Investigation On The Mechanism Between Vortex And Cavitation In Propelling Pump Tip Clearance With Front Guide Vanes

With the development of China's maritime power strategy from shallow sea to deep sea and the urgent need of anti-submarine technology,the propelling pump with its excellent hydrodynamic and stealth performance has been widely used in surface and underwater combat platforms.However,the propelling pump belongs to axial(or mixed partial axial)turbomachinery,and there is a gap between the blade tip and the inner wall surface of the catheter.Due to the existence of tip clearance,the fluid in the tip clearance will interact with the main flow in the impeller passage in the driven of pressure difference to coil and form a tip clearance vortex.Cavitation occurs when the pressure in tip clearance vortex decreases further.Cavitation will not only affect the efficiency of propelling pump and produce vibration,but also induce noise,which will greatly degrade the safety and stealth performance of the combat platform.In the present paper,a certain type of propelling pump model with 1mm tip clearance size is taken as the main research object.The characteristics of the external flow field of the propelling pump under different inlet flow conditions are investigated by means of numerical simulation and model test to have a deeper understanding of the characteristics.On this basis,for a typical operating condition with certain advance coefficient,the flow structures in the tip clearance vortexes cavitating flow field of the propulsion pump,the interaction and evolution of tip clearance vortexes and cavitation at different rotational speed cavitation numbers are investigated.The development characteristics of tip clearance vortexes field and its internal mechanism are illustrated.The mechanism of tip clearance vortexes loss is revealed.The main research contents are as follows:(1)Three sets of grids encrypted by Richardson extrapolation method are selected and their sensitivity is analyzed.Combined with high precision test,the accuracy of shear stress transport-curvature correction(SST-CC)model and large eddy simulation(LES)wall-adapting local eddy-viscosity(WALE)model is verified.On the basis of verifying the accuracy of the models,three operating conditions with different rotational speed cavitation numbers under the condition that the advance coefficient J=0.6 are selected for investigation.Analyses are focused on the performance,characteristics of flow field,cavitation volume in impeller domain and the pressure pulsation at the tip clearance.A typical rotational speed cavitation number condition is selected for analyzing temporal and spatial evolution of tip clearance cavitation.(2)The identification effect of tip clearance vortexes field of propelling pump are investigated based on different vortex identification methods.The vortex identification methods include Q criterion,?ci criterion,?2 criterion,? and Liutex vortex identification method.After a good vortex identification method is obtained,the evolution process of tip clearance vortexes cavitating field of propelling pump and the characteristics of tip vortex core under different rotational speed cavitation numbers are analyzed.The tip vortex core position,radius and intensity at different rotational speed cavitation numbers are compared to understand the influence of cavitation.(3)Based on the vorticity transport equation in cylindrical coordinate system,the interaction mechanism between vortex and cavitation in the tip clearance vortexes field of propelling pump is studied to grasp the law of vorticity transport evolution in tip clearance flow field caused by the development of tip clearance vortexes cavitation.The turbulence intensity distribution in the tip vortex field is studied,and the analysis is focused on the influence of turbulence normal stress terms(ur'ur'),(u?'u?'),(uz'uz')and turbulence shear stress term<ur'uz')on the each term in the turbulent kinetic energy transport equation.The effect of the development of tip clearance vortexes cavitation on the production of turbulent kinetic energy,which is an important term in the turbulent kinetic energy equation,in tip clearance field is studied in detail.The transport characteristic of turbulent kinetic energy in the development process of tip clearance vortexes is revealed in depth.(4)The inception position of the tip clearance vortexes of propelling pump and the fluctuation mechanism of tip clearance vortexes are studied at different rotational speed cavitation numbers to grasp the mechanism of the fluctuation of tip clearance vortexes.The instability and the breaking of tip clearance vortexes are investigated based on the Hall vortex core equation.To estimate the tip clearance loss caused by the instability and the breaking of tip clearance vortexes,quantitative analysis is launched based on Denton's tip clearance loss model,and the main parameters affecting tip clearance loss are analyzed as well,so as to provide theoretical basis for the control of tip clearance loss in the propulsion pump in the future.The mismatching of tip clearance flow velocity and main flow velocity will result in mixing loss.The mixing loss outside the tip clearance is investigated based on the theory of entropy generation.The influence of turbulent dissipation and viscous dissipation on the mixing loss is analyzed,and the influence of the development of tip clearance vortexes cavitation on mixing loss is analyzed quantitatively.