Vena contracta effect

The situation of a gas passing through a sudden contraction is quite different from that of a sudden enlargement. The streamlines entering the contraction continue to converge until reaching a minimum cross-sectional area. This phenomenon is called a vena contracta effect, which is attributed to the inability of the fluid to turn the sharp 90° comer illustrated in Figure 3.18. 

Figure 3.18. vena contracta effect of fluid flow through a sharp-edge exit [46]... 

Holes should not be located closer than one Z)in from any sharp bend or tee in the header/manifold to prevent solids from being sucked into the manifolds due to the vena contracta effect. 

Under uniform hopper draw-down conditions, an idealised shear zone may be assumed to exist as shown in Fig. 3. The shear zone is assumed to be linear and defined by the release angle f. It is also assumed that ihe velocity profiles are approximately linear as illustrated. In the extended skirtplate zone, the velocity profile is substantially constant with the bulk solid moving at a average velocity equal to the belt velocity. Since the average bulk solid velocity at the exit end of the hopper skirtplate zone is less than the average velocity in the extended skirtplate zone, there will be a Vena contracta effect with the bed depth y less than the bed depth H at the exit end of the feeder. 

Cavitation and Flashing From the discussion on pressure recoveiy it was seen that the pressure at the vena contracta can be much lower than the downstream pressure. If the pressure on a hquid falls below its vapor pressure (p,J, the liquid will vaporize. Due to the effect of surface tension, this vapor phase will first appear as bubbles. These bubbles are carried downstream with the flow, where they collapse if the pressure recovers to a value above p,. This pressure-driven process of vapor-bubble formation and collapse is known as cavitation. 

The bnttons at the end of the lines in Figure 5.23 for the nozzle and venturi correspond to critical (choked) flow. Althongh there are no buttons shown on the lines for the orifice, choked flow does occnr in orifices as well. However, the choked flow is not as reproducible for orifices because of the variable effects of the vena contracta. 

Figure 4.4 shows the effect of an abrupt inlet to the pipeline. Notice the vena contracta formed just downstream of the inlet. 

---