Effect Of Tip Clearance And Blade Number Ratio On The Performance Of Contra-rotating Axial Flow Propulsion Pump

At present,the water jet propulsion pump is used in many new ships and submarines.Contra rotating axial flow water-jet propulsion pump is widely used because of its large flow,high head,wide stable operation area and good anti cavitation performance.According to the structural characteristics of the contra rotating axial flow pump,the rear rotor is located at the outlet of the front rotor,so the reasonable matching design of the two-stage rotor is of guiding significance to realize the miniaturization and high efficiency of the water-jet propulsion pump.In this paper,a set of contra rotating axial flow propulsion pump is designed according to the thrust requirements of a certain submarine.Taking it as the research object,based on the numerical simulation technology,the influence of the tip clearance and blade number ratio of two-stage rotor on the pump's external characteristics,internal flow field and the pump's injection performance in the whole basin is studied systematically.In the first place,considering the minimum clearance allowed by the cavitation performance of axial flow pump,the coefficient of tip clearance s is defined.Nine groups of design parameters are selected,?= 1,2.5,4,5,6.25,7.5,8.5,9.5,10.5,respectively.Comparing the hydraulic performance and propulsion performance of pump under different tip clearances.In the next place,in order to describe the influence of different tip clearance ratio of the front rotor on the performance of the pump,the tip clearance ratio coefficient k is defined.Four groups of data are selected,k = 0.5,0.8,1.25 and 1.5 respectively.The hydraulic performance and propulsion performance of the pump are analyzed when only the front rotor is increased without changing the rear rotor.Lastly,the ratio coefficient of the number of blades X is defined to study the influence of the number of blades of the front and rear rotor on the pump performance.In the process of the study,the influence rules of the number of blades on the pump hydraulic performance under the design parameters of X<1 and X>1 are summarized.The results are summarized as follows:1.With the increase of ?,the curve of scraping rate develops from the quadratic function of opening downward to the "M" type.Through the distribution of internal flow field,it is found that with the increase of tip clearance,the interference effect between two-stage impellers is enhanced,and the "interference vortex" develops to the front rotor area.The maximum thrust coefficient is obtained at ?=4.2.The curve of the scraping rate under the condition of small flow showsirregular distribution,and the scraping rate of the rear rotor under the condition of large flow is less affected by the tip clearance of the front rotor.The thrust coefficient reaches the maximum value at k = 0.8,and the thrust efficiency continues to decrease by about 3%.3.In order to avoid the large impact of the fluid flowing out of the front rotor when entering the rear rotor,resulting in energy loss,the difference between the number of blades of the two stage impeller should not be too large.When X< 1,the change trend of the pressure load distribution curve of the two stage impeller is relatively gentle,with strong working stability and excellent pump performance.The thrust efficiency will be reduced by increasing the number of rear rotor blades,but it will be improved by increasing the number of front rotor blades.In this paper,the concept of scraping rate is introduced to correlate the blade pressure difference with the scraping effect of rotor case,and the problem that the influence of two factors on clearance leakage is difficult to describe is solved,Considering the influence of blade chord length on its performance,the blade structure of impeller is redesigned according to the number of blades.In the later stage of this study,the impeller needs to be optimized according to the obtained pressure load distribution curve.