Research On The Control Of S-shaped Characteristics And Pressure Pulsations Of Pump-turbines

To expand the operating range of pumped-storage units and ensure the safety and stability of the unit in partial output conditions and transient operations,it is necessary to investigate the hydrodynamic characteristics of the pump-turbine in off-design conditions in generating mode.This dissertation attempts to establish an understanding of the stability mechanism of the hydraulic system and the hydrodynamic characteristics of the pump-turbine through methods including prototype experiments,model test research,internal flow field simulation of a pump-turbine,and hydraulic design of a pump-turbine runner.In addition,the relationship between geometric design and the S-shaped characteristic curves of the pump-turbine as well as internal flow characteristics is explored.The following results have been achieved:(1)Reasons for significant amplitude and wide frequency composition of pressure pulsations during the transient operation of the prototype pump turbine(emergency load rejection and power failure)were analyzed.Some factors influencing significant pressure pulsations were pointed out.The exponential relationship between amplitude of the blade rotation frequency in the vaneless space and speed factor was obtained in turbine mode.Taking a 300 MW capacity and medium specific speed prototype pump turbine as a case,an attempt was made to analyze the causes of severe pressure pulsation during the transient process of a pumped-storage power station.Taking the emergency load rejection and power failure process at the rated operating conditions of a single unit as an example,the main components of the pressure pulsation at measuring points such as the spiral casing inlet,draft tube inlet,and vaneless area and the variation of their intensity during the transients are clarified.The influence of different initial output on characteristics of pressure pulsation in time domain and frequency domain in turbine mode was analyzed.The influence of the parallel operation of two units in the same hydraulic unit on characteristics of the pressure pulsation was analyzed.It was also mentioned that the intensity of pressure pulsation in the rotor-stator interference was significantly more intense during parallel operation,either in the steady state or in emergency load rejection.The influence of different sampling frequencies on the time-averaged pressure and pressure pulsation in the same transient process was pointed out.It is found that there might exist the corelation between the instantaneous amplitude of the blade passing frequency 9f_n and the speed factor(reflecting the Strouhal number)during the time period when the guide vane opening varied slowly after the load rejection of the unit.(2)The variation of global parameters and the characteristics of pressure pulsations in the vaneless space of a model pump turbine in various transient processes were investigated in detail.A novel transient process that can be utilized to dynamically explore the S-shaped characteristic curves was proposed and experimentally confirmed.The applicable range of the exponential relationship between the amplitude of blade passing frequency and speed factor was further studied.Based on the pumped-storage test rig at Wuhan University,steady-state and various transient experiments of a pump-turbine runner with 9 equal blades were carried out.It was shown that the hump and hysteresis of the energy characteristic curves of the pump-turbine in pump mode were due to the pressure change in the draft tube inlet.Through the transient process of power failure,the dynamic characteristic curves of the pump-turbine in three operating conditions,and the time-frequency characteristics of the pressure pulsation in the vaneless space were presented.The feasibility of dynamically measuring the performance curve of the pump-turbine through dynamic adjustment of the sphere valve and a start of the pump-turbine with low pump head was experiementally analyzed.Through the point-to-point measurement experiment of the sphere valve,the steady-state characteristic curve of the pump-turbine was obtained.In the runaway test,the external characteristic parameters of the runner and the pressure pulsation in the vanless space were analyzed.The feasibility of obtaining the characteristic curve of the pump-turbine in turbine mode by dynamic adjustment of the sphere valve was analyzed,and it was pointed out that the maximum intensity of rotor-stator interaction did not vary significantly with the variation of the full stroke time of the sphere valve.A new type of transient process that dynamically obtains the complete S-shaped characteristic curves under the premise of a fixed rotational speed of the unit was presented—constant-speed oscillation.It was shown that the root cause of the dynamic instability of the constant-speed oscillation was the positive slope of the characteristic curves at no-flow.When trying to force two units running in parallel to experience constant-speed oscillation at the same time,it is observed that the unit with more intense S-shaped characteristics tended to operate only in the reverse pump mode,showing its pump effect.However,the unit with weaker S-shaped characteristics would swtich back and forth from reverse pump mode,turbine brake mode,and turbine mode.The exponential relationship between the amplitude of the blade passing frequency 9f_n and the speed factor was verified through steady-state and various transient experiments.The applicable scope of this relationship was also discussed.