*3.3. Vector and Pressure Distribution in the Middle Section of the Backflow Channel*

The backflow channel is a key component for water circulation in the self-priming pump. Figure 9 presents the vector and pressure distribution in the middle section of the pump in the three self-priming stages at *t* = 0.2, 2, and 6 s. It can be seen that due to the work of the rotating impeller, the pressure on the right side of the backflow channel is higher than the left side (B1 and F1), so that the water in the gas-water separation cavity can flow back to the impeller inlet through the backflow channel. In the initial self-priming stage (*t* = 0.2 s), the gas-water mixture flowing out of the diffuser enters the lower part of the gas-liquid separation cavity (A1). The gas-water mixture presents three flow states. Firstly, the water in the gas-water mixture flows to the backflow hole by gravity (B1). Secondly, the gas in the water in the gas-water mixture flow freely (C1). Thirdly, because the backflow hole is located on the left side of the gas-liquid separation cavity and the backflow capacity of the backflow channel is limited, resulting in a large vortex region on the right side (D1). In the middle self-priming stage (*t* = 2 s), the flow region of the whole gas-liquid separation cavity is divided into two parts (E1–E2). In the lower region, the water flows down to the backflow hole, while in the upper region, the gas flows upward to the pump outlet. In the final self-priming stage (*t* = 6 s), the pressure difference between the two sides of the backflow channel increases.

Pressure Contour (kPa)

**Figure 9.** Vector and pressure distribution of the backflow channel in the self-priming process. (**a**) *t* = 0.2 s; (**b**) *t* = 2 s; (**c**) *t* =6 s.

In the initial self-priming stage, the water flowing back from the backflow channel is diverted at the intersection of the self-priming cover (F1). Some water flows to the impeller inlet (G1) and the remainder flows to the upper side of the self-priming cover. In the middle and final self-priming stages, as the water column in the inlet section rises and the pressure at the impeller inlet decreases (G1, G2, G3), the pressure difference between point F and point G rises continuously. The water flowing back from the backflow channel merges with the water on the upper side of the self-priming cover at point F, and flows to the inlet of the impeller. The above results show fully that the backflow capacity of the self-priming pump is gradually enhanced in the three self-priming stages.
