Large Eddy Simulation On The Cavitation Characteristics Of Jet Pumps

The jet pump is widely used due to its simple structure,reliable operation,convenient operation and maintenance,etc.However,the cavitation often occurs in jet pumps,and involves complex phenomena such as multiphase turbulence,shear layer and backflow,resulting in complex cavitating flow structures in jet pump.The internal flow mechanism of cavitation in jet pump is insufficient.Based on this,This thesis uses the large eddy simulation method to carry out detailed simulation research on the cavitatng flow structure and flow mechanism in different stages of the jet pump with emphasis on the unstable limit condition and reverse flow condition.The main results and conclusions are as follows:(1)This thesis introduces the LES equations and cavitation model,and generates structured meshes for typical jet pump model.Three sets of meshes have been used to do mesh-independent research.The basic hydrodynamic performances of jet pump and the average wall pressure distribution are compared with the test.The results show that the large eddy simulation can accurately simulate the performance of the jet pump.(2)This thesis uses large eddy simulation to simulate and analyze four flow stages at the downstream condition in jet pump: the cavitation inception stage,cavitation developing stage,unstable limit cavitation stage and stable limit cavitation stage.The results show that the cavitation evolution obtained by the large eddy simulation agrees well with the test,especially the periodic shrinkage and regrowth process of the cavitation cloud during the unstable limit cavitation condition.The local low pressure in the shear layer leads to the initialization of cavitation.In the first half of the cavitation development stage,an annular cavitation cloud structure appears,but it is extremely unstable and has no obvious periodic feature.The length of the cloud appears to shrink and grow periodically.The rib-shaped vortex cavitation sometimes develops between two adjacent annular vortex cavitations,and the cavitation cloud developed to the diffuser tube is truncated and falls off due to the re-entrant jet,and finally cavitation disappears with the high pressure.As the pressure at the outlet decreases,the cavitation cloud pulsation frequency first increases and then decreases,but the overall pulsation frequency belongs to the low frequency.(3)Simulation and analysis have been carried out for the initial and transport phases of the jet pump backflow cavitation.The results show that the initial stage of backflow cavitation is mostly small vacuoles,and is mainly distributed around the inlet of the throat.At the transport stage of backflow cavitation,the cavitation cloud in the jet pump will completely break away from the inlet of the throat and develop upstream.Then the cavitation cloud will be broken,and the partially broken cavitation cloud will be transported to the nozzle exits and then collapses.The potential flow core region in the initial stage of backflow cavitation will extend into the throat,while the potential flow core region in the transport phase has been offset by the backflow.The affected area of the recirculation is obviously extended upstream.As the flow ratio decreases,the high pressure zone in the throat gradually moves forward,the pressure near the inlet of the throat gradually increases,and the pressure gradient also increases,and the cavitation primary position approaches the nozzle outlet.The occurrence of cavitation in the initial stage of backflow cavitation is resulted by the difference between the two fluids and the long-term low pressure.As the flow ratio decreases,backflow cavitation enters the transport phase,and the main cause of cavitation is the high velocity gradient of the reverse shear layer.(4)Based on the vorticity transport equation,the interaction between cavitation and vortex is analyzed.The results show that the evolution of the vortex in the jet pump mainly includes the pairing and merging of vortex rings,the generation of flow vortices and the appearance of complex vortex structures,and eventually forming a fully developed turbulent flow field with a large number of small eddy structures.In the reverse flow condition,the vortex structure in the jet pump is concentrated in the nozzle outlet to the throat inlet area.As the flow ratio is reduced,the backflow is intensified,and the vortex structure in the throat is gradually reduced to almost disappear.However,the vortex structure in the suction chamber becomes more and more complicated,and its leading edge is also developing upstream.Analysis using the vorticity transport equation shows that cavitation has a great influence on the vorticity distribution and transport in the jet pump.During the development of cavitation in the entire jet pump,the vortex stretching term dominates the vorticity transport,and the vortex dilatation term and the baroclinic torque term also play an important role in the evolution of the vortex in the cavitation flow.