**4. Results and Discussion**

#### *4.1. Throttling Characteristics*

In general, the flow coefficients of profile B, C, and E decrease with the increase of flow velocity, which indicates that the greater the flow velocity, the greater the turbulent fluctuation, the more pressure loss is formed, which is in accordance with the general law (Figure 11). The flow coefficients of profile A increase obviously with the increase of Reynolds number, because the ridge of profile A is too steep and the flow velocity is lower. As a result of the blocking effect, it is not easy for the fluid to flow through

the ridge. On the contrary, when the flow velocity is high, the fluid momentum is large and easy to pass, so the pressure drop decreases and the flow coefficient increases. The flow coefficient of profile d does not change obviously with the increase of Reynolds number. The possible reason is that the side of the ridge of profile d is gentle and easy for the fluid to pass through, but the width of the ridge top is wide. The narrow passage leads to an approximate sharp angle, which leads to an excessive bending angle of the flow passage, approaching 180 degrees. The fluid is not easy to turn, resulting in pressure loss. The two factors restrict each other, resulting in the flow coefficient almost unchanged.

**Figure 11.** The results of the Comparison of Throttling Characteristics of Five Profiles. (**a**) *Cv*-*Re* of five different ridge profiles; (**b**) ζ-*v* of five different ridge profiles.

By comparing the curves of profile A, B and C, it can be known that with the gradual increase of the flatness of the side wall of the ridge, the flow coefficient becomes larger and the flow performance is better at the same flow velocity; by comparing the curves of profile C, D and E, we can know that the appropriate increase of the width of the top of the ridge is beneficial to the fluid steering, flow passage and flow coefficient. The circulation performance is better. Therefore, the top of the ridge should be too narrow, and the side walls of the ridge should be too steep.

Comparing the streamline diagrams of four types of surfaces (Figure 12), it can be seen that there are large vortices near the right side of the ridge and the upper wall of the exit, which is the direct cause of pressure loss. The former is determined by the shape of the ridge, and the latter is caused by the protrusion of the pipe wall on the right side of the upper wall hindering the flow. In addition, there

are small eddies on the edge of the diaphragm and valve body installation. In addition, although there are two large eddy zones, the shape and size of the eddy zones are also different. When the ridge is too steep or the ridge top is too narrow, leading to too large bending, the scale of the right-side eddy on the ridge is very large, and the shape of the eddy is nearly circular, resulting in a great pressure loss. In contrast, the right-side eddy zones of the ridge of B and E are relatively flat. It can be inferred that increasing the flatness of the side of the ridge can reduce the vortices to a certain extent; for the vortices near the upper wall at the exit, the roundness of the protrusion of the inner wall of the upper wall can be appropriately increased or a more suitable shape can be designed to weaken the vortices.

**Figure 12.** The streamline diagrams of four types of surfaces.
