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[en] Conventional representations of the various operation modes of a pump-turbine (4-quadrant characteristics) have important disadvantages. While curves of Q11 vs n11 have singularities at E=0 and multiple values in the 'unstable' ranges, the curves EnD(QnD) get singular at n=0. As a remedy, one may split the characteristics into separate parts, and switch between them. Another approach introduced by P. Suter (1966, ) defines a different set of variables which avoids singularity and always remains unique-valued. Suter described this artifice for non-regulated pumps; but using it for regulated machines without modifications is not practical due to large distortions at small guide vane opening. A decisive improvement has been described by C.S. Martin . It avoids the distortion of the head-vs-flow curves at low load. The present paper describes how further improvement is possible, in particular with regard to the representation of torque. A modified torque parameter is obtained by subtracting the shutoff torque; this parameter can be handled in the same practical way as the discharge. Other improvements concern the correction for leakage at small guide vane opening, and the treatment of very small and zero opening. These details are concerned with the problem of closed gate where Suter's concept does not work. Applications are demonstrated, not only how to represent the hydraulic performance (head vs. discharge and torque vs. discharge), but also for other characteristics, such as the development of pressure and pressure pulsation in various locations, or the steady-state and unsteady guide vane torque. The advantage of a set of continuous, single-valued functions for all those physical properties greatly simplifies computation of their behavior during transients. Moreover, the, Suterized' properties of pump-turbines of different specific speed are less different from each other than the conventional ones, a fact that facilitates application of available test data for later projects.
[en] Highlights: • An interesting hysteresis phenomenon was analyzed using entropy production theory. • A function was used to calculate the entropy production in the wall region. • Generation mechanism of the hump and hysteresis characteristics was obtained. - Abstract: The hydraulic loss due to friction and unstable flow patterns in hydro-turbines causes a drop in their efficiency. The traditional method for analyzing the hydraulic loss is by evaluating the pressure drop, which has certain limitations and cannot determine the exact locations at which the high hydraulic loss occurs. In this study, entropy production theory was adopted to obtain a detailed distribution of the hydraulic loss in a pump-turbine in the pump mode. In the past, the wall effects of entropy production were not considered, which caused larger errors as compared with the method of pressure difference. First, a wall equation was proposed to calculate the hydraulic loss in the wall region. The comparison of hydraulic loss calculated by entropy production and pressure difference revealed a better result. Then, through the use of the entropy production theory, the performance characteristics were determined for a pump-turbine with 19 mm guide vane opening, and the variation in the entropy production was obtained. Recently, an interesting phenomenon, i.e., a hysteresis characteristic, was observed in the hump region in pump-turbines. Research shows that the hysteresis characteristic is a result of the Euler momentum and hydraulic loss; the hydraulic loss accounts for a major portion of the hysteresis characteristic. Finally, the hysteresis characteristic in the hump region was analyzed in detail through the entropy production. The results showed that the hump characteristic and the accompanying hysteresis phenomenon are caused by backflow at the runner inlet and the presence of separation vortices close to the hub and the shroud in the stay/guide vanes, which is dependent on the direction of discharge.
[en] This work investigates the influence of water compressibility on pressure pulsations induced by rotor-stator interaction (RSI) in hydraulic machinery, using the commercial CFD solver ANSYS-CFX. A pipe flow example with harmonic velocity excitation at the inlet plane is simulated using different grid densities and time step sizes. Results are compared with a validated code for hydraulic networks (SIMSEN). Subsequently, the solution procedure is applied to a simplified 2.5-dimensional pump-turbine configuration in model scale with an adapted speed of sound. Pressure fluctuations are compared with numerical and experimental data based on prototype scale. The good agreement indicates that the scaling of acoustic effects with an adapted speed of sound works well. Finally, the procedure is applied to a 3-dimensional pump configuration in model scale. Pressure fluctuations are compared with results from prototype measurements. Compared to incompressible computations, compressible simulations provide similar pressure fluctuations in vaneless space, but pressure fluctuations in spiral case and penstock may be much higher. With respect to pressure fluctuation amplitudes along the centerline of runner channels, incompressible solutions exhibit a linear decrease while compressible solutions exhibit sinusoidal distributions with maximum values at half the channel length, coinciding with analytical solutions of one-dimensional acoustics.
