Cavitating flow around a hydrofoil was simulated using a transport equation-based model with consideration of the influence of noncondensable gases. The cavity length and the pressure distributions on the suction side can be well predicted for stable cavities using the standard renormalization-group (RNG) $κ-ε$ turbulence model with proper noncondensable gas mass fraction. The unstable cavity shedding at lower cavitation numbers was not well predicted by the standard RNG $κ-ε$ turbulence model. A modified RNG $κ-ε$ turbulence model was evaluated by comparing the calculated spatial-temporal pressure distributions on the suction wall with experimental data. The results showed that the predicted cavity growth and shedding cycle and its frequency agree well with the experimental data. However, the pressure increase caused by interaction of the reentrant flow and the cavity interface is overestimated, which caused the time-averaged pressure on the front part of the hydrofoil to be overestimated. The time-averaged pressure on the rear of the hydrofoil was low because the small cavity shedding on the rear part of the cavity was not predicted.

Three-dimensional numerical simulations are carried out by commercial software STAR-CCM+ to investigate the hydraulic transient and flow behaviours of a prototype pump-turbine during the normal shutdown process. The global characteristics and pressure fluctuations are analysed as the pump-turbine runs through the turbine mode, into the turbine braking mode, and finally into the reversed pump mode. As the guide vanes close, flow rate and runner torque both decrease while the rotational speed drops in response to the negative torque on the runner. The proposed approach accurately predicts the transient characteristics during the shutdown process and provides insight into the inner flow behaviour of a pump-turbine. Thus, this study has the potential to facilitate both exploration and diagnostics of faults of turbo machines during transient operation.

Unsteady flow phenomena like rotating stall frequently occur in centrifugal pumps under off-design conditions. Rotating stall could lead to flow instabilities and pressure pulsation, which affect the normal operation of pumps. The mechanism of rotating stall has not been sufficiently understood in previous researches. In this study, the impact of rotating stall in the impeller on centrifugal pump stability and pressure pulsation is numerically investigated. This paper aims to detect the unsteady flow characteristics inside the centrifugal pump by computational fluid dynamics technology, to analyze pressure pulsations caused by rotating stall and to explore the propagation mechanism of rotating stall. Unsteady numerical simulations are performed by ANSYS 16.0 to model the unsteady flow within the entire flow passage of a centrifugal pump under 0.4QBEP and 0.6QBEP working conditions. Through flow characteristics research, the generation and propagation of rotating stall are discovered. Flow separation appears near the leading edge of the pressure side and transforms into vortices, which move along the passage. Meanwhile, the stall cells rotate circumferentially in the impeller. Additionally, frequencies and amplitudes of pressure pulsations related to rotating stall are investigated by spectrum analysis. The results detect a possible characteristic frequency of rotating stall and show that the interaction between stall cells and the volute tongue could have an influence on rotor–stator interaction (RSI).

Three-dimensional numerical simulation for a prototype pump-turbine has been performed to investigate the transient characteristics and flow behaviors in the startup process. The rotational speed, which varies with runner torque and inertial of the rotating systems, is obtained by hydraulic-force coupling method. To simulate the guide vane motions, dynamic overset mesh technique is applied with good maintenance of mesh quality. Comparisons with the existing experimental results verify the validity of the current method in predicting the startup transient characteristics of a pump-turbine with good accuracy. During the startup transient, the guide vanes open according to a prescribed sequence, which brings in increasing flow rate to the unit. The runner accelerates due to growing water power energy exerted on the runner blades. When approaching the speed no-load condition, a ring-shaped flow enlacing the entire vaneless space, that we term “water-ring,” is found to block the through-flow along the circumference, and the pump-turbine features a prominent partial pump flow state in the runner, which equalizes the torque to no load. Thus, the runner rotational speed stops increasing and stabilizes toward rated speed. The prediction shows that the pressure fluctuation amplitude of the speed no-load can be 3.8 to 8 times that of the full load, presenting evidently stronger instability at the speed no-load.

After long-term operation, the material loss due to slurry erosion often leads to significant changes in the impeller geometry. This change can, in turn, affect the flow characteristics and the erosion mechanism in a centrifugal pump. To investigate this matter, we consider two geometric models based on a prototype stainless steel impeller, which failed due to huge erosion problems from the pulp slurry. These two models are different in the degree of wear, with one model at the pre-erosion stage and the other with worn blades. For both models, the flow characteristics have been obtained in the Eulerian reference frame by means of the SST k-ω model. Then, in order to determine the erosion patterns, the particles have been tracked in the Lagrangian reference frame. In this way, the influence of the flow characteristics on the erosion patterns has been analyzed. Results show that the geometric variations greatly affect the flow characteristics, and consequently the erosion patterns. Particularly, it has been found that the clearance flow plays a significant role in defining erosion characteristics, such as erosion distribution, areas, and rates. Interestingly, the simulation results for current study show that the erosion rates after long-term operation greatly decrease near both the outlet edge and the blade bottom. It follows also that for accurate erosion predictions, these geometric variations have to be considered in the numerical model. This paper provides a better understanding of the complex erosion mechanism in centrifugal pumps, which can help to reduce the wear in future designs.

Pressure oscillations caused by vortex rope were measured in the draft tube of a prototype Francis turbine. The three-dimensional, unsteady Reynolds-averaged Navier-Stokes equations with the RNG $κ−ϵ$ turbulence model were solved to model the flow within the entire flow path of the prototype hydraulic unit including the guide vanes, the runner, and the draft tube. The model was able to predict the pressure fluctuations that occur when operating at 67–83% of the optimum opening. The calculated frequencies and amplitudes of the oscillation show reasonable agreement with the experiment data. However, the results at 50% opening were not satisfactory. Pressure oscillations on the runner blades were found to be related to the precession of vortex ropes which caused pressure on the blades to fluctuate with frequencies of $−fn+fd$ ($fn$ is the rotational frequency and $fd$ is vortex procession frequency). The peak-to-peak amplitudes of the pressure oscillations on the blades at the lower load conditions (67% opening) were higher than at higher load conditions (83% opening). Fluctuations on the suction side tended to be stronger than on the pressure side.