Investigation of Tandem-Blade Impellers on the Performance and Flow Characteristics of the Centrifugal Pump

The hydraulic performance and operational stability of centrifugal pumps are closely governed by the internal flow characteristics of the impeller. Under off-design conditions, conventional impellers tend to experience flow separation, vortex-induced energy loss, and intensified pressure pulsations, which impair performance and induce vibration. To improve these limitations, this study introduces a tandem-blade impeller configuration, where the front and rear blades cooperate to redistribute aerodynamic loading and enhance energy transfer. Performance tests and high-fidelity numerical simulations analyses were conducted on conventional and tandem impellers with varying blade numbers to evaluate hydraulic performance, vortex evolution and unsteady excitation forces. The results show that the tandem impeller realizes a notable head increase of approximately 1-2m, particularly evident at moderate and high flow rates. This improvement arises from more progressive pressure loading along the blade passages and a more uniform exit velocity field. Furthermore, the tandem impeller achieves two-stage pressure rise, which weakens abrupt loading zones and alleviates mid-span flow separation typically observed in conventional impellers. Vortex identification based on the Ω-criterion reveals that the tandem impeller induces stronger suction surface separation vortex within the gap. The development of these vortices is dominated by vortex stretching and rotational inertia effects, leading to enhanced vortex intensity and broader spatial distribution. These flow structures significantly modify the unsteady pressure field, as confirmed by FFT and continuous wavelet transform analysis, where the tandem impeller exhibits higher amplitude low-frequency components and more pronounced time-varying spectral energy, particularly under part-load conditions. In terms of force response, the tandem impeller shows improved radial force and axial force balance near the design and overload conditions. However, under part-load conditions, the enhanced low-frequency excitation leads to larger radial and axial force fluctuations, which may negatively impact mechanical stability. Overall, the tandem impeller demonstrates clear advantages in improving hydraulic head and outlet flow uniformity, but its gap-induced unsteady flow behavior requires further optimization, especially for stable operation under part-load conditions.