(3)The trade-off between geometric parameters of pump-turbines in improving the hydraulic performance and the operation stability was presented.The influence of geometric parameters of pump-turbines with splitter blades on the S-shaped characteristics was clarified.According to the design parameters and geometric parameters such as runner inlet diameter,outlet diameter,guide vane height and other geometric parameters of model runner A that have been studied by model tests,a number of pump turbine runners were designed.In total,two numerical orthogonal tests and a sensitivity test were completed,to to analyze the influence of geometric parameters on the S-shaped characteristic curves of the pump-turbine.The influence of blade number,wrap angle,blade thickness,short blade inlet position,blade placement angle on the high-pressure side of the runner?₂,wrap angle gradient from the upper crown to the shroud on the high-pressure side of the runner,blade thickness distribution,blade camber on the net head and efficiency in pump mode,net head and efficiency in turbine mode,specific speed,speed factor in no load and no flow were analyzed.In addition,the influence of these geometric parameters on the contradiction of improving the hydraulic performance and stability of the pump turbine was analyzed.Then,in order to decouple the specific influence of each parameter from the orthogonal experiment,through the parameter analysis of equal blades and the design of splitter blades,the influence of the high-pressure side blade placement angle?₂,low-pressure side blade placement angle?₁,wrap angle gradient,wrap angle,and blade camber,the relative length of splitter blades,the total number of blades,and the relative ratio of main blades to splitter blades on the shape of S-shaped characteristic curves and the efficiency curves of the pump-turbine were discussed.(4)The design of pump-turbine runners with splitter blades to improve the global parameters and pressure pulsations in transient process was confirmed.The conversion of the dominant frequency of rotor-stator interaction in the vaneless space of runners with splitter blades was discovered.Based on the selected pump-turbine with splitter blades,detailed model tests were carried out to verify the improvement of the water hammer pressure,the rise of rotational speed and pressure fluctuations during transient operations.In addition,the internal flow characteristics of the runner when the pump-turbine was operated in the region of S-shaped characteristic curves were shown by numerical simulations.Meanwhile,the conversion of the dominant frequency in the vaneless space was investigated.From steady experiments,it was pointed out that the designed pump-turbine with splitter blades had stronger flowrate capacity than the reference runner,and it had higher efficiency under the conditions of large flowrate and high output.In designed net head,the variation of the intensity of the dominant frequency in the vaneless space with the guide vane opening was analyzed.In experiments varing net head with a constant guide vane opening,the conversion of the dominant frequency in the vaneless space was observed.It was shown that under designed net head and large flowrate conditions,the dominant frequency of pressure pulsations in the vaneless space corresponded to the number of main blades or splitter blades.As the flowrate decreased,the blade passing frequency corresponding to the total number of blades gradually became the dominant frequency.By conducting transient experiments including the start-up,emergency load rejection with guide vanes stalled,dynamic measurements through the control of sphere valve,constant speed oscillation,etc.,the variations of global parameters and transient pressure pulsations of the pump-turbine with 4 main blades and 4 splitter blades were shown in detail.Subsequently,the S-shaped characteristics and pressure pulsations characteristics of the pump-turbine with 4 main blades and 4 splitter blades were quantitatively compared with the reference runner.It was found that the designed pump-turbine with splitter blades had a slightly higher amplitude of pressure pulsations in the vaneless space and a slightly lower efficiency under designed conditions.However,the amplitude of pressure pulsations of the pump-turbine with 4 main blades and 4 splitter blades in off-design conditions were lower than the reference runner,and its S-shaped characteristic curve was gentler.Through the experiment of emergency load rejection with the same guide vane closing scheme in designed net head,the improvement of the water hammer pressure,the rise of rotational speed and pressure fluctuations of the pump-turbine with 4 main blades and 4 splitter blades was quantitatively shown.Through numerical simulations,it was observed the conversion of the dominant frequency in the vaneless space of pump-turbines with splitter blades was not only reflected in the pressure mode in the vaneless space,but also reflected in the circumferential distribution of flow parameters including relative flow angle and vortex intensity?_z.