[en] The stresses and relative stiffness of the head cover bolts in a pump turbine have been calculated using ANSYS. In the numerical model established, the head cover, stay ring and bolts have been included. Results show that the bolts bend due to the preload and the hydraulic pressure acting on the head cover. Consequently, stresses in the mid sections of the bolts are uneven. It is also found that the relative stiffness of the bolts and flange vary with the working condition and preload, which indicates that the whole model including the bolts is a nonlinear system. Therefore, this study concludes that the relative stiffness of the bolts and flange for pump turbines should not be assumed to be a constant value at the design stage. (paper)
[en] In recent years an increased interest in pump-turbines has been recognized in the market. The rapid availability of pumped storage schemes and the benefits to the power system by peak lopping, providing reserve and rapid response for frequency control are becoming of growing advantage. In that context it is requested to develop pump-turbines that reliably stand dynamic operation modes, fast changes of the discharge rate by adjusting the variable diffuser vanes as well as fast changes from pump to turbine operation. Within the present study various flow patterns linked to the operation of a pump-turbine system are discussed. In that context pump and turbine mode are presented separately and different load cases at both operation modes are shown. In order to achieve modern, competitive pump-turbine designs it is further explained which design challenges should be considered during the geometry definition of a pump-turbine impeller. Within the present study a runner-blade profile for a low head pump-turbine has been developed. For the initial hydraulic runner-blade design, an inverse design method has been applied. Within this design procedure, a first blade geometry is generated by imposing the pressure loading-distribution and by means of an inverse 3D potential-flow-solution. The hydraulic behavior of both, pump-mode and turbine-mode is then evaluated by solving the full 3D Navier-Stokes equations in combination with a robust turbulence model. Based on this initial design the blade profile has been further optimized and redesigned considering various hydraulic pump-turbine requirements. Finally, the progress in hydraulic design is demonstrated by model test results which show a significant improvement in hydraulic performance compared to an existing reference design.
[en] One of the most important controlled parameters of thermal power units is the boiler drum water level. Disturbances of feed water flow rate could cause instability of the drum water level. This study proposes the Gray correlation compensation (GCC) control technology for the Boiler feed water pump turbine (BFPT) to solve this problem. Simulation results indicate that the GCC controller outperforms the traditional proportional-integral-derivative controller when it encounters different disturbances. Furthermore, the GCC controller can rapidly switch to the high-pressure steam source to ensure that the drum water level is in the secure range during steam source switching of the BFPT
[en] In order to get the accurate hump characteristic curve of a pump turbine in pump mode, three dimensional steady simulations were carried out using SST k-ω turbulence model with cavitation model and without cavitation model under different operation condition points with 19 mm guide vanes opening. A refinement grids were generated to adapt the turbulence model. The results obtained with cavitation model show a better agreement with experiments. The detailed analysis was undertaken to find out the relationship between the cavitation and hump region. It is concluded that the hump characteristic is related with cavitation
[en] Sea water is used as the ultimate heat sink for Cirus and Dhruva reactors. Sea water is pumped through a common header to both the reactors using vertical turbine pumps located in a 40 feet deep chamber. A traveling water screen with 10 mm mesh is provided in the chamber at the upstream side of the pump. This screen is rotated at specific intervals with washing water jet supply on, for cleaning the screen of any entrapped matter. During the monsoon in year 2004, on one occasion, while the screen was being operated, its shear pin got broken. On inspection it was found that few baskets had got bent, preventing its movement along the haulage chain links through the guide brackets. The bent baskets were in almost completely choked condition with large collection of plastic sheets and it appeared that they got bent due to the resultant high liquid differential pressure across it. The matter was investigated in detail. This paper deals with the observations, conclusions and remedial measures w.r.t the bending incident of the traveling water screen baskets. (author)
[en] Highlights: • The authors present measurements at test rig of 12 pumps running as turbine. • The pump operating mode of these machines has been correlated to that as turbine. • A procedure selecting the best pump as turbine for a hydrological site is set up. • The procedure is recursive giving the possibility to refine the preliminary choice. • An exemplificative case study is illustrated with the aim to explain the procedure. - Abstract: The use of hydraulic pumps running as turbines offers several advantages with respect to traditional turbines. The possibility of predicting the performances of a pump running as a turbine and of selecting the suitable machine for a given hydropower site, in an easy and reliable way, is a still open issue. The present work makes a contribution to the solution of this subject both through experimental and theoretical activities. The experimental activity is carried out by the characterisation of 12 pumps measured, at test rig both in their natural operation, as pumps and in reverse mode, as turbines. The numerical activity starts from the definition of correlations regarding the heads and capacities conversion factors, useful for the preliminary selection of a suitable pump to run as turbine in a microplant for a given hydropower site. Then, a statistical method involving polynomials was implemented, allowing the performance curves to be provided. The agreement between experimental data and numerical results is comparable to that obtained by others authors. Finally, a comprehensive procedure has been set up, refining the selection by evaluating likewise if the efficiency of the selected pump to use as a turbine, at operating point of the proposed plant, is acceptable. The results related to a study case are shown with the aim to highlight the easiness of the proposed method, having in mind that it does not represent the generality of the possible applications of pumps running as turbines.
[en] The static stress of bolts connecting head cover and stay ring in a pump turbine has been analyzed through ANSYS. The 3D flow of the pump turbine was simulated to get the pressure on head cover and stay ring. The stresses of bolts under 2 loading conditions were calculated: preload only as well as both preload and hydraulic pressure. Results showed that the stress of bolts was greater than head cover and stay ring. Stress concentration of bolts appeared at the position where diameter changed. Due to the influence of hydraulic pressure, the stress of bolts became larger. Head cover and stay ring deformed at the position where stiffness was poor. (paper)