*3.2. Streamline and Gas–Water Two-Phase Distribution in the Middle Section of the Pump*

From the previous analysis, it can be found that the whole self-priming process of the self-priming pump is divided into three stages: the gas-suction stage due to the impeller's rotating role in the initial self-priming stage, gas-suction stage due to gas-water mixing and the role of gas–water separation in the middle self-priming stage, and gas-suction stage due to the water flowing from the inlet elbow into the pump in the final self-priming stage.

In order to comprehensively study the gas-liquid flow of the pump in the self-priming process, the middle section of the entire pump that best reflects the flow characteristics was selected. Figure 8 shows the streamline and gas-water two-phase distribution in the middle section of the pump at several moments of the self-priming process. The following three self-priming stages of the multistage self-priming pump were obtained through numerical calculation: initial (*t* ≤ 1 s), middle (1 s < *t* < 5 s), and final (*t* ≥ 5 s) self-priming stages. Three moments were selected for analysis for each self-priming process. As shown in Figure 8a,b, the time that the impeller rotates for a cycle is about 0.02 s, and the rotating impeller worked on the water to make it flow quickly to the outlet pipe in the initial self-priming process. Hence, the gas in the inlet section entered the gas-water mixture cavity, mixed with the water, then continuously flowed through the impeller and diffuser (A1, A2, A3). The upper part of the gas-water mixture from the diffuser flowed to the outlet pipe (B1, B2, B3), and the lower part flowed to the lower side of the gas-water separation cavity (C1, C2, C3).

Figure 8d–f shows the streamline and gas-water two-phase distribution in the middle section of the pump at *t* = 2, 2.02 and 2.04 s. It can be seen that compared with the initial self-priming stage, the change of gas-water two-phase distribution in the middle self-priming is slow, and the gas-water two-phase boundary line in the pump is based on the uppermost end of the lower half of the impeller (D1–D2). The upper half part is mainly the gas phase, and the lower part is mainly the water phase, mainly because the amount of water flowing out from the impeller is basically the same as that from the backflow channel into the impeller, and the amount of exhausting gas is basically the same as the amount of inhaling gas, resulting in that the gas-water two-phase boundary is basically stable. Figure 8g,i indicate that in the final self-priming stage, the entire pump was mainly full of water. A large amount of gas was exhausted from the pump within a short period. In addition, since the backflow channel is only in communication with the lower half of the self-priming cover, a small portion of the gas is sealed in the upper half of the backflow channel and a large resting bubble region is formed (E1, E2, E3). The gas is finally exhausted and even a small amount of gas is not exhausted.

Void Fraction Contour

**Figure 8.** Streamline and gas–water two-phase distribution in the middle section of the pump at several moments of the self-priming process. (**a**) *t* = 0.1 s; (**b**) *t* = 0.12 s; (**c**) *t* = 0.14 s; (**d**) *t* = 2 s; (**e**) *t* = 2.02 s; (**f**) *t* = 2.04 s; (**g**) *t* = 6 s; (**h**) *t* = 6.02 s; (**i**) *t* = 6.04 s.
