The Study Of Transient Characteristics On Screw Mixed-flow Water-jet Pump During Startup Period

Water-jet pumps are widely used in high performance warships and new submarines.The instantaneous impact and power overload of the water-jet pump during startup period seriously endanger the stability and reliability of the system.Therefore,It is great significant to study the water-jet pump during startup.In this thesis,the study object is the screw mixed-flow water-jet pump.The quasi-steady state method is used to calculate the water-jet pump model firstly,and the reliability of numerical method is verified by comparing the numerical results with the experimental results.Secondly,considering the influence of impeller rotational acceleration and fluid acceleration on transient flow field,the impeller speed control realized by user defined function,and the sliding mesh technique is used to numerically calculate the start-up process of water-jet pump.The transient characteristics of screw mixed-flow water-jet pump are analyzed from the aspects of external characteristic,the evolution of internal flow field,blade load and radial force.The influence of startup time and different startup mode on the transient characteristics of water-jet pump during startup period is further studied.The specific study conclusions are as follows:1.the external characteristic curves are obtained by numerical simulation and experiment based on water-jet pump of multi-flow field under different rotating speed during mooring state.And it shows that the flow rate of different rotating speeds varies linearly with the rotating speeds,the increasing trend of the head,thrust and torque is parabolic.Comparing the numerical results with the experimental results we can reveal that the numerical calculation results are close to the experimental values.2.The quasi-steady state numerical method is used to simulate the four different rotating speeds of 450,750,1200 and 1500r/min,and the internal flow field distribution of different operating points under the design rotating speed is predicted.The results of calculations reveal that: the distribution flow field of water-jet pump at the design rotating speed is greatly affected by different flow rate conditions.The pressure fluctuation of water-jet pump was strongly affected by radial position and flow rate change.The amplitude value of pressure fluctuation locating at the interface of impeller and guide vane is the largest.The dominant frequency and period are only related to the number of impeller blades,and not affected by flow and rotating speed changes.The external characteristic parameters of water-jet pump at different rotating speed are consistent with the change of the rotating speed,and the internal flow field distribution is similar.3.The transient numerical method is used to simulate the startup process of water-jet pump.The results of calculations reveal that: the external characteristic parameters of water-jet pump show obvious transient effect during startup period,and the change of the transient flow field with time lags behind rotating speed during startup period.There is a deviation between the calculation results of the transient method and those of the quasi-steady state method.During the acceleration of the impeller,the blade load value increases rapidly with the increase of the rotating speed.When the rotating speed is stable,the distribution of blade load remained unchanged.The radial force changes slowly in the early stage of the impeller acceleration and the period of radial force is large.When the impeller completed the acceleration,the radial force got extreme value,and then periodically changed.During different startup times,the flow field evolution of water-jet pump has similarity.When the rotating speed of impeller changes in a straight line,the external characteristic parameters has extreme values,and the extreme value is related to the startup time.When the rotating speed of the impeller changes exponentially,there is no obvious extreme value of the external characteristic parameters after the rotating speed is stabilized,and the flow in the pump almost reached a steady state when the impeller finished accelerating.