Research on the unsteady flow characteristics of high specific speed axial flow impellers with small aspect ratio and double blades.

Abstract

High specific speed impeller with low aspect ratio is ideal for high-speed pump-jet propulsion. This study investigates its unsteady hydrodynamic characteristics using experimental and numerical methods. At the design flow rate (Q_d), the pressure amplitude at the blade's leading and trailing edges varies significantly along the streamline. Blade shape minimally affects flow velocity but significantly impacts leading-edge pressure fluctuations. For the mainstream, the highest fluctuation peak occurs at 0.9Q_d. The pressure amplitude gradually decreases from blade's shroud to hub. Along circumferential direction the minimum pressure amplitude is located in the middle of the two blades. In gap flow, the leading-edge pressure peaks at Q_d, with fluctuations primarily at 5 times and 10 times the rotation frequency. Meanwhile, pressure fluctuations in the tip clearance's height direction exhibit a consistent distribution, reaching their maximum at the leading edge. Vortex structure analysis using various Q criteria reveals that increasing Q enhances vortexes in the impeller while reducing them in the diffuser. Moreover, flow rates result in a simultaneous decrease in vortexes within both components, while the pressure distribution on the isotropic vortex surface remained stable.