| Pumped-storage technology is one of the most ideal methods of large-scale peak shaving and power storage in the electricity grid.Due to dramatic increase in renewable energy sources in the electricity grid,such as the wind and solar power,which are easily affected by environmental state,pump-turbines as key parts of pumped-storage power plants are supposed to frequent start-up and shut-down to balance rapid fluctuations of the loads.In such case,pump-turbines should have better flexibility of the operating,and wider operating range.The hump and S-shaped characterisitcs are key technologies to restrict the flexibilty and stable operating range.Nowadays,pump-turbines trend toward higher specific speeds,higher heads and larger capacities,which lead to more serious instability problems of the hump and S-shaped characteristics.Consequently,the studies on hump and S-shaped instabilities become the core and key technology in the filed of pump-turbines.It is a worldwide problem,which attracts the world’s attention and has been invesitged in-depth in the field of hydraulic machinery.In present study,with respect to the largest capacity pumped-storage power plant installed in China,the hump characteristic instability was investigated through combination of the theoretical analysis,model testing and numerical simulation.The aim is to obtain the flow mechanism and find better analysis methods and technology to resolve the problems in pump-turbines.In addition,the work could provide some basis theoretical guidences to promote the localization process of pumped-storage power plants in China.Main studies are as follows:(1)To validate the exsitence of the hysteresis characteritic in the hump region,expeirmental method was adopted to study the energy characteristcs and pressure fluctation characteristics with different guide vane openings using the designed system.Experimental results show there occur hump characteristics with different guide vane openings and accompaning with the hysteresis phenomena.To obatin frequency characteristic and ensure the sources of the unsteady flow patterns,the analysis methods of FFT(fast Fourier transform),STFT(short time Fourier transform)and cross power spectrum were introduced.Analysis shows that the hump characterisitc as well as the hysteresis phenomenon are related to low-frequency fluctuations in stay/guide vanes and vanless space,which are originated from rotating stall or unsteady vortex motion.(2)To analyze the flow mechanism of the hump characteristic as well as the hysteresis phenomenon,steady state simulations were carried out using two-equation turbulence models SST k-ω.Performance characteristics at 13 mm,19mm and 25 mm guide vane openings were predicted through the numerical simulations,which show great agreement with the experimental data.Euler theroy and hydraulic loss analysis in detail were conducted at 19 mm guide vane opening.Results show that the hump characteristic close to BEP mainly comes from the increase in the hydraulic loss;the second one far away from BEP results from the decrease in Euler momentum and increase in the hydraulic loss.The difference of the Euler momentum and hydraulic loss in the two directions leads to the hysteresis phenomena,which increases the unsteady region in the hump region.In addition,numerical analysis were performed at the same operating condition with different guide vane openings to obtain the influence of guide vane opening setting on the hump characteristic.(3)To accurately determine the sources of hydraulic loss resulting from the hump characteristic,entropy production therory was adopted from the view of the second law of thermodynamics.Considering the wall effect of local entropy production rate,the wall eqaution was prosposed to solve the problem of inaccurate calculation of hydraulic loss in the wall region.The results show the hump characterisitc mainly results from from backflow at the runner inlet near the shround and flow separation as well as vortex motion in the stay/guide vanes through the entropy production analysis.(4)Unsteady simualtions were performed at 19 mm guide vane opening to invesitage pressure flucutation characteristic and obtain the generation mechanism of low-frequency pressure fluctuation in the hump region.The results show that the low-frequcny pressure flucutations are orginated from rotating stall and shock in the stay vanes at the operating point 0.92 QBEP in the first hump region,while those come from rotating stall in the guide vanes at the operat ing point 0.37 QBEP in the second hump region.At the rest operating points,low-frequency pressure fluctuations are casued by the vortex motion and shock in stay/guide vanes.(5)To obtain the transient flow characteristics of the hump characteristic as well as the hysteresis phenomenon,transient simulations of valve rapid startup and shutdown processes were conducted.In combination with the methods of continuous Wavelet analysis,using the flow field,pressure field and hydraulic loss distribution,the occurrence,development and cessation of backflow and Dean vortex in the draft tube,as well as rotating stall,separation vortex and backflow vortex in stay/guide vanes were presented in the transient processes.Finally,the relation between performance characteristics and undesirbale flow was determined.The results show that transient processes enlarge the unstable hump region,which leads to more severe hysteresis characteristics. |
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