Daqing Qin

Numerical investigation on transient flow of a high head low specific speed pump-turbine in pump mode

Transient process of a pump-turbine, in which the inlet discharge decreases rapidly in pump mode, usually appears due to its frequent switch between pump mode and turbine mode. In order to ensure safe and stable operation, the mechanism of flow characteristics during the transient process was investigated combining numerical and experimental methods. First, steady and unsteady simulations were carried out using the shear-stress transport (SST) k-ω turbulence model under 19 mm guide vane opening. External and internal characteristics under different discharges were presented using numerical simulations based on experimental validation. The results show that the position and area of flow separation as well as appearance of vortices vary with time in the transient process. Finally, the variation of the transient frequency characteristics was obtained through short time Fourier transform. It indicates that main frequencies are blade passing frequency and its harmonic frequencies at the beginning of the transi...

Flow characteristics prediction in pump mode of a pump turbine using large eddy simulation

To obtain more accurate flow characteristics of pump turbines, the method of large eddy simulation with wall-adapting local eddy viscosity model is applied in simulating several operating points in the pump mode. Firstly, based on the experimental validation, the method of large eddy simulation could better predict the external performance and internal flow characteristics in a pump turbine in the pump mode compared with the method of Reynolds-averaged Navier–Stokes with two-equation turbulence model shear stress transport k–ω. Then, flow characteristics under 1.00QBEP (best efficiency point), 0.91QBEP, 0.88QBEP, and 0.85QBEP operating points are investigated to find out the causes of the head drop in the energy-discharge curve through large eddy simulation. The detailed analysis reveals that the head drop at the point 0.85QBEP is related to the recirculation flow at the runner inlet. Finally, unsteady studies confirm that vortex movement at the runner inlet lead to the variation of the amplitudes and directions of the velocity, which generates the rotation of the separation vortices in the runner and stay vane channels.

Entropy production analysis for hump characteristics of a pump turbine model

The hump characteristic is one of the main problems for the stable operation of pump turbines in pump mode. However, traditional methods cannot reflect directly the energy dissipation in the hump region. In this paper, 3D simulations are carried out using the SST k-ω turbulence model in pump mode under different guide vane openings. The numerical results agree with the experimental data. The entropy production theory is introduced to determine the flow losses in the whole passage, based on the numerical simulation. The variation of entropy production under different guide vane openings is presented. The results show that entropy production appears to be a wave, with peaks under different guide vane openings, which correspond to wave troughs in the external characteristic curves. Entropy production mainly happens in the runner, guide vanes and stay vanes for a pump turbine in pump mode. Finally, entropy production rate distribution in the runner, guide vanes and stay vanes is analyzed for four points under the 18 mm guide vane opening in the hump region. The analysis indicates that the losses of the runner and guide vanes lead to hump characteristics. In addition, the losses mainly occur in the runner inlet near the band and on the suction surface of the blades. In the guide vanes and stay vanes, the losses come from pressure surface of the guide vanes and the wake effects of the vanes. A new insight-entropy production analysis is carried out in this paper in order to find the causes of hump characteristics in a pump turbine, and it could provide some basic theoretical guidance for the loss analysis of hydraulic machinery.

Analytical solution of Reynolds equation under dynamic conditions

The flow in the clearance heavily influences the lubrication performance of journal bearing. Therefore, it is important to study the flow mechanism in journal bearing. As known, the pressure distribution in bearing clearance can be described with Reynolds equation. In this paper, unsteady form of Reynolds equation was simplified and nondimensionalized at first, based on certain assumptions. Then, the equation was solved with regular perturbation method, obtaining the analytical solution. At the end, the results of our work were compared with the results of “infinitely short journal bearing” model, Vignolo et al. (Approximate analytical solution to Reynolds equation for finite length journal bearings. Tribol Int 2011; 44: 1089–1099), and numerical simulations. The comparison implies that our results show more coincidence with the numerical results in a wider range.

Pressure fluctuation prediction in pump mode using large eddy simulation and unsteady Reynolds-averaged Navier–Stokes in a pump–turbine

