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

## TL;DR

This paper studies the unsteady flow behavior in axial flow impellers used in high-speed pump-jet propulsion systems.

## Contribution

The study reveals how blade shape and flow rates affect pressure fluctuations and vortex structures in axial flow impellers.

## Key findings

- Blade shape significantly impacts leading-edge pressure fluctuations but has minimal effect on flow velocity.
- Pressure fluctuations peak at 0.9 times the design flow rate and are strongest at the blade's leading edge.
- Vortex structures increase in the impeller and decrease in the diffuser as flow rate increases.

## 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 (Qd), 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 0.9Qd. 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 Qd, 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.