| Waterjet propulsor is higher and lighter than the traditional propeller,hence it is widely used in the new and advanced vessels.In fact,waterjet pump as the major compent is installed downstream of the intake duct,where the suction flow is nonuniform and unfavourable for the pump actually performance.Insufficient understanding of head and instability deviation between the uniform and nonuniform suction flows is a major problem encountered in the design and application of waterjet pumps.In essence,this paper aims to understand and clarify the generation mechanism of performance breakdown with nonuniform inflow,which plays an important role in waterjet development.This paper is carried out based on the Natural Science Foundation of Jiangsu Province(BK20151342).In accordance with the principle of finding,investigating and solving problem,particular attention are devoted to the inner flow structure and stall behavior in the waterjet pump under nonuniform inflow,then the internal correlation between the nonuniform inflow and performance breakdown is revealed.Based on the generation mechanism,two methods are suggested to improve and enhance the hydraulic performance of waterjet pump under nonuniform inflow: one is active control of the nonuniform suction flow and the other is to increase the resistance of waterjet pump to the nonuniform inflow.The main conclusions and results are summarized as follows:1.Based on the previous research on waterjet pump and relevant report about compressor,theoretical,experimental and numerical investigation were combined and conducted to illustrate the performance breakdown of an axial waterjet pump under nonuniform inflow and explain its generating mechanism.In this study,numerical simulations were performed to compare pump performance with two different computational domains.One is a single waterjet pump under uniform inflow and the other is a complete waterjet propulsion system with nonuniform inflow.Numerical results of single waterjet pump were compared with the experimental curve and a desirable agreement was obtained;hence,the simulation model is suitable for the following numerical analysis.Moreover,pump performance comparison between the uniform and nonuniform inflow indicates that large nonuniformity of suction flow(about 0.459)causes a substantial drop in head of 8.8%,at the design point.2.In order to explain the generation mechanism of head breakdown in a waterjet pump,the nonuniform inflow structure and its responding flow mechanism are discussed at the design point.The primary feature of nonuniform inflow is a distinct total pressure and swirl distortion upstream the impeller,which evolves into a circumferential vortex(CV)near the top.Additionally,detailed comparisons of pressure field,separation pattern,vortex structure,and blade loading were conducted to clarify the internal correlation between the nonuniform inflow and head breakdown: different blades or cascades operate at different off design points due to the redistributed flow rate by CV.In detail,the hub section of blade 2 operates at a small flow rate condition,a spanwise vortex shedding from the hub triggers a sharp downward peak of blade loading in the hub section.Moreover,the shroud section of blade 2 works on a large flow rate condition,a concentrated vortex deviates from the suction surface and induces an entire reduction in blade loading of the shroud section.These remarkable reductions in the blade loading eventually generate a head deviation.3.In order to elucidate stall behavior of waterjet pump under nonuniform suction flow,numerical simulation in the range of 1 m/s to15 m/s was carried out in terms of performance and inner flow structure.The original waterjet pump is divided into two circumferential and parallel tubes via parallel compressor theory: tube L corresponds to the distorted sector and tube H is the clean sector.Depending on the operating conditions of two tubes(occurring stall or not),stall development of waterjet pump under nonuniform suction flow consists of partial stall,stall inception and full stall.The mechanism dictating flow details in waterjet pump,including the degree of inlet distortion,inlet velocity and total pressure distibution,stall cells and theoretical head breakdown boundary-layer separation,are systematically investigated.Results indicate that spike stall inception occurs in waterjet pump with uniform inflow;while an additional distorted disturbance(CV)interacts with the previous spike disturbance: if both disturbances suppress with each other,stall cells induced by CV are locked in the distorted sector,so this behavior is named as partial stall;if both disturbances are independent,stall cells begin to translate around the annual,so waterjet pump approaches to the stall inception;if both disturbances promote each other,stall sprawls from the distorted sector to the clean sector,so this behavior is named as full stall.4.Nozzle diameter and ship speed are synchronously changed to control the nonuniform suction flow and remain the design flow rate.Numerical simulation of waterjet pump encountering with these different nonuniform inflows were performed at the design point.Moreover,modified pump efficiency curve indicating the head drop is obtained from the comparison between the previous numerical head and the uniform value,in which the nonuniformity is chosen as the identified index: large nonuniformity produces substantial head drop;nonuniformity equaled to 0.25 is the critical point of actual head gain and decline.In the waterjet pump design process,the nonuniformity of suction flow from the intake duct is tested or simulated,then the modified pump efficiency is calculated to modified the design head.5.Active control of the nonuniform suction flow aims to decrease the nonuniformity,resulting in the improvement of hydraulic performance of waterjet pump with nonuniform inflow.Therefore,intake duct is optimized against origins of nonuniform inflow,such as secondary flow induced by S type bend and the blockage and disturbance of upstream shaft.The reducing curved angle is confirmed to suppress the secondary flow and then improve the suction flow,further pump head has an increase of 3% at the design point.A permanent maglev waterjet pump without shaft is designed and compared with the original pump.Results shows that shaftless pump encounter with better suction flow than the original,nearly 3% increase of thrust is observed at the design point.Furthermore,adjustable nozzle of jet pump is applied in the waterjet pump;according to the ship speed,the nozzle area is adjusted to control the nonuniform suction flow.6.The reduction of impeller diameter in waterjet axial flow pump is helpful to diminish the secondary flow formed by the bend and delay stall occurrence,consequently the ability of waterjet pump resistance to the nonuniform inflow is enhanced.Energy coefficient indicating energy conversion performance is equal to head coefficient multiplied by flow rate coefficient;energy coefficient comparison between theoretical maximum value and statistics of the existing excellent pumps corroborates the feasibility of reducing impeller diameter,hence the compact configuration is able to strengthen energy conversion performance.Based on the maximum energy coefficient,the design method of waterjet pump is improved,its designed pump accompanied with other products reveal advantages of compact waterjet axial flow pump: miniaturization and light weight,high pump efficiency and power density,strength energy conversion performance and capacity of resistance to the nonuniform inflow.Meanwhile,compact waterjet pump extends to the mixed flow pump(special speed even lower than 500),the new pump combines axial flow configuration with mixed flow pump performance,then broadens the high and low ship speed limits.Further,aft-loaded distribution of blade loading is used in the maximum energy coefficient method to reduce the airfoil cavitation coefficient and improve cavitation performance of compact waterjet pump.The final low special speed,compact type,waterjet axial flow pump designed by the maximum energy coefficient method and aft-loaded distribution,possesses advanced overall performance,such as high energy conversion and cavitation performance.The engineering application can offer a reference of extending the design method into conventional pumps. |
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