Compressibility effects and rotational dynamics of stall cells in a pump turbine diffuser during hump instability

The hump phenomenon significantly impacts the operational stability of pump turbines, with rotating stall in the diffuser channels being a key unsteady flow structure and a primary cause of hump instability. This study investigates the compressibility effects and rotational behavior of stall cells in the diffuser during hump instability conditions using proper orthogonal decomposition and two-dimensional frequency domain visualization methods. The stall evolution process is analyzed in terms of spatial distribution, intensity, and frequency domain characteristics. The results reveal that the compressibility of the working fluid partially suppresses the transition of rotating stall cells into stable stall cells. Meanwhile, the circumferential propagation of stall cells is identified as the primary contributor to significant hydraulic losses and a critical factor in the hump phenomenon. Consequently, suppressing the rotational behavior of stall cells not only effectively reduces hydraulic losses and mitigates the hump phenomenon, but also expands the unit's safe operating range and enhances its performance under off-design conditions, offering potential strategies for improving pump turbine stability.