Influence of Span Angle Variations on Fluid Flow Characteristics in Gerotor Screw Vacuum Pumps

Abstract Positive displacement pumps, like screw pumps, are often used for rough vacuum in low to medium vacuum ranges, starting at atmospheric pressure, where their performance is key to overall efficiency. This study investigates a screw vacuum pump with a gerotor-type design. A fluid model of the gerotor screw vacuum pump (GSVP) is developed, and the impact of span angle (SA) design on the fluid flow characteristics of the GSVP is analyzed using computational fluid dynamics (CFD). To evaluate the GSVP's pumping performance under atmospheric pressure, the study assumes air as the working fluid, with an inlet pressure of 1 atm and an outlet pressure of 150 kPa in the CFD model, thereby simulating the gas extraction process from the chamber at 1 atm pressure. Three span angle configurations (24-deg, 36-deg, and 48-deg) are examined to observe variations in flow characteristics while keeping the outer rotor's cross-sectional area, rotor length, and screw helix angle consistent. Simulation results indicate that increasing the span angle in the GSVP results in a higher average flowrate, higher outlet pressure, lower average flow velocity, and lower outlet temperature. A larger SA structure also achieves smoother fluid flow and more stable operation. For fluid moments, both inner and outer rotors experience relatively small magnitudes, suggesting a minimal impact of fluid flow on the rotors. These findings suggest that a larger span angle in the GSVP design improves pump efficiency and flow stability compared to smaller span angles.