Research On Distortion Characteristics And Control Methods Of Inlet Channel And Outlet Flow Of Waterjet Propeller

As a new type vessel propulsion,water-jet propulsors have been widely used in military and civilian high performance ships because of their superior operation performance.However,due to the asymmetry of its own geometry,the flow distortion out of the intake duct leads to redistribution of the inlet flow field of the water-jet pump,and such influence on the pump can not be ignored.With the support of Jiangsu Natural Science Foundation?BK20190847?and China Postdoctoral Science Foundation?2019 M661744?,this paper considers the existing flush type intake duct as the research object,sets its outlet non-uniformity and the minimal pressure at lip as the optimization objectives,combines Radial Basis Function Neural Network and NSGA-II algorithm,and seeks to obtain geometric characteristic parameters which improve the optimization objectives simultaneously.The main research work and contents of this paper are summarized as follows:?1?Since the lack of research on the outflow distortion of water-jet intake duct,this field is developed and improved based on the S curved inlet in jet turbine.By exploring the method to determine the capture area of intake duct,the virtual stream tube is solved,and then it is combined with the intake duct and regarded as S-bend inlet.With reference to the outflow distortion characteristics,types and evaluation indicators of the S curved inlet in jet turbine,this paper makes a thorough study on the S-bend inlet to find the intercommunity of different fields.?2?The distortion control problem of the intake duct is transformed into an optimization problem including minimizing the outlet non-uniformity?and maximizing the minimal pressure at lip p_minin simultaneously.The main geometric characteristic parameters are selected as the decision variables:ramp angle?,the lip height h,the radius of the upper lip R₃,the radius of the lower lip R₄.The sample space is constructed by Latin Hypercube Sampling after determining the range of the variables?N=115?.The Radial Basis Function Neural Network is utilized as the fitness evaluation model to train and test data.Finally,a multi-objective optimization model based on the NSGA-II algorithm is established and the optimal individual is obtained from the Pareto frontier.In the optimal model,p_min increases 72.4%to-24488.93Pa and?has a drop of 74.17%to 0.1571,the optimization effect is obvious.?3?Numerical simulations of the intake duct before and after optimization at different ship speeds are carried out.Then,analyzing the causes of decrease of outlet non-uniformity from outlet axial velocity distribution and vortex characteristics:the secondary flow in the intake duct is suppressed,the disturbance of the shaft is weakened,the high intensity concentrated vortex is disassembled and the outlet non-uniformity decreases.Analyzing the causes of the improvement of the efficiency and the minimum pressure at lip from turbulence characteristics and energy transportation:the lip radius increases,centrifugal force is weakened,pressure recovery is enhanced and the minimum pressure at lip increases.Meanwhile,due to the adjustment of geometric parameters could suppress the flow separation on the upper wall of the intake duct,turbulence loss reduces,energy transport ability and conversion efficiency of kinetic-pressure energy enhances,and the efficiency is increased by 5%.?4?The original and optimized intake duct are installed at the upstream of the water-jet pump separately,and the matching degree between the intake duct and the pump after the optimization and its gain effect on the external characteristics of the pump are explored:the operating flow rate increases by 96 m³/h,the actual head increases by 3.6%,and the operating efficiency increases by 4.8%.Quantify the total pressure and swirl distortion of the intake duct before and after optimization to assess the distortion.The circumferential/radial total pressure distortion of the optimized model reduce obviously,the quality of the outflow is improved,and the non-uniformity on the pump inlet is reduced by 0.168.The inflow field of impeller is divided into flow area?hub and middle flow surface?and blockage area?shroud?as the span coefficient is 0.75.The mapping relationship between increasing head and internal flow structure is constructed by combining the static pressure distribution of pump blade and blade load:in the flow area,the span separation vortex and wake are suppressed,the static pressure difference at the blade inlet increases,and the blade load coefficient curve of the optimized model changes to the front load type,the work capacity is enhanced,the blade load/circulation distribution increases,and the pump head raises.In the blockage area,the area of maximum boundary vorticity flux region decreases in the optimized model,the axial torque increases,the blade effective output work increases and the pump head raises.Meanwhile,the backside streamline curvature and centrifugal force increase,the backside static pressure drops,static pressure difference rises,blade load coefficient curve shifts upward,the work capacity is enhanced,the blade load/circulation distribution increases,and the pump head raises.