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To reveal the effect of cavitation on the energy conversion characteristics of helical axial multiphase pump (Abbreviated to multiphase pump), the cavitation flow in multiphase pump is simulated. The results show that with the decrease of cavitation number σ, cavitation firstly extends along the streamwise of blade suction side, and then turns to the pressure side of blade. When cavitation develops to fracture stage, the suction side of the blade is completely covered by bubbles; The vapor volume fraction in the impeller is almost 0 at the inception and critical cavitation stages; When cavitation develops to fracture stage, cavitation extends to the whole impeller passage. The output power of impeller is mainly contributed to the power done by the pressure. The power done by the viscous force in the critical fracture cavitation stage is slightly reduced. While, the power done by the pressure in the fracture cavitation stage decreases greatly. With the evolution of cavitation, the turbulence dissipation loss in the impeller decreases gradually. Moreover, in the critical fracture and fracture cavitation stages, the friction loss increases greatly compared with the previous two cavitation stages, resulting in an increment of the total energy loss.

A major challenge in the operation and development of the high-head water resources, especially in the mountains with high sediment yield, is the erosion of Pelton turbine components. In this work, a numerical study was carried out based on sediment properties measured in field conditions, such as particle distributions and concentrations, to analyze the erosion mechanism of a prototype Pelton turbine injector. The Volume of Fluid (VOF) method was combined with a Lagrangian particle tracking approach to simulate the air-water-sediment flow, followed by the application of Mansouri’s model to estimate the erosion. The predicted erosion patterns were in good agreement with field observations, especially in physically reproducing the asymmetrical erosion distribution on the needle surface. To elucidate this asymmetry, fundamental analysis of the flow patterns including the vortex structures and the secondary flow on the particle behaviors was carried out. Interestingly results were found about the secondary flow induced by the von Karman vortex shedding, which increased the particle separations, and consequently, enhanced the erosion in shedding areas. The current work may provide important engineering insights to reduce erosion of components with inner obstructions.

In order to analyze the energy conversion characteristics of the impeller blades in multiphase pump quantitatively, the impeller performance in design working condition was studied under water and gas-liquid two-phase flow, respectively. The results show that the capacity of impeller energy conversion gradually increases when the blade wrap angle changed from 0 to 60°; when the wrap angle is larger than 60°, the capability began to degenerate gradually, and the energy transfer performance of the pressure surface began to degrade. The enhancement of the energy transfer performance of the impeller pressure surface had a positive impact on the impeller energy conversion performance, but the enhancement of the energy transfer performance of the suction surface had an adverse effect on the impeller energy conversion performance. Under the gas-liquid two-phase case, the impeller power capability degraded, the energy conversion performance of impeller was worse than that of water alone. The research results had essential theoretical significance for the design of hydraulic optimization of the multiphase pump.

Inlet gas void fraction (IGVF) played an important role on the flow characteristics in a multiphase pump. To reveal the effect of inlet gas void fraction on the flow characteristics in the tip clearance, a combination of numerical simulation and experiment was carried out and the reliability of numerical method was verified by comparing with the experimental data of the flow field by using high-speed photography. The results showed the accumulated gas was mainly at the impeller inlet near the pressure side (PS), tip clearance near the tip and the suction side (SS). When the IGVF increased, there was an obvious stratified structure and the separated vortex in the tip clearance. Compared to the water case, the gas caused the tip leakage flow velocity to decrease from the blade inlet to the streamwise coefficient of 0.2, and to increase from the streamwise coefficient of 0.2 to the blade trailing edge. At the same time, the IGVF had a significant influence on the tip leakage vortex (TLV) structure and trajectory, and the streamlines and vorticity distribution corresponding to the wake and the TLV were changed. Moreover, the flow characters and the structure of the TLV were more complicated under gas-liquid condition.

Because of the clearance and pressure difference between the pressure side (PS) and the suction side (SS), the tip leakage flow developed and evolved into tip leakage vortex (TLV). The relationship between the swirling strength of TLV and the pressure difference between the PS and the SS was analyzed. The velocity distribution in the tip clearance was also investigated in a multiphase pump. The reliability of numerical method was verified using high-speed photography. The trajectory of TLV was found to be distributed in a wavy shape and the swirling strength of TLV was positively correlated with the pressure difference between the PS and the SS. The tip separated vortex was formed when the tip leakage flow entered the tip clearance, and the TLV was formed when tip leakage flow moved out of the tip clearance. The low-pressure region in the tip clearance consisted of two parts. The part near the PS was caused by the tip separated vortex, and the other part near the SS was caused by the TLV. In addition, the flow separation phenomenon occurred at the radial coefficient of 0.97 and led to the increase of the turbulent energy loss of the pump.

This paper presents entire flow path simulations in a prototype four-nozzle Pelton turbine under three water heads, from pipe flow to nozzle jet water sheet flow on rotating bucket and drop from bucket brim on the air. Different type flows in the Pelton turbine are analyzed by adopting the three-dimensional transient air-water two-phase flow simulation method. Hydraulic performance of stationary parts and rotating runner are evaluated along the entire flow path. Flow analyses indicate that the pressure pulsation is very low and the water flow could be regarded as steady flow in the stationary parts. The hydraulic loss in these parts drops as the water head increases due to a reduction of the frictional loss. Afterwards, the interactions between the jets and buckets in the rotating runner are discussed. The pressure pulse on the bucket surface pulsates with the spreading of the water sheet flow and takes up 10%–25% water energy. The hydraulic performance of the bucket is highest at optimal water head and decreases as the water head varies. Unsteady flow analyses show that there is a potential for interference between the two adjacent jets if the water head or the angle between the two jets decreases.

For tidal power stations, turbines operate in a wide head range and sometimes near the free surface due to tide changes, which commonly consists of short intake channel. When they operate under low water levels, free surface flows would be a major concern for hydraulic stability. The present paper establishes a simulation method for predicting all flow fields in tidal power stations. Two-phase simulation model is introduced with the acceleration of gravity and the free surface, which consists of a turbine unit and two reservoir models (sea side and reservoir side). Hydraulic performance of a prototype turbine was predicted by using the proposed model, based on water levels of both two sides. The predicted unit performance agreed well with the test result. The flow fields were compared with ones by two other simulation methods. The proposed method could predict non-uniform intake flow inducing head losses by the sudden contraction and the gravity force, which sufficiently influenced turbine internal flows. Consequently, head drops in each part of the turbine were distributed differently, under the same net head. The present model provides a foundation for predicting prototype turbine performances under not only tide changes but also air-water interactions.

For tidal power stations, the turbine operates under low-head near the free surface. The water levels change both the reservoir and sea sides, furthermore, the intake channel length is short. In this paper, the objective is to investigate the effect of surface vortices on operating conditions for tidal power units. Two-phase flows were simulated with a simplified reservoir model. The numerical method was verified with experimental data. In the reservoir model, the predicted surface vortex agreed well with various analytical models. From the numerical results, two-phase flows in the reservoir includes surface vortices sufficiently influenced the intake flows. It induces the non-uniformity of the pipe flow, which is by the strong vorticity and the air content in the water. The pipe entrance loss also directly influences operating conditions. Surface vortices influenced the net head and the flow rate passing through a pipe, and these effects increase exponentially as the water level decreases or the water level difference increases. Consequently, predicting the net head and the flow rate of the prototype could be complicated for site tests, and it would be also difficult to accurately predict prototype performances with the typical model test and numerical simulation methods for tidal power units.