Research On The Influence Of Inlet Vortex On The Performance Of Axial-flow Pump And Pump Device

Axial-flow pump is widely used in low head pumping stations for cross-basin water diversion,irrigation,drainage and municipal engineering in plain and low-lying areas because of its advantages such as large flow,low head and the simple structure.For the managers of large and medium-sized low head pumping stations,the most worrying problem is that the unit vibration and performance plummet in the process of operation,and the inlet vortex is one of the most important reasons for the instability of the axial-flow pump device.Because of the inlet vortex has been generated before entering the impeller,it will not only cause the flow pattern disorder at the impeller inlet plane,serious even may be gas inhaled,but also inevitably aggravate the impact of cavitation and rotating stall when pump device under special operating conditions,may resulting in superimposed or even beyond the expected damage.Therefore,eliminating the harm of inlet vortex to axial-flow pump and pump device is a necessary prerequisite to achieve the design goal of safe,stable and efficient operation of an axial-flow pump.In order to find out the propagation and evolution law of inlet vortex in the axial-flow pump,reveal the interaction between inlet vortex and high speed rotating impeller,explore the damage reason of inlet vortex on the instability performance of the axial-flow pump,and establish the relationship between the evaluation of inlet flow state and the performance of the axial-flow pump device,in this paper,study the flow phenomenon and hydraulic performance of axial-flow pump and pump device with or without inlet vortex by ways of theoretical analysis,model test and numerical simulation.The influence of inlet vortex on the external characteristics of axial-flow pump and pump device was tested by energy performance test,the flow and evolution process of inlet vortex in the pump was recorded by high speed camera,the disturbance ability of inlet vortex in the flow field was compared and analyzed by non-contact LDV flow field measurement technology;the three-dimensional unsteady characteristics of axial-flow pump and pump device under inlet vortex were studied by numerical simulation,the results of different vortex identification criteria were compared and analyzed to judge the applicability of vortex identification criteria in high speed rotating impeller.The main results are as following:1.The importance of uniform flow pattern at impeller inlet plane of axial-flow pump is presented,and the evaluation standard of flow pattern is tested and revised.The vertical pump device with elbow inlet passage and the horizontal pump device with shaft well inlet passage were taken as the research objects,and the inlet passage was divided into three kinds of grids respectively,to test the reliability of evaluation indexes such as the uniformity of axial velocity distribution,the weighted average angle of velocity and the hydraulic loss.The modified formula of velocity distribution uniformity greatly reduces the dependence on grid scale,the grid size of inlet passage will affect the pressure distribution at the leading edge of the blade,inlet passage in engineering applications should also be divided into grids of sufficient density to more accurately reflect the detailed flow.2.The energy performance test of an axial-flow pump with or without inlet vortex and the velocity field measurement of the impeller inlet and impeller was carried out.The morphology and evolution of the inlet vortex under different conditions and at different times were recorded by a high speed camera.The model test results show that the energy characteristic curves after the installation of the vortex generator are generally shifted downward compared with those before the installation,the vortex generator of the same size will make the energy performance of the axial-flow pump decline more seriously at higher rotating speeds;the vortex generator can induce the inlet vortex in front of the impeller,which disrupts the flow field at the impeller inlet,and the induced vortex can enter the impeller stably;the disturbance ability of vortex generator to flow field is closely related to the flow condition.The smaller the flow is,the weaker the disturbance of the flow field will be.However,inlet back flow of small flow will interact with the disturbance of the vortex generator.In different positions,the vortex induction may not only inhibit the back flow,but also may promote the back flow.At different speeds,the water flow inside the pump disturbed by the vortex generator does not conform to the similarity law.3.The three-dimensional steady and unsteady numerical simulation of an axial-flow pump with and without inlet vortex is carried out,which complements the deficiency of model test research.The numerical simulation results show that the accuracy of the numerical simulation results is verified by both the external characteristics of the model test and the internal flow field measurement.The inlet vortex generated by the vortex generator and the attached bottom vortex in the inlet pool are similar in shape and pressure gradient distribution characteristics.Identification results of Q criterion and Liutex criterion in the impeller are very close.Liutex criterion can reduce the shear interference on the blade surface,and can also identify strong vortex and weak vortex at the same time.After entering the rotating impeller,the induced vortex is quickly cut off by the rotating blade,and the cut-off vortex gradually decreases in strength.The induced inlet vortex and impeller interfere with each other,when the induced inlet vortex velocity at the monitoring point is in the same direction as the mainstream velocity,it can promote the flow at the point;when the induced inlet vortex velocity at the monitoring point is in the opposite direction to the mainstream velocity,it can inhibit the flow at the point.4.The influence of induced inlet vortex on the performance of the vertical axial-flow pump is analyzed.The results of the model experiment with and without vortex generators and the pressure pