Numerical investigation of the effect of weir installation angle on the air-separation performance of a self-priming pump

A self-priming centrifugal pump is designed to automatically evacuate air from the suction line and establish a stable liquid flow without external priming. During the start-up phase, the pump handles a gas–liquid mixture that often leads to suction failure; thus, the separation tank must effectively decouple the air from the recirculating water. While the air–water separation weir is a critical component for this process, its geometric optimization has lacked comprehensive numerical quantification. This study investigates the impact of the weir installation angle (0°, 9°, and 18°) on air removal efficiency using two complementary numerical approaches: an unsteady two-dimensional volume of fluid model is used to qualitatively examine the transient air–water interface during the initial priming stage, while a three-dimensional dispersed phase model is used as the quantitative basis for assessing micro-bubble removal performance. The two-dimensional volume of fluid results indicate that a 9° angle promotes rapid initial air exhaust by aligning the internal flow. However, at 18°, excessive turbulence induces air phase fragmentation, causing undesired recirculation that slightly reduces the initial exhaust efficiency. Conversely, the dispersed phase analysis shows that separation efficiency ( η sep ) peaks at 96% for the 18° case, compared to approximately 70% for the 0° and 9° cases. This indicates that, among the three tested configurations, the 9° case is more favorable for initial evacuation, whereas the steeper 18° case is more favorable for guiding fine micro-bubbles toward separation zones during steady operation. These results show that the preferable weir-angle configuration depends on the prioritized performance metric within the tested design space—the initial priming speed versus the long-term micro-bubble removal. The combined interpretation of qualitative volume of fluid interface observations and quantitative dispersed phase particle-tracking metrics provides a physically consistent framework for evaluating self-priming behavior.