*3.1. Gas-Water Two-Phase Distribution in the Inlet and Outlet Sections of the Pump*

Figure 7 presents the gas-water two-phase distribution in the inlet and outlet sections of the pump at several moments of the entire self-priming process based on CFD. The red area represents gas, and the blue area represents water. a–f show the gas-water two-phase distribution at *t* = 0, 0.1, 0.2, 0.3, 0.4 and 0.5 s after the start of the pump. Due to the rotation of the impeller, the water in the pump flows rapidly to the outlet section, and the gas in the inlet section rapidly rushes to the pump, and the water column in the inlet and outlet sections rises continuously. Since the void fraction of the impeller is getting larger and larger, the work capacity of the impeller is gradually weakened, resulting in a slower rise in the water column in the inlet and outlet sections (see A0 to A5 and B0 to B5). At *t* = 0.5 s, the height of the water column in the inlet and outlet sections remains essentially constant (see A4 to A5 and B4 to B5). At this time, the self-priming process due to the impeller rotational role is substantially complete. Moreover, the gas in the inlet section begins to enter into the outlet section at *t* = 0.2 s.

Figure 7g–j show the gas-water two-phase distribution at *t* = 1, 2, 3, 4 s after the self-priming pump is started. It can be seen that the water column in the inlet section is rising and close to the gas-water mixture cavity (see A7 to A10). That's because some gas-water mixture in the gas-water mixture cavity enters into the gas-water separation cavity successively through the impeller and diffuser. In the gas-water separation cavity, the gas-water mixture is in a free-projecting state. Under the action of buoyancy, the lighter gas flows upwardly into the outlet section, and the heavier water flows downward, and flows back to the impeller inlet through the backflow channel for the next cycle. Repeatedly, the total amount of gas in the pump body steadily decreases, and the gas in the inlet section is continuously replenished to the inside of the pump, thereby causing the water column in the inlet section to continuously rise. Compared to the previous self-priming process, the rising rate of the water column in the current self-priming process is significantly smaller, mainly because the maximum self-priming rate in the previous self-priming process is approximately equal to the pump's internal maximum flow rate, while the gas and the water are repeatedly mixed and separated, and the gas is exhausted a little bit in the current self-priming process. When 0.5 s < *t* ≤ 2 s, the gas in the pump has not completely escaped from the water column in the outlet section. As the gas in the water column increases, the water column rises continuously (see B5 to B7). When 2 s < *t* ≤ 4, the gas escapes from the water column of the outlet section. The current self-priming process is a gas-suction stage due to the role of gas-water mixture and gas-water separation.

Figure 7k–m shows the gas-water two-phase distribution at *t* = 5, 6, 7 s. It can be seen that the water column in the inlet section rises continuously and enters the pump at *t* > 4 s, so the impeller void fraction decreases continuously and the work capacity of the impeller is enhanced, leading to a continuous rise in the water column in the outlet sections (see B9 to B11). Compared with the previous self-priming process, the exhaustion rate of the gas in the current self-priming process is significantly larger and most of the gas in the pump is exhausted in a short time. Finally, the entire self-priming process ends and the self-priming centrifugal pump enters the normal working condition.

Air Volume Fraction Contour

**Figure 7.** *Cont.*

**Figure 7.** *Cont.*

(**m**)

**Figure 7.** Gas–water two-phase distribution in the inlet and outlet sections of the pump at several moments of the entire self-priming process. (**a**) *t* = 0 s; (**b**) *t* = 0.1 s; (**c**) *t* = 0.2 s; (**d**) *t* = 0.3 s; (**e**) *t* = 0.4 s; (**e**) *t* = 0.5 s; (**f**) *t* = 1 s; (**g**) *t* = 2 s; (**h**) *t* = 3 s; (**i**) *t* =3 s; (**j**) *t* = 4 s; (**k**) *t* = 5 s; (**l**) *t* = 6 s; (**m**) *t* = 7 s.
