*2.2. Establishing the Calculation Domain*

As shown in Figure 3, the calculation domain of the multistage self-priming pump includes inlet section, gas-water mixture cavity, self-priming cover plate, impeller, pump cavity, diffuser, gas-water separation cavity, backflow channel and outlet section. The inlet and outlet pipe are too long, so they are not completely shown.

**Figure 3.** Calculation domain of the self-priming centrifugal pump by using Pro/Engineering software (Pro/E 5.0, Parametric Technology Corporation, Boston, MA, USA). 1. Inlet section; 2. Gas-water mixture cavity; 3. Self-priming cover plate; 4. Pump cavity; 5. Impeller; 6. Diffuser; 7. Gas-water separation cavity; 8. Outlet section; 9. Backflow channel.
