| Axial flow pump is widely used in South to North Water Diversion Project,ship water jet propulsion,marine energy utilization and other national strategic engineering fields.Owing to the inevitable existence of tip clearance,tip leakage flow is a typical flow phenomenon in axial-flow pump,which is often accompanied by complex cavitation structures such as tip clearance cavitation,TLV cavitation,shear layer cavitation,sheet cavitation on the suction surface,as well as the suction-side-perpendicular cavitating vortices(SSPCV).Cavitation can threaten the stable operation of an axial flow pump due to its adverse impacts such as blockage in the passage and efficiency loss.In this dissertation,the numerical simulation was combined with the high-speed photography and transient pressure measurements to study the characteristics of tip leakage vortex and its induced unsteady cavitating flow at different flow rates and cavitation conditions based on the numerical investigation on the unsteady vortex cavitating flow of the Delft Twist 11 hydrofoil.The correlation between different cavitation structures and pressure fluctuation was analyzed,as well as the hydraulic oscillation induced by complex cavitation cloud in the tip region.The inception,development,collapse of the SSPCV and induced flow instability were also clarified.The main contents of this dissertation are as follows:1.The PANS method was combined with the homogeneous cavitation model to simulate the unsteady cavitating flow around the Delft Twist 11 hydrofoil.The accuracy of the numerical method was verified by the unsteady shedding of the cavitation on the surface captured by visualization experiments.The vorticity dynamics of the shedding cavitation were analyzed by the vorticity transport equation,which verified the applicability of the vortex-dynamics analysis method in the axial flow pump.And the interference mechanism of the re-entrant jet and side jet on the primary and secondary shedding of the cavitation on the suction surface was also revealed.2.The evolution of the tip leakage vortex and its induced different cavitation structures in an axial flow pump were studied by the high-speed photography experiment.It was found that cavitation inception was easier at partial loading conditions,and the starting point of the cavitation moved backward along the blade chord with the increase of the flow rate.A triangular cavitation cloud was formed at the tip region as the cavitation number is low.SSPCV shed from the trailing edge of the cavitation cloud and migrated in the passage with an angle approximately perpendicular to the blade.The mapping relationship between tip cavitation structures and pressure fluctuation was studied by the combination of high-speed photography and transient pressure measurement experiment.The pressure increased from the suction surface to the adjacent pressure surface.The TLV cavitation appeared to be a narrow and long range of low pressure when the pump operated at overload conditions.The TLV cavitation and blade tip were connected by the shear layer cavitation to form a triangular cloud cavitation as the flow rate and cavitation number decreased,showing as a larger low-pressure region The SSPCVs were formed at the cost of reducing the triangular cavitation area,which led to a decrease in the low-pressure range.The oscillating tail of TLV cavitation was mixed with the SSPCVs as SSPCVs migrated to the adjacent blade PS,which caused pulsations in the recovery process of pressure.3.The performance tests show that the head and efficiency decreased in varying degrees,and the cavitation performance also decreased significantly with the increase of tip clearance.Due to the increase of tip leakage,the mixing effect between the clearance flow and the main flow was more intense,which aggravated the pressure instability in the tip region.Specific Chromatograms of tip cavitation with different blading densities were set up by the dimensionless flow rate and cavitation number.With the increase of blading density,the range of slight cavitation in the tip region increased,and the cavitation deterioration was accelerated at partial loading conditions.And SSPCV is easier to be cut off by adjacent blades during migration,which leads to cavitation instability in the adjacent flow passage.4.The temporal and spatial evolution of tip leakage vortex and the generation mechanism of unstable cavitation cloud,as well as the characteristics of the induced pressure fluctuation were studied by the three-dimensional unsteady cavitating flow in an axial flow pump.It was an accelerating process for tip leakage flow to enter the gap under the drive of pressure difference.With the continuous loss of energy,the leakage flow velocity decreased,and the tip leakage vortex was formed by the entrainment between the leakage flow and the main flow.The tip leakage vortex and its induced cavitation structure led to severe fluctuation of tip pressure field.The periodic shedding of SSPCV caused obvious low-frequency signals in the tip pressure pulsation.5.The generating position of the SSPCV and the change of the vortex vector were analyzed by the numerical results.The spatiotemporal evolution of the SSPCV could be divided into three stages: Generating stage,Shedding stage,Dissipating stage.Meanwhile,it was also pointed out that the inception mechanism of the SSPCV was induced by multiple cavitation structures near the blade tip.Small scale water vapor structure was generated on the cavitation surface under the combined action of the main stream,re-entrant jet,and radial jet,which led to the formation of fracture points and the periodical shedding of SSPCV.The correlation between vorticity as well as its transport terms in different directions and the evolution of SSPCV were analyzed using the vorticity transport equation in the rotating coordinate system.The vorticity at SSPCV region was mainly contributed by the tangential and axial vorticity components,in which the circumferential vorticity was the main component.The shedding SSPCV migrated and twisted gradually in the passage,forming a shape perpendicular to the blade,which led to the decrease of the tangential vorticity and the increase of the axial vorticity.The interference between SSPCV and the adjacent blade was investigated by the distributions of the vorticity components,velocity field,turbulent kinetic energy,shear strain rate,and blade loading.It was clarified that the adjacent flow passage was blocked by the SSPCV,resulting in the change of the inlet flow angle.In addition,the sudden drop of the blade loading as well as the flow instability in the adjacent flow passage caused by the SSPCV were revealed.The results obtained in this dissertation clarified the problem of flow instability induced by the shedding of complicated cavitation in the tip region of an axial flow pump,which provide a theoretical basis for the investigation on the control methods of the unstable tip cavitation,and provide technical guarantee for the design and stable operation of axial-flow pump. |
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