For pump–turbines, most of the instabilities couple with high-level pressure fluctuations, which are harmful to pump–turbines, even the whole units. In order to understand the causes of pressure fluctuations and reduce their amplitudes, proper numerical methods should be chosen to obtain the accurate results. The method of large eddy simulation with wall-adapting local eddy-viscosity model was chosen to predict the pressure fluctuations in pump mode of a pump–turbine compared with the method of unsteady Reynolds-averaged Navier–Stokes with two-equation turbulence model shear stress transport k–ω. Partial load operating point (0.91QBEP) under 15-mm guide vane opening was selected to make a comparison of performance and frequency characteristics between large eddy simulation and unsteady Reynolds-averaged Navier–Stokes based on the experimental validation. Good agreement indicates that the method of large eddy simulation could be applied in the simulation of pump–turbines. Then, a detailed comparison of variation for peak-to-peak value in the whole passage was presented. Both the methods show that the highest level pressure fluctuations occur in the vaneless space. In addition, the propagation of amplitudes of blade pass frequency, 2 times of blade pass frequency, and 3 times of blade pass frequency in the circumferential and flow directions was investigated. Although the difference exists between large eddy simulation and unsteady Reynolds-averaged Navier–Stokes, the trend of variation in different parts is almost the same. Based on the analysis, using the same mesh (8 million), large eddy simulation underestimates pressure characteristics and shows a better result compared with the experiments, while unsteady Reynolds-averaged Navier–Stokes overestimates them.

Influence of guide vane setting in pump mode on performance characteristics of a pump-turbine

Performance characteristics in pump mode of pump-turbines are vital for the safe and effective operation of pumped storage power plants. They are resultant of Euler head (power input) and hydraulic losses (power dissipation). In this paper, 3-D steady simulations were performed under 13mm, 19mm and 25mm guide vane openings (GVOs). Three groups of operating points under the three GVOs were chosen based on experimental validation to investigate the influence of guide vane setting on flow patterns upstream and downstream. Analysed results show that, the guide vane setting will obviously change the flow pattern downstream, which in turn influences the flow upstream. It shows a strong effect on hydraulic losses in guide and stay vanes. In addition, at the large part load conditions, the change of GVO will increase the relative flow angle at the runner outlet. As a consequence, it decreases the Euler head. However, at other operating conditions, it only has a little influence on Euler head. Flow patterns in pump mode are very dependent on the GVO and discharge.

Investigation into the flow details of runner region in a pump turbine at off-design conditions

Due to the rapid development of large thermal power stations and nuclear power stations, it requires that the power grid should provide greater capability and flexibility. Security, stability, and economy in operation should also be taken into consideration. The pumped storage power station which has double peak regulating capability is a good choice to realize this goal. However, the existence of S shape characteristic of a pump turbine in generating mode at off-design condition makes it difficult to connect to the power grid. So it is necessary to analyze the reason of S shape characteristic in pump turbine by experimental investigation and numerical simulations. To achieve the above-mentioned object, numerical simulations of a pump turbine’s internal flow field were performed. The generating mode was analyzed by unsteady computational fluid dynamics simulation. The evolution of flow details in runner regions under the S shape characteristic conditions was observed by changing the guide vane opening. The unsteady flow structure and load on the blades in regions of S shape characteristic were obtained, which will contribute to the design of hydro-turbine.

CFD detection for inner flow and presure fluctuation in a hydraulic tubine

Pressure fluctuation is a common problem in large-scale hydroturbine, which will affect the performance of hydroturbines, such as negatively affecting the efficiency, increasing the damage of components and decreasing the life span, bringing up great potential troubles to the operation safety of large-scale hydroturbines. To study the pressure fluctuation in the hydroturbine, CFD calculation was performed on the Francis hydroturbine. The model of Francis hydroturbine studied in the paper is Baihetan HEC-a1014-type. The computations were carried out in the whole runner blade passage and the boundary conditions were set as the same as the experimental conditions. The unsteady flow of the turbine was computed. The effects of turbulence were modeled with RNG κ-ε turbulence model. The inner flow field and the pressure fluctuation were obtained from the calculation using the solver of Fluent. After analyzing the results, conclusions were drawn out.

Numerical Simulation and Experimental Study of the Pressure Fluctuation in Water Turbines

Pressure fluctuation is a common problem in large-scale hydraulic turbine, which will affect the performance of the water turbines, such as negatively affecting the efficiency, increasing the damage of related components and decreasing the life span, bringing up great potential troubles to the operation safety of large-scale water turbines. The mechanism how the pressure fluctuation appears is due to the low-frequency pressure pulsation in the draft tube and the mid-frequency pressure pulsation generated by dynamic and static interferences before the runner transmit upstream, and the low-frequency and mid-frequency pressure pulsation in the front of the runner, guide vanes, fixed vanes and the inlet of the volute. To study the relationship between the capacity and the pressure fluctuation of the water turbine, the CFD calculation was performed on the water turbine and the results of the calculation were compared with the measurements in this study. The model of water turbine studied in this paper is Baihetan HEC_1014-type. The computations were carried out in the whole runner blade passage and the boundary conditions were set as the same as the experimental conditions. The unsteady state flow of the turbine was computed. The effects of turbulence were modeled with standard κ-e turbulence model. The inner flow field and the pressure fluctuation were obtained from the calculation using the solver of Fluent. And the results of simulation are compared with the experimental results.Copyright