Z G Zuo

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.

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 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 Predictions of the Incipient and Developed Interblade Vortex Lines of a Model Francis Turbine by Cavitation Calculations

The existence of runner interblade vortices can cause instability problems in Francis turbines, for example, pressure fluctuations, vibrations of runners, noise, and so forth. It is favorable in engineering practice to have the knowledge of the appearance of the incipient and developed inter blade vortex lines on the Hill diagram in unit parameters. Most numerical research on the inter blade vortices has been focused on the study of the characteristics of pressure fluctuations by single-phase flow calculations. However, since the two vortex lines are distinguished by observations of visible vortices, which contain cavitating flows, it is clear that cavitation calculations are needed for their predictions. A method by solving RANS equations with RNG k – ∊ turbulence model and ZGB cavitation model was proposed for the predictions in a Francis model turbine. Modifications of the turbulence viscosity were made for better simulation results. Vorticity criterion was chosen to identify the vortices. The fact that the results of cavitation calculations have a better agreement with experimental observations than single-phase calculations proves the validity of this prediction method.

Pressure fluctuation prediction of a model pump turbine at no load opening by a nonlinear k-ε turbulence model

In this paper, a new nonlinear k-e turbulence model based on RNG k-e turbulence model and Wilcoxs k-ω turbulence model was proposed to simulate the unsteady flow and to predict the pressure fluctuation through a model pump turbine for engineering application. Calculations on a curved rectangular duct proved that the nonlinear k-e turbulence model is applicable for high pressure gradient flows and large curvature flows. The numerically predicted relative pressure amplitude (peak to peak) in time domain to the pump turbine head at no load condition is very close to the experimental data. It is indicated that the prediction of the pressure fluctuation is valid by the present nonlinear k-e method. The high pressure fluctuation in this area is the main issue for pump turbine design, especially at high head condition.

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.

Numerical simulation on dimension decrease for annular casing of one centrifugal boiler circulation pump

Primary formulation derivation indicates that the dimension of one existing centrifugal boiler circulation pump casing is too large. As great manufacture cost can be saved by dimension decrease, a numerical simulation research is developed in this paper on dimension decrease for annular casing of this pump with a specific speed equaling to 189, which aims at finding an appropriately smaller dimension of the casing while hydraulic performance and strength performance will hardly be changed according to the requirements of the cooperative company. The research object is one existing centrifugal pump with a diffuser and a semi-spherical annular casing, working as the boiler circulation pump for (ultra) supercritical units in power plants. Dimension decrease, the modification method, is achieved by decreasing the existing casings internal radius (marked as Ri0) while keeping the wall thickness. The research analysis is based on primary formulation derivation, CFD (Computational Fluid Dynamics) simulation and FEM (Finite Element Method) simulation. Primary formulation derivation estimates that a design casings internal radius should be less than 0.75 Ri0. CFD analysis indicates that smaller casing with 0.75 Ri0 has a worse hydraulic performance when working at large flow rates and a better hydraulic performance when working at small flow rates. In consideration of hydraulic performance and dimension decrease, an appropriate casings internal radius is determined, which equals to 0.875 Ri0. FEM analysis then confirms that modified pump casing has nearly the same strength performance as the existing pump casing. It is concluded that dimension decrease can be an economical method as well as a practical method for large pumps in engineering fields.

Numerical simulation on casing modification of a boiler water circulation pump

In this paper, hydraulic performance comparisons are made by numerical simulation method on boiler water circulation pump with casings of different shapes. The existing pump adopts a semispherical casing and a garlic-shaped casing. Results show that in the garlic-shaped casing noticeable swirling vortex can be found in the top region of the discharge nozzle, and semispherical casing has better performance in hydraulic efficiency and head.