Study On The Inner Flow And Cavitation Characteristics Of Water-jet Propulsion

Water-jet propellers are gradually being used in high-performance ships instead of traditional propellers due to their advantages of simple transmission mechanism,high working speed and strong anti-cavitation capability.However,in the process of ship sailing,accelerating and turning,the water-jet propeller is easy to operate under cavitation condition,cavitation bubbles generated on the surface of the blades seriously block the flow channel,resulting in a decline in its operating efficiency.At present,high power density has become a development direction of water-jet propulsion.scholars at home and abroad have made research on this goal,but the impeller of water-jet propulsion is very easy to cavitate when it runs at high speed.The occurrence of cavitation will damage the flow passage components,increase the vibration and noise,and reduce the performance of the thruster.The article is based on the National Natural Science Foundation(No.51879120),this paper uses numerical simulation method to analyze the internal cavitation caused by the high speed differential pressure of the impeller of water-jet propeller and the influence of different nozzle shapes on the performance of water-jet propeller.The main achievements of this paper are as follows:(1)Taking the axial-flow water-jet propeller as the research object,3D modeling,structural grid division and grid independence verification are carried out for it.Using CFD software to calculate the steady numerical value of the water-jet propeller at different speeds,and the simulated thrust and change are compared with the test,and it is found that the two are basically consistent,which shows that the results of numerical simulation have better accuracy and reliability.(2)Through the numerical calculation of the water-jet propeller at different speeds,it is found that cavitation occurs when the impeller of the water-jet propeller runs at high speed.The results show that the cavitation occurs near the top of the suction surface near the inlet of the blade,and with the increase of the rotating speed,the cavitation area extends to the outlet of the blade and between the adjacent blades;because of the existence of the inlet passage,the flow forward of the impeller inlet isnot uniform,and the distribution of the cavitation on the blade is not completely consistent.At the tip clearance,the most serious point of the tip leakage is about 15%of the blade rim from the inlet edge.At the same time,due to the influence of impeller cavitation,the leakage of the rim flow surface in the cavitation wake area of the suction surface of the blade increases sharply.(3)In order to study the transient cavitation flow field of the water-jet propeller,the unsteady calculation of the water-jet propeller under different cavitation conditions is carried out.It is found that most of the cavitation bubbles in the flow field of the water-jet propeller is stably attached to the leading edge area of the impeller inlet at the design speed,companied by the periodic growth and collapse of small-scale cavitation bubbles in the tail,and the total volume fraction of the cavitation bubbles in the impeller passage changes periodically.The pressure pulsation at the inlet of the impeller of the water-jet propeller is mainly affected by the turbulence caused by the channel bend and the shaft;the pressure pulsation at the edge of the impeller and between the impeller and the guide vane is jointly affected by the turbulence caused by the impeller,the guide vane,the channel bend and the shaft,of which the turbulence caused by the channel bend and the shaft is the most dominant;the pressure pulsation at the nozzle is only affected by the impeller.The frequency of radial force on impeller is mainly affected by the turbulence caused by the bend and shaft of runner,and the main frequency amplitude increases first and then decreases under the influence of rotating speed and cavitation conditions.(4)The steady numerical simulation is carried out for three different types of nozzles,and the internal velocity,pressure,turbulent kinetic energy and vortex core distribution are compared and analyzed.The results show that the linear type and double twist type nozzles can be selected in the design speed,but the calculation parameters of the linear type nozzle are relatively good;and the linear nozzle is more suitable for the water-jet propeller at the high speed,and its water-jet propulsion efficiency is the highest.