Study On The Characteristics Of The Flow Around The Stall Of The Hydraulic Airfoils And Cascades

Stall is a key problem that restricts the stability of the axial flow pump under small flow condition.The internal flow mechanism and excitation mechanism of the axial flow pump stall are very complicated.Hydrofoils and cascades are the main elements of the hydraulic components in the axial flow pump.The hydraulic performance and excitation characteristics are important references for hydraulic design of the axial flow pump.The study of stall flow characteristics involve strong crossover in many disciplines,and it is an urgent problem for academia and engineering to reveal its mechanism and control its stall effectively.Under the support of the Chinese Postdoctoral Science Foundation,the NACA6508 hydrofoil and its constructed cascade are researched as the research object.Based on the numerical simulation and experimental measurement,the steady and unsteady flow characteristics of the hydrofoil and cascade are studied under the different flow conditions.Meanwhile,the effect of corrugated hydrofoil on controlling stall is preliminarily explored.The main results and conclusions are as follows:1.The validity of the k-ω SST turbulence model is verified based on the lift and drag of the hydrofoil and the velocity measurement in the cascade wake region.Further,the flow structure around the hydrofoil and the cascade under the different flow conditions is calculated.In addition,the variation of the drag coefficient,the flow structure and the surface pressure coefficient with the flow conditions are analyzed.A quantitative relationship between critical stall angle and blade density is established,and the induced mechanism of hydrofoil and cascade stall is preliminarily revealed.The study shows that the increase of the cascade density can delay the stall of the cascade,and the transition point of the pressure coefficient at the suction surface can be used as the criterion of the stall.The expansion of the angle area at the suction surface leads to the stall of the hydrofoil and the cascade.Moreover,the coverage and strength on the suction surface of hydrofoil and cascade are quite different in stall condition.2.The large eddy simulation(LES)method is used to calculate the unsteady flow around hydrofoil and cascade under the different flow conditions.Unsteady characteristics of the lift and drag coefficient and the evolution of the large-scale eddy are analyzed.Combining the pressure pulsation in the inlet flow area and wake region and the synchronous measurement of the vibration acceleration,the time-frequency characteristics are analyzed and the vibration acceleration level is evaluated in the specific frequency band.The research shows that the second characteristic frequency of the stall is excited by the evolution of the separation vortex,the wide-band characteristics are obvious in the low frequency band.in addition,the main characteristic frequency decreases with the increase of the attack angle;the pressure fluctuation level in the wake vortex evolution region is approximately exponential growth with the increase of the attack angle,and the maximum point of the vibration water level is slightly larger than the critical angle;stall separation pattern of hydrofoil and cascade are different.3.Based on the numerical simulation,the control effect of the wavy structure at the suction surface under different wave numbers and relative amplitude on the hydrofoil stall is analyzed.And the influence of the corrugated structure at the suction surface on the lift and drag coefficient of the hydrofoil is clearly defined.Combined with velocity field and limit flow spectrum analysis,It is found that using corrugated structure can effectively control stall by restricting the diagonal zone of the suction surface.