Optimizing cascade density in mixed flow pump-as-turbine: Investigating blade number and wrap angle interactions

In order to investigate the performance impact of the synergistic effect of blade number and blade wrap angle on mixed flow pump-as-turbine (MF-PAT), four sets of experiments were designed in this paper. By combining numerical calculations and experiments, the concept of cascade density l/t was introduced as a measure of the combination parameters of blade number and blade wrap angle. Results demonstrate that cascade densities within the range of 0.4–0.5 optimize both peak efficiency and operational stability. Specific combinations yielding superior performance include Z = 6 with β ≈ 60° (η = 72.42%, representing a 6.26% efficiency gain over baseline) and Z = 7 with β = 50°–60° (η = 70.6%–71.9%, corresponding to 4.44%–5.75% improvements). When the cascade density is low, increasing the blade number appropriately can improve turbine performance. However, when the cascade density is high, increasing the blade number has little effect. Increasing the blade wrap angle appropriately can also improve turbine performance. The internal flow analysis reveals that increasing cascade density can reduce flow separation and improve the fluid flow state. Turbulence loss analysis shows that higher cascade density leads to more uniform loss distribution and reduced turbulence losses. This study provides valuable insights into the optimal design of MF-PAT impellers for improved efficiency and performance.