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

Abstract

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 (ωs = 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.

Hysteresis Characteristic in Hump Region of a Pump-Turbine Model

“…All the signals are unsteady and normalized according to Equation (8) as unsteady pressure coefficients. The intensity of pressure fluctuations is expressed as the standard deviation according to Equation (9). Figure 7 gives the standard deviation of pressure fluctuations for every monitoring point varying with the operating condition in the two directions under 19 mm GVO.…”

“…However, it is not necessary condition to form the hysteresis loop. Based on the above research, they [8,9] continued to perform the experiments of unsteady pressure fluctuations, and proposed that the hysteresis characteristic which pressure difference of the pump and impeller experiences is related to the occurrence and cease of the backflow at the impeller inlet and outlet.…”

CFD flow analysis in the centrifugal vortex pump

“…Boundary conditions were set far enough from the pump stage so that their impact on the flow could be neglected. At the entrance to the domain, a pressure-inlet boundary condition was used, at the exit, an outlet-vent boundary condition was used, because it allows adjustment of the loss coefficient at the exit, (Kaupert and Staubli, 2003). The boundary condition outlet vent is defined by pressure loss that is proportional to dynamic pressure:…”

Hysteresis Characteristic in the Hump Region of a Pump-Turbine Model

“…However, this is not necessary condition to form a hysteresis loop. Based on the above research, the same authors continued to perform unsteady pressure fluctuation experiments, and proposed that the hysteresis characteristic which the pressure difference of the pump and impeller experiences is related to the occurrence and cessation of backflow at the impeller inlet and outlet [8,9].…”