When  the  water  flows  through  the  right angle

turns, the pressure drops and the turbulent kinetic ene- rgy increases. Thus a series of low pressure zones are formed in the corners. The generation of the vortex in- creases the local energy loss, hence a low pressure zone is produced. The feature of this passage is that the effective controls of the pressure drop and velocity can be made by regulating the sizes of the sections of the channel, the shapes of the turns, and the distance between two turns. As the size and the density of the vortex are controlled effectively, a severe cavitation can be avoided, as occurs frequently in the multi-ori- fice control valves or some single stage throttling con- trol valves.

Another two cases based on the boundary condi- tions in Table 2 are also simulated and the results are shown in Fig.6 and Fig.7.

Fig.7 Flow characteristics of labyrinth passage for Case 3

Figure 6 shows the pressure contours and the velocity vectors based on the following boundary con- ditions: the inlet velocity is 5 m/s (Reynolds number Re = 8.304 104 ), and outlet pressure is 102 kPa. Figure 7 shows the pressure contours and the velocity vectors based on the following boundary    conditions:

In  Case 2,  the  pressure drops from 355  kPa   to

193 kPa in the “series passage”, which contributes 64.0% of the total pressure drop. While in Case 3, the pressure drops from 355 kPa to 287 kPa in the “series passage”, which accounts for 60.7% of the total pre- ssure drop. These two cases are shown in Fig.6(a) and Fig.7(a), respectively. It indicates that the “parallel passage” can be used to regulate the flow rate in the passage, and if it is necessary to control the pressure drop, the flow rate in the “parallel passage” will be higher than that in the “series passage”. Thus the “series passage” is mainly used to control the pressure drop. So combining the “series passage” with the “pa- rallel passage” can meet both requirements for the pressure drop and the flow rate in engineering applica- tions.

The numerical results also show that when the inlet fluid velocity increases, the flow resistance will increase correspondingly, thus a higher pressure drop will follow, but its magnitude varies step by step and evenly. Therefore, the severe partial cavitation and erosion can be avoided in the wall of the passage. That is, reducing the inlet flow rate and the maximum flow rate is of benefit to alleviate the cavitation and erosion in the passage.

The pressure drops in the “series passage” in all 3 cases see two features of this labyrinth passage: (1) the most pressure drops happen when the fluid flows through the “series passage”, and the percentage will increase with the increase of the flow rate. The expla- nation is as follows. (2) One main stream from the inlet is pided into two parts at the section “ L6, R ” in Fig.2.

When they flow through two symmetrical and parallel passages, the velocities are deduced to a lower level, thus the throttling capability is weakened even the narrowest size is 0.0025 m, which is smaller than that one in the “series passage”(0.0040 m, as shown in Table 1). (3) The pressure drop performance relies heavily on the flow rate. As the labyrinth passage in- vestigated here yields a local pressure loss, only a higher inlet pressure can promote the flow rate.论文网

Judging from the concentration of the stream lines shown in Fig.5(b), Fig.6(b) and Fig.7(b), the vor- tex zone will change also according to the inlet velo- city, and if the velocity is higher, the area of the vor- tex zone will expand and the turbulence becomes more severe.

3.3 The downwards serrated curve for pressure drop in passage

Another feature of the labyrinth passage is the right angle turns and the connected expansion sections