A novel approach is proposed to predict alternating stall in a centrifugal pump impeller with even blades by introducing a low-pressure ratio, which is defined as the ratio of the deviation of the low-pressure zones of adjacent impeller passages. The threshold of 2/3 is shown to be a good quantity with which to accurately and quantitatively predict alternating stall and even critical alternating stall (CAS). The effectiveness of this new approach is validated by comparison with previous findings obtained under quasi-steady conditions. Large eddy simulation data for a six-blade centrifugal pump impeller are used to predict the CAS under transient conditions, with the transient conditions corresponding to a sinusoidal flow rate with an equilibrium value of 0.5Qd (where Qd is the design load) and an initial phase of zero combined with different oscillation amplitudes. The low-pressure ratio frequency equals the flow rate frequency, approximately 2 Hz. The phase of the low-pressure ratio lags behind the flow rate. When the oscillation amplitude is larger than 0.15Qd, a non-stall state occurs during the dropping stage of the flow rate. The flow rates corresponding to the CAS during the dropping and rising stages, respectively, increase and decrease as the oscillation amplitude increases.
