commonly used to manufacture packaging featuring a short cycle time and where colour changes are infrequent. In both designs the melt flows via semicircular channels located in the nozzle face. 

For friction loss in laminar flow through semicircular ducts, see Masliyah and Nandakumar AlChE J., 25, 478-487 [1979]) for curved channels of square cross section, see Cheng, Lin, and On ]. Fluids Eng., 98, 41-48 [1976]). 

The Humphreys spiral concentrator is a spirally shaped channel or launder with a modified semicircular cross section, as illustrated in Fig. 19-30. The standard spiral consists of five complete turns, but three-turn units are used in some instances when an unusually rapid and clean separation takes place, as in second-stage or cleaner spirals. There is a drop of 0.34 m (13.5 in)/turn as the flowing pulp progresses from the top to the bottom of the spiral. One spiral concentrator occupies about 0.37 m" (4 ft") of floor space and about 2.1 m (7 ft) of headroom measured from feed to discharge box. The optimum particle-size range of feed particles for spirals is about 10 to 200 mesh (2 to 0.074 mm). 

Before closing this chapter, we feel that it is useful to list in tabular form some isothermal pressure-flow relationships commonly used in die flow simulations. Tables 12.1 and 12.2 deal with flow relationships for the parallel-plate and circular tube channels using Newtonian (N), Power Law (P), and Ellis (E) model fluids. Table 12.3 covers concentric annular channels using Newtonian and Power Law model fluids. Table 12.4 contains volumetric flow rate-pressure drop (die characteristic) relationships only, which are arrived at by numerical solutions, for Newtonian fluid flow in eccentric annular, elliptical, equilateral, isosceles triangular, semicircular, and circular sector and conical channels. In addition, Q versus AP relationships for rectangular and square channels for Newtonian model fluids are given. Finally, Fig. 12.51 presents shape factors for Newtonian fluids flowing in various common shape channels. The shape factor Mq is based on parallel-plate pressure flow, namely,... 

Figure 4.40 Schematic of the microbubble column , a numbered-up Taylor-flow microreactor with a mixing for each reaction channel (left) scanning electron micrograph of the mixing element, top view (right). L liquid C gas. The small channels with the semicircular openings are the gas feed and the larger rectangular ones are for liquid feed (by courtesy of VDI Verlag) [275],...

P.E. Geyer, D.F. Fletcher, B.S. Haynes, Laminar flow and heat transfer in a periodic trapezoidal channel with semicircular cross-section, Int. J. Heat Mass Transfer. 2007, 50. 3571-3480. 

The St. Jude Medical valve has two semicircular leaflets, which divide the area available for forward flow into three regions two lateral orifices and one central orifice. The major part of the forward flow emerged from the two lateral orifices. The measurements at peak systole along the centerline plane, 8 mm downstream of the valve, showed a maximum velocity of 220 and 200 cm/s for the lateral and central orifice jets, respectively. The velocity of the jets remained about the same as the flow traveled from 8 to 13 mm downstream (Figure 77.5). The velocity profiles showed two defects, which corresponded to the location of the two leaflets. The velocity measurements indicated that the flow was more evenly distributed across the flow chamber during the deceleration, than during the acceleration phase. Regions of flow separation were observed around the jets adjacent to the flow channel wall as the flow separated... 

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