Yuekun Sun

Distribution of Pressure Fluctuations in a Prototype Pump Turbine at Pump Mode

Pressure fluctuations are very important characteristics in pump turbines operation. Many researches have focused on the characteristics (amplitude and frequencies) of pressure fluctuations at specific locations, but little researches mentioned the distribution of pressure fluctuations in a pump turbine. In this paper, 3D numerical simulations using SSTk − ω turbulence model were carried out to predict the pressure fluctuations distribution in a prototype pump turbine at pump mode. Three operating points with different mass flow rates and different guide vanes’ openings were simulated. The numerical results show how pressure fluctuations at blade passing frequency (BPF) and its harmonics vary along the whole flow path direction, as well as along the circumferential direction. BPF is the first dominant frequency in vaneless space. Pressure fluctuation component at this frequency rapidly decays towards upstream (to draft tube) and downstream (to spiral casing). In contrast, pressure fluctuations component at 3BPF spreads to upstream and downstream with almost constant amplitude. Amplitude and frequencies of pressure fluctuations also vary along different circumferential locations in vaneless space. When the mass flow and guide vanes’ opening are different, the distribution of pressure fluctuations along the two directions is different basically.

Numerical simulation of pressure fluctuation of a pump-turbine with MGV at no-load condition

In order to analyse the pressure fluctuation caused by misaligned guide vanes (MGV) during starting period at no-load condition, 3-D (three dimensional), unsteady flows in a pump-turbine were numerically studied. Pressure fluctuations of different points at no-load condition are obtained. Fast Fourier Transform(FFT) was used to analyse the frequency spectrum of pressure fluctuations. The amplitude and dominant frequency of pressure fluctuation at vaneless space between the runner and guide vane, as well as the inlet of draft tube, was investigated. The amplitude of pressure fluctuation of the pump-turbine with MGV device is twice that of synchronous vanes. This might be caused by the non-uniform flow in the pump-turbine due to the pre-opened guide vanes. The pump-turbine with synchronous vanes has a low frequency which is 0.33fn, while the low frequency changes into 0.63fn when the MGV device is used. The vortex rope in the draft tube is large than that of synchronize vanes. Resultsof pressure fluctuations with synchronous vanes agree with each other between computational and testing results. The numerical study of pressure fluctuations with MGV can provide a basic understanding for the improvement of the instability of a pump-turbine.

Instability study of a pump-turbine at no load opening based on turbulence model

A linear eddy viscosity model and a non-linear eddy viscosity model were used to simulate a centrifugal pump at an off-design point. Compared with the inner flow of experimental results, turbulence model was accurate in the calculation of the rotating stall phenomenon in the runner of low flow conditions. The turbulence model and three dimensional (3-D), unsteady flow in a pump-turbine was used to study the instability of a high-head pump-turbine at no-load opening. Fluid coupling and dynamic mesh were used to simulate the change of runners rotational speed. Stall phenomena in the runner caused by a large incidence angle of flow at the region between stay vanes and guide vanes were analyzed. Calculations based on different moments of inertia were accomplished in order to investigate the influence of moment of inertia to the stall phenomena in the runner. The Rayleigh criterion was introduced to analyze the instability of the stall phenomena in the runner. The explicit characteristics such as the flow-rate, rotational speed, torque of the runner etc. were analyzed. Results show that the moment of inertia has great influence on the stall phenomena in the runner. The Rayleigh criterion can be used to evaluate the instability of the pump-turbine at no-load opening. The flow in the pump-turbine at runaway speed is more stable when the moment of inertia increases. The study of a pump-turbine at no-load opening can provide a basic foundation for the improvement of S characteristics.

Numerical study of vortex rope during load rejection of a prototype pump-turbine

A transient process of load rejection of a prototype pump-turbine was studied by three dimensional, unsteady simulations, as well as steady calculations.Dynamic mesh (DM) method and remeshing method were used to simulate the rotation of guide vanes and runner. The rotational speed of the runner was predicted by fluid couplingmethod. Both the transient calculation and steady calculation were performed based on turbulence model. Results show that steady calculation results have large error in the prediction of the external characteristics of the transient process. The runaway speed can reach 1.15 times the initial rotational speed during the transient process. The vortex rope occurs before the pump-turbine runs at zero moment point. Vortex rope has the same rotating direction with the runner. The vortex rope is separated into two parts as the flow rate decreases to 0. Pressure level decreases during the whole transient process.The transient simulation result were also compared and verified by experimental results. This computational method could be used in the fault diagnosis of transient operation, as well as the optimization of a transient process.

Numerical simulation of the influence of distributor pitch diameter on performance and pressure fluctuations in a pump-turbine

In order to analyse the influence of distributor pitch diameter on performance and pressure fluctuations in a pump turbine, a numerical model based on a pumped storage power station was built to develop the numerical simulation. Steady and unsteady flows were simulated using the SST k-ω turbulence model and SIMPLEC Pressure-Velocity coupling scheme. The performance, inner flow and pressure fluctuations between runner blades and guide vanes of both turbine and pump mode was contrasted in different distributor pitch diameters. The result shows there was a maximum total efficiency in a given distributor pitch diameter instead of the design diameter. Amplitudes and frequencies of pressure fluctuations on this diameter and design diameter were analysed, minor differences were observed. This position can be considered to help improving the flow of the pump turbine.

Numerical study of pressure fluctuations transfer law in different flow rate of turbine mode in a prototype pump turbine

Numerical simulation using SST k-w turbulence model was carried out, to predict pressure fluctuation transfer law in turbine mode. Three operating points with different mass flow rates are simulated. The results of numerical simulation show that, the amplitude and frequency of pressure fluctuations in different positions are very different. The transfer law of amplitude and frequency of pressure fluctuations change with different position and different mass flow rate. Blade passing frequency (BPF) is the first dominant frequency in vaneless space, while component in this frequency got smaller in the upstream and downstream of vaneless space when the mass flow is set. Furthermore triple blade passing frequency (3BPF) component obtained a different transfer law through the whole flow passage. The amplitude and frequency of pressure fluctuations is also different in different circumference position of vaneless space. When the mass flow is different, the distribution of pressure fluctuations in circumference is different. The frequency component of pressure fluctuations in all the positions is different too.