Effect Of Impeller Tip Clearance On Performance Of Water-jet Propulsion With A Screw Mixed-flow Pump

The smooth operation of Water-jet propulsion with the screw mixed-flow pump is the key to ensure the safe running of the ship with water-jet propulsion.The distance between the tip of the rotating blade and the stationary part is called the tip clearance.The leakage vortex of the tip clearance will not only change the external characteristics of the pump,but also cause leakage vortex cavitation in the impeller,reducing the anticavitation performance of the pump,then causes blade cavitation and vibration.In this paper,the screw mixed-flow pump is taken as the research object,and the tip clearance coefficient is defined as the ratio of the tip size to the blade thickness.Based on the CFX solver and the SST k-ω turbulence model,the numerical calculation is carried out to study the effect of the tip clearance coefficient on the performance of the screw mixed-flow pump.The main research contents are as follows:1.Based on the definition of the tip clearance coefficient,five blade thickness schemes were formulated,and it was found that reducing the blade thickness could improve the anti-cavitation performance of the screw mixed-flow pump,and increasing the blade thickness would reduce the anti-cavitation performance of the screw mixedflow pump.In order to further improve the anti-cavitation performance of the screw mixed flow pump,this paper selects a screw mixed flow pump with a better anticavitation performance of 2mm blade thickness for follow-up research.2.According to the definition of the tip clearance coefficient,six coefficient schemes are formulated to analyze the changes of the head,efficiency,axial force,thrust and leakage of the screw mixed-flow pump under different tip clearance coefficients.When the tip clearance coefficient τ=0.08,the efficiency of the screw mixed-flow pump is the highest,and the efficiency is increased by 1.25%.With the increase of the tip clearance coefficient,more axial force is converted into thrust,and the relationship between the leakage ratio of the tip clearance and the tip clearance coefficient presents an approximate exponential function change trend.When the tip clearance coefficients τ=0.08 and 0.125,the leakage flow of the tip clearance has less blocking effect on the impeller inlet,and the influence range of the tip leakage vortex is small.3.Through the calculation of the cavitation of the screw mixed-flow pump under different tip clearance coefficients: With the increase of the tip clearance coefficient,the anti-cavitation performance of the screw mixed-flow pump gradually decreases when the tip clearance coefficient is in the range of 0.08≤τ≤0.25 and 0.375≤τ≤0.625;In the range of tip clearance coefficient 0.25≤τ≤0.375,the anti-cavitation performance of the screw mixed-flow pump is gradually improved When τ=0.375,the anti-cavitation performance of the screw mixed-flow pump is significantly improved,and it is the optimal tip clearance coefficient for the anti-cavitation performance of the screw mixed-flow pump.4.Through the transient numerical simulation of the spiral mixed-flow pump with different tip clearance coefficients,it is found that in one rotation cycle,the peak radial force of the impeller presents Sine function with the tip clearance coefficient approximately.By comparing the vortex identification methods—Q method and Ωmethod,it is found that the Q method can quickly and accurately identify the vortex structure.When the tip clearance coefficient τ=0.08,the evolution of the vortex structure inside the impeller and guide vane of the screw mixed-flow pump is analyzed,and it is concluded that the vortex tube structure in the impeller breaks at 4/6T,and reforms at 5/6T.The vortex tube is formed at 1/2T.Through the above analysis,it can be concluded that the tip clearance coefficient τ=0.08 can maximize improve the efficiency of the screw mixed flow pump and improve the anti-cavitation performance of the screw mixed flow pump.Therefore,the optimal tip clearance coefficient of the screw mixed flow pump is determined.τ= 0.08.