just as shown in Fig.2, where there are 10 stages of the pressure drop with much lower velocities than those where there are only three to four stages.

In this section, we will also focus on the follo- wing questions:

(1) Why does the pressure drop in the “series pa- ssage” contribute the most amount of the total pressu- re drop?

In section 3, the numerical results in 3 cases all show that the pressure drop in the “series passage” is much greater than that in the “parallel passage”. The first reason is that the flow rate in the “series passage” is double of that in the “parallel passage”. So the velo- city there is much higher than that in the “parallel pa- ssage” even the section area of the latter is greater. And the second reason is that there are 6 right angle  turns in the “series passage” while only 4 in the “parallel passage”. These turns will dissipate much energy and generate a larger pressure drop.

(2) The main functions of the “series passage” and the “parallel passage” for the pressure dropping.

The numerical results indicate if only use the “series passage” to generate the designed pressure drop, it will need less drop stages and shorter length, while the flow rate might not meet the requirements. On the contrary, if only use the “parallel passage” to generate the designed pressure drop, maybe the flow rate is easily to reach, however, it will need more drop stages, and lead to a greater size. So we may say that the “series passage” is mainly responsible for the pressure dropping while the “parallel passage” is mainly re- sponsible for the flow-rate regulation.

(3) The issues that remain to be solved and the future work for the CFD on the flow in the narrow passage.

In this work, only the single liquid phase is inve- stigated when it flows through the passage, the vapor phase is not considered. As the liquid water simulated here is in the state of a mediun temperature and pre- ssure, it is not easy to have the cavitation caused by the phase transfer, and the results also show that the

pressure at any location in the passage is not lower than the saturated vapor pressure (15oC, 1.7 kPa), so the occurrence of cavitation is avoided.

While in engineering applications, the operation temperature may be higher than 250oC, and the opera- tion pressure may be higher than 17 MPa, so the cavi- tation can not be avoid. However, even with the multi-

phase model, as is widely used to simulation the cavi- tation, many difficulties still exist. As the passage is so narrow that the gradient of the density, the velocity and the pressure will become much greater at the in- terface of two phases. Much work is being carried out and there will be some promising results in the future work.

5. Conclusions

In this paper, a tortuous labyrinth passage consi- sting of a series of right angle turns in a disk of high pressure control valve is studied. The numerical me- thod is used to simulate the distribution of the velocity and the pressure drop in the passage. Based on the numerical results, the following conclusions can be drawn.

(1) When the fluid flows around each right angle turn, a separation occurs near the back edge. There are 6 vortexes in the “series passage” and 4 vortexes in the symmetric positions in the “parallel passage”. These 10 vortexes correspond to the 10 stages of the pressure drop.

(2) In the tortuous labyrinth passage, the velocity is continuously reduced to a reasonable level at the passage outlet. As the pressure drop takes place, the velocity is controlled by the designed area. There is no cavitation occurring, as is induced by the phase transi- tion.