Analysis On Inflow Characteristic Of Pump-turbine In Hump Region

Recently,a lot of new energy sources such as tidal energy,solar energy,and wind energy have been introduced into the power system,making its stability requirements more stringent.Pumped storage units are responsible for peak and valley filling and power quality improvement in the power system,and its stability and safety have become more important.The hump characteristic of pump-turbine under small flow rate is one of the unstable characteristics that hinders the normal operation of the unit.Research on it can provide a basis for the stable operation of a pump-turbine.In order to reveal the internal flow characteristics,external characteristics and their relationship in hump region,this paper takes a pump-turbine as the research object,and uses the SST k-co turbulence model to carry out three-dimensional full-flow channel unsteady simulation at the opening of guide vane of 14°.The main research contents and conclusions are as follows:(1)The internal flow of each component of the unit in pump mode,especially under the hump region operating condition is discussed.The results show that there are three groups of high-speed areas separated by 120°in the circumferential direction of runner outlet in hump region,which is caused by the uneven distribution of the tangential velocity.There are also three sets of stalled flow channels separated by 120° in the circumferential direction of the cascade channels,and the stall flow channels are filled with undesirable flows such as flow separation,vortex or backflow.Therefore,the hump characteristic of this unit is related to the uneven distribution of the runner outlet and the stall phenomenon of the cascade channel(2)Based on analysis methods such as pressure standard deviation,peak-to-peak pressure and Fast Fourier Transform,the pressure pulsation characteristics,pressure pulsation sources and propagation rules of the unit under different working conditions are explored.The results show that the low-frequency pressure pulsation is domain frequency at the cascade channel under low-flow conditions.Besides,the domain frequency of the cascade channel is 0.2fn under the hump region condition.As flow rate increases,the internal flow pattern is improved,and its low-frequency pressure pulsation gradually weakens until it disappears,and blade passing frequency is the main frequency.The domain frequency of the pressure pulsation of runner is the rotating frequency and its harmonic frequency,but as the flow rate decreases,the low-frequency pulsation and the rotating frequency pulsation interact,and the pressure pulsation spectrum becomes more complicated in runner.(3)The rotating stall propagation mechanism and propagation characteristics are studied,and the pressure pulsation characteristics caused by rotating stall is explained based on the velocity coefficient distribution.The results show that in the range of 0.82Qb to 0.76Qb operating conditions,three groups of stall clusters with a phase difference of 120° are distributed uniformly in the circumferential direction of the cascade channel.Secondly,if a flow channel stalls,it will be blocked by unstable flow structures.Then the blocked fluid flows into the adjacent flow channel,leading to increase in the angle of attack of the fluid in the front flow channel and decrease in the angle of attack of the fluid in the rear flow channel.It will promote the occurrence of the former first-track stall group and decline of the latter first-track stall group,and gradually form the rotating stall phenomenon in this way.Finally,combined with the evolution rule of the stall cluster,after five rotation cycles,the three groups of stall clusters in the cascade channel all rotate 120°,that is,the three stall clusters rotate a total of one week.It can be inferred that the 0.2fn low-frequency pulsation in vane zone is caused by the low-speed rotation of the stall clusters.