Research On Flow Instability Mechanism Of A Mixed-flow Water Jet Pump

The mixed-flow water jet pump has the advantages of small vibration and noise,small attachment resistance and strong anti-cavitation performance,which has become the priority development direction of science and technology and military industry in various countries.Due to the geometric restriction of the intake duct and the disturbance of the driving shaft,the flow separation phenomenon will occur in the intake duct of the water jet propulsion during the actual operation,and the non-uniform inflow with asymmetric circumferential distribution will be formed.Non-uniform inflow not only leads to flow instability,induced performance degradation and insufficient thrust,but also leads to uneven load distribution of each blade,increasing vibration and noise.Meanwhile,the water jet propulsion is often in part-loading condition,which is easy to induce unstable flow such as rotating stall,which further worsens the internal flow field.Therefore,it is of great significance to investigate the flow instability mechanism and reveal the internal relationship between the non-uniform inflow and the flow response.By changing the inflow conditions of the water jet pump,the internal flow characteristics under different inflow patterns are studied.The main research work and achievements are as follows:(1)The numerical calculation model of the water jet propulsion is established.The Wieghardt equation is introduced into the CEL language to realize the accurate simulation of the internal flow.The performance characteristics of the water jet pump,such as head,efficiency,thrust,are obtained at different speeds.The difference of the flow separation in the intake duct at and the inducement of vortex initiation are revealed,and the non-uniformity at different speeds is evaluated quantitatively by the physical parameters.The results show that at high speed,the curvature of the slope and the disturbance effect of the driving shaft make much flow direction vortices converge at the upper slope of the intake duct,the flow pattern distribution of the intake duct is highly uneven,and the head loss is large.At low speed,the bottom boundary layer and the flow separation effect of the lip make a large number of vortices accumulate at the bottom of the intake duct.The degree of non-uniformity increases with the rise of speed,the loss of inflow is serious at high speed,and the thrust is easy to be insufficient.(2)The matching relationship between non-uniform inflow and unstable flow is established,and the difference of flow distribution and blade loading in each passage is analyzed.Based on the analysis of pressure fluctuation,the transient propagation characteristics of unstable flow in the pump are revealed.The results show that,at high speed,the uneven degree of inflow leads to the obvious non-uniformity in impeller.The high-energy fluid of the inflow surface passes through the overcurrent passage,which promotes the formation of large-area leakage vortex in the rim region.The low-energy fluid flows through the blocked passage and forms large-area vortex near the pressure surface near the hub side.With the decrease of speed,the nonuniformity of flow distribution in the pump decreases with the decrease of the nonuniformity of the inflow surface.Based on the analysis of the characteristics of unsteady pressure pulsation at various speeds,the frequency of fluid induced vibration in the pump induced by nonuniform inflow is always axial frequency.With the decrease of speed,the amplitude of pressure pulsation and the amplitude of main frequency in impeller are greatly weakened.The non-uniform inflow caused by the geometric restriction of the intake duct is the basic reason for the non-uniform distribution of the flow pattern and the flow instability in the pump.(3)The performance,flow pattern and stall characteristics of the water jet pump are studied based on the condition that the water jet propulsion usually operates under the part-loading condition.The results show that,due to the influence of geometry and speed,the drop amplitude of head in hump region induced by rotating stall is further extended,and the stall effect is intensified.According to various evaluation indexes,the flow characteristics of each working condition in the hump region are distinguished,and the corresponding stall state under each working condition is defined.Through the capture of stall vortex,it is found that it is closely attached to the suction surface of the trailing edge,across the whole impeller passage,and has a trend of extending to the leading edge of the downstream blade.Under the action of the entrainment and blockage of the stall vortex,the flow path of the incoming flow will show multiple deflections.Part of the incoming flow will flow to the downstream passage under the combined action of the blockage effect of the stall vortex and the impact of the leakage flow,and the other part will return to the inlet pipe.With the occurrence of rotating stall,the reverse flow and the flow loss at the inlet pipe increases sharply,which further worsens the inflow of impeller.The inverse flow induced by rotating stall is the fundamental cause of energy dissipation and performance degradation under stall conditions.(4)Based on the actual inflow process of the water jet propulsion device,the influence of the intake duct mode on the internal flow characteristics of the water jet pump is investigated.The results show that,under the large flow rate,the unsteady flow in the pump increases significantly and the head loss increases significantly under the influence of non-uniform inflow.Under the small flow rate,the geometric structure disturbance effect of the intake duct disappears basically,and the high-speed inverse flow occupies the main disturbance inducement.Due to the reduction of counter flow area and backflow flow,the flow pattern and head in the pump are better than that in uniform inflow.Under stall conditions,the degree and scale of stall vortex under intake duct mode are weaker than that of uniform flow.Based on wavelet analysis,the transient propagation characteristics of stall cell under critical stall point are analyzed.After a period of low frequency and high amplitude single stall cell disturbance,the water jet pump will gradually turn to the circumferential disturbance of multiple stall cells with high frequency,and the unstable flow pattern in the pump will be further intensified.Under intake duct mode rotating stall always keeps a high amplitude and low frequency disturbance,which shows the transient characteristic of the single stall cell propagating in impeller.