| With the implementation of the national strategy of the"14th Five-Year Plan",the"Long-Range Objectives Through the Year 2035",and"Carbon Peak in 2030 and Carbon Neutrality in2060",the installed scale of wind and photovoltaic,which are clean and renewable energy,has increased substantially.More and more hydropower stations are put into operation as"regulators"of the power system,which means that the operation conditions of the hydraulic turbine,the core equipment of hydropower,are adjusted more frequently,and the probability of running under the conditions of variable flow,speed and head is higher,which puts forward strict requirements for the high operation stability of the unit.Due to the requirement of high operation stability,this paper takes HLA551-LJ-43 turbine as the research object,based on Liutex vortex identification method and entropy production theory,the characteristics of flow field,the propagation of pressure pulsation,the formation,development and rupture of the vortex structure,cavitation,and their effects on the hydraulic loss are analyzed under full working conditions.The main results are as follows:(1)The analysis results of flow field and pressure pulsation show that:(a)With the decrease of the angle of attack of the guide vane,the head of the upstream stay vane is not partially blocked due to the flow rate reduction,and the high-speed zone and low-pressure zone of its backwater will gradually disappear;The circular motion of the fluid is enhanced and streamline is lengthened,which intensifies the impact of the fluid on the water surface of the guide vane,making the flow state more disordered.(b)Under the influence of the upstream flow state,a local low-pressure zone gradually appears on the back of the downstream runner blade,and the eccentric phenomena of low-pressure and stall appear in the straight cone section and elbow section of the draft tube.(c)Whether under rated condition or under partial flow condition,the vortex rope frequency(0.5f_nunder rated condition and 0.25f_n under partial flow condition)appears in the downstream draft tube due to the propagation of pressure fluctuation in the spiral case domain,which seriously affects the hydraulic stability of the whole unit.(2)The analysis results of vortex structure and cavitation characteristics based on Liutex theory show that:(a)With the decrease of the angle of attack of the guide vane,in the vane domain,there is always axial vortex generated by the impact of fluid on the wall,while the streamwise vortex at the end of the guide vane changes with time,and is in a state of continuous formation and rupture.(b)In the runner domain,cavitation and inter-blade passage vortex are formed in the low-pressure zone of the upstream face of blade and the water release cone domain.(c)In the draft tube domain,the shape of vortex rope in draft tube produced under different working conditions is different.When working under rated conditions,it will produce stable spindle vortex rope.When the flow rate decreases to 81%of the optimal efficiency point,the spiral rope is formed.With the decrease of the opening,the volume of vortex rope increases greatly,forming a larger cavity vortex zone.When the opening decreases to theα=8°,the original shape of vortex rope is destroyed,and the broken vortex fills the straight cone section and elbow section of the whole draft tube.(3)The results of hydraulic loss analysis based on entropy production theory show that:(a)Under different upstream inflow conditions,the hydraulic loss of the volute and the vane domain basically has no obvious change.The hydraulic loss in the spiral case is very small,less than 6%.The hydraulic loss of the stay vane is mainly caused by impact phenomenon and flow separation,which mainly occurs at the head end and tail end.The hydraulic loss of the guide vane is mainly caused by static and dynamic interference and the streamwise vortex caused by the mutual extrusion of the fluid and the extended flow,which mainly occurs on the side facing the water surface.(b)Under rated working conditions,the hydraulic loss in the runner is mainly caused by the tip leakage vortex.However,under partial flow conditions,the hydraulic loss in runner is mainly caused by vortex,and the energy dissipation in runner accounts for over 24.4%of the whole runner.(c)Under the condition of partial flow,The hydraulic loss in the flow channel mainly occurs in the draft tube domain and in the draft tube accounts for more than 60%of the whole channel due to the large volume of draft tube,large circumferential velocity component and the influence of vortex rope.(4)The analysis results of variable speed condition show that:(a)Reducing the rotating speed can effectively improve the flow field in the runner domain,thus reducing cavitation and hydraulic loss,effectively eliminating the helicity of the vortex rope in draft tube,which is beneficial to the stability of the unit.(b)Increasing the rotating speed can eliminate the original vortex in the blade passage,but it will produce a new vortex structure near the tip of blade,which will increase the original hydraulic loss. |
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