*3.5. Vector and Gas-Water Two-Phase Distribution in the Middle Section of the Gas-Water Mixture and Separation Cavities*

As an important part of the self-priming pump, the gas-water mixture cavity mainly provides space to promote the mixing of gas and water. Figure 12 shows the vector and gas-water two-phase distribution of the gas-water mixture cavity in the self-priming process. As shown in Figure 12a, there is a clear boundary layer in the gas-water mixture cavity in the initial self-priming stage, and the gas is in the upper layer, while the water is in the lower layer. Moreover, when *t* = 0.4 s, a large amount of gas from the inlet section enter the gas-water mixture cavity and flows downward in three directions (A1, B1 and C1). The gas in the middle direction flows directly downward, the gas in the left and right directions flows down the wall of the gas-liquid mixture cavity, and some of the gas flows back along the boundary layer to the middle portion of the gas-water mixture cavity (B2 and C2), and finally merges with the inflowing gas in the middle direction (A2), and the other gas is mixed with water at the boundary layer to enter the water region (B3 and C3). As can be seen from Figure 12b, the gas-water boundary layer becomes unclear in the middle self-priming stage. The gas from the inlet section enter the gas-water mixture cavity in the middle direction (D1), one part is mixed with water at the boundary layer to enter the water region (D2), and the other part is refluxed in the left and right direction (D3 and D4), forming a large number of vortices. Since the inhaling rate of the gas in the middle self-priming stage is much smaller than that in the initial stage, three parts of gas are combined in the middle of the gas-water mixture cavity in the initial self-priming stage, while a strand of gas is divided into three parts in the middle self-priming stage. As shown in Figure 12c, the gas-water boundary layer has disappeared in the final self-priming stage. Compared with the water region, the flow field in the gas region is more disordered, indicating that the vortex is more likely to be generated in the gas region.

Void Fraction Contour

**Figure 12.** Vector and gas-water two-phase distribution of the gas-water mixture cavity in the self-priming process. (**a**) *t* = 0.2 s; (**b**) *t* = 2 s; (**c**) *t* = 5 s.

The gas-water separation cavity mainly provides space to promote the separation of gas-water mixture. Figure 13 illustrates the vector and gas-water two-phase distribution of the gas-water separation cavity in the self-priming process. As shown in Figure 13a, the void fraction of the gas-water separation cavity is not high overall and exhibits a non-uniform distribution in the initial self-priming stage. Under the influence of the rotation of the impeller, the gas-water mixture rotates counterclockwise in the annular cavity and escapes from the separation cavity, which is consistent with the rotating direction of the impeller (A1, B1, C1 and D1). As can be seen from Figure 13b, the gas-liquid separation cavity plays the role of the separation of gas and water in the middle self-priming stage. The gas is basically distributed in the upper half of the gas-water separation cavity due to the influence of gravity. Although the internal flow field of the entire gas-water separation cavity is disordered, the flow direction of the annular cavity is counterclockwise. As shown in Figure 13c, since the water from the inlet section starts to enter the pump, a large amount of gas-water mixture flows in the gas-water separation cavity, which not only causes a decrease in the void fraction, but also causes the gas to spread throughout the gas-water separation cavity.
