| Axial flow pumping stations are widely used in plain areas because of the high flow rate and low head,and some pumping stations have the function of reverse power generation in addition to forward pumping.The use of axial flow pumping station reverse power generation has the advantage of not being restricted by the height difference of the terrain and lower cost,which can provide clean hydropower energy for the plain areas.The internal flow pattern is more complicated than the pumping condition,and there exist significant pressure pulsation and backflow vortex,which will cause the instability of the whole system,resulting in the degradation of hydraulic performance and vibration of the unit structure.The complex alternating stress and inertia force on the impeller may cause fatigue damage or even cracking of the impeller blade.In order to provide a basis for safe,reliable operation and structural optimization of the unit during reverse power generation of the axial flow pumping station,this paper analyzes the internal flow characteristics and structural dynamic characteristics of the impeller shaft system during reverse power generation of the vertical axial flow pump unit mainly by numerical simulation method supplemented by model tests,and the main contents and conclusions are as follows:(1)The unsteady flow characteristics of the blade zone,the fluid force of the impeller and guide vane and the pressure pulsation characteristics of the typical section during the reverse power generation of the axial flow pump device were analyzed.The increase of the flow rate causes the contraction of the pressure gradient area in the blade zone,while the pressure gradient between the inlet side and the outlet side of the impeller increases significantly.The low-pressure area range of the suction surface of the impeller blade expands continuously,while the change of the flow rate has little effect on the pressure distribution of the impeller blade pressure surface.Affected by the rotor-stator interaction effect between the impeller and guide vane,the dominant frequency of the impeller and guide vane of the fluid force is the impeller rotational frequency and its harmonic frequency.The effect of rotor-stator interaction gradually increases from the guide vane inlet,guide vane outlet,impeller inlet to impeller outlet.The pulsation amplitude of guide vane inlet and impeller inlet shows a positive correlation with the flow rate,and the change of flow rate has a more significant effect on the pressure pulsation amplitude of guide vane outlet and impeller outlet.(2)The transient vortex rope evolution mechanism,pressure pulsation characteristics and causes of vortex rope formation in the dustpan-shaped conduit during reverse power generation of the axial flow pump unit were described.The three-dimensional geometry of the vortex rope in the dustpan-shaped conduit at each flow rate has obvious differences.At 0.8Qbep and 1.1 Qbep,the vortex rope is a long spiral extending downward,while at 1.0Qbep,the vortex rope is gradually broken along the main flow direction.The cross influence of the velocity circulation at the impeller outlet and the axial flow of the main stream leads to the formation of the spiral vortex rope.The axial velocity low velocity region of the flow in the dustpanshaped conduit fits the position of the vortex core of the long strip vortex rope.The tangential velocity at the vortex core is the smallest,and the maximum tangential velocity appears at the edge of the vortex rope,where the larger velocity gradient and pressure gradient make the turbulence spread more intensely.Further downstream,the rotational intensity of the vortex core of vortex rope is weakened,and the rotor-stator interaction effect in the dustpan-shaped conduit is weakened with the increase of the distance from the impeller.The rotation of the vortex rope leads to a large amount of low-frequency pulsation in the dustpan-shaped conduit,and the dominant frequency of the pressure pulsation spectrum near the impeller is 3 times the rotational frequency,while the dominant frequency of the pressure pulsation near the vortex rope is 0.5 times the rotational frequency.(3)Based on the entropy production theory,the energy dissipation law in the full flow conduit under each flow rate were analyzed when the axial flow pump device reverses for power generation.The entropy production in the impeller domain tends to increase and then decrease with the increase of flow rate,while the entropy production in the dustpan-shaped conduit tends to decrease and then increase,and the flow rate has a more significant effect on the entropy production in the dustpan-shaped conduit.The turbulent dissipative entropy production occupies the main position in the whole flow conduit,and the wall entropy production of the whole flow conduit shows a positive correlation trend with the flow rate.The turbulent dissipative entropy production of the impeller mainly occurs near the leading and trailing edges of the blades,and the turbulent dissipative entropy production of the dustpan-shaped conduit is mainly concentrated in the mainstream region.Flow impact,flow separation,vortex and backflow are the main reasons for inducing high entropy production rate and energy loss.(4)The dynamic characteristics of the impeller shaft system structure were analyzed by the one-way transient fluid-structure coupling method,and the equivalent force and deformation law of the impeller blade at various flow rate were described.The periodic fluctuation range of the equivalent stress value of the impeller blade is obviously larger under the partial optimal efficiency condition.Under the influence of the rotor-stator interaction between the impeller and guide vane,the dominant frequency of the equivalent stress of the impeller blade is the impeller rotational frequency and its frequency doubling.The maximum deformation of the blade exists at the tip of the blade trailing edge,and the dominant frequency of the blade deformation at various flow rate is 3 times the impeller rotational frequency.The added mass and flow damping of the fluid will reduce the natural frequency of the impeller shafting,and the impeller shafting is not easy to resonate in the wet mode. |
