The Unsteady Pressure Field in a High Specific Speed Centrifugal Pump Impeller—Part II: Transient Hysteresis in the Characteristic

Hysteresis in a pump characteristic results from instability phenomena involving complex three dimensional flow with recirculation. The unsteady flow field on the top and bottom branches of a hysteresis loop in a high specific speed (ω_s = 1.7) centrifugal pump characteristic was experimentally evaluated. A hypothesis for recirculation zones and prerotation as power dissipaters is proposed for explaining the discrepancy in the pressure and shaft power hysteresis. The experimental investigation was performed in both the rotating and stationary frame. In the rotating frame 25 miniature pressure transducers mounted in an impeller blade passage were sampled with a telemetry system. In the stationary frame a fast response probe was implemented. The changing impeller flow field manifested itself between the two branches of the hysteresis with increasing stochastic pressure fluctuations. Using this information the position, size, and strength of the impeller recirculation was quantitatively determined. Theoretically the rate of change of useful hydraulic power in the hysteresis regime during transient pump operation was found to be a function of throttling rate. Quasi-steady behavior existed for slow throttling, |dφ/dt| < 0.005 s⁻¹. A second-order nonlinear dependence on the throttle rate was determined for the change of useful flow power during the commencement/cessation of the impeller recirculation.