*3.4. Streamline and Gas-Water Two-Phase Distribution in the Middle Section of the Impeller and Di*ff*user*

The impeller and diffuser are the core components of the pump. A radial diffuser with outward and return vanes was used, and the middle section of the impeller coincided with the middle section of the outward diffuser. Figure 10 presents the streamline and gas–water two-phase distribution in the middle section of the impeller and outward diffuser in the three self-priming stages. As shown in Figure 10a,b, the void fraction of the impeller and outward diffuser increased rapidly in the initial self-priming stage. When *t* = 0.2 s, most of the space of the first-stage impeller was filled with gas. That is, only a small amount of gas–water mixture was present at the inlet and pressure surface of the impeller blade (A1 and B1), whereas plenty of gas existed near the suction surface of the impeller that had a large number of vortices, similar to a type of "dead water zone" (D1). The gas-water mixture from the impeller inlet flowed along the pressure surface to the impeller outlet. Upon entering the outward diffuser, the velocity of the gas–water mixture decreased, so more water existed at the impeller outlet and outward diffuser (C1). When *t* = 0.4 s, the void fraction of the impeller and outward diffuser is further reduced, and the region containing the gas-water mixture at the inlet and pressure surface of the impeller blade is significantly decreased (A1 and A2). As shown in Figure 10c,d, in the middle self-priming stage, most of the space in the impeller was filled with gas, and a small amount of water was present at the inlet and pressure side of the impeller blade. Eventually, the exhausting and inhaling rates gradually reached a state of dynamic equilibrium, and the void fraction of the impeller and outward diffuser is basically stable. Compared with the initial self-priming stage, and the vortex region in the impeller is slightly reduced in the middle self-priming stage (D1 and D2). As shown in Figure 10f,g, the water in the inlet section has begun to enter the impeller in the final self-priming stage, and the void fraction of the impeller is drastically reduced. When *t* = 6 s, the impeller is basically filled with water; however, there is still some gas in some flow channels of the impeller, indicating that the exhausting process of the impeller has unsteady characteristics. In summary, the key to the self-priming of the pump is that the rotating impeller forces a small amount of gas-water mixture at the impeller inlet to flow along the blade pressure surface to the impeller outlet and into the outward diffuser.

The return diffuser is an important component of the diffuser. It not only introduces the liquid into the outlet section, but also eliminates the rotational component of the liquid. Figure 11 presents the flow line and gas-water two-phase distribution in the middle section of the return diffuser in the three self-priming stages. As shown in Figure 11a,b the void fraction of the return diffuser increases sharply (A1 and A2) in the initial self-priming stage, and the return diffuser is substantially filled with gas at *t* = 0.4 s, and there are many vortices in the gas region. As can be seen from Figure 11c,d, compared with the initial self-priming stage, the void fraction of the return diffuser is decreased (A3). As shown in Figure 11e,f, a large amount of water enters the return diffuser in the final self-priming stage, and the void fraction of the return diffuser is further decreased (A4). In general, the streamline and gas-water mixture in the return diffuser do not evenly distributed along the circumference. The gas first gathers in the region close to the suction surface of the return diffuser (B1), while the water first gathers near the pressure surface (see C1), and the gas-liquid mixture from the outward diffuser flows along the pressure surface of the return diffuser, which is consistent with the gas-water two-phase distribution in the impeller.

**Figure 10.** Streamline and gas–water two-phase distribution of the impeller and outward diffuser at several moments of the self-priming process. (**a**) *t* = 0.2 s; (**b**) *t* = 0.4 s; (**c**) *t* = 1 s; (**d**) *t* = 2 s; (**e**) *t* = 5 s; (**f**) *t* = 6 s.

**Figure 11.** Streamline and gas-water two-phase distribution of the return diffuser at several moments of the self-priming process. (**a**) *t* = 0.2 s; (**b**) *t* = 0.4 s; (**c**) *t* = 1 s; (**d**) *t* = 2 s; (**e**) *t* = 5 s; (**f**) *t* = 6 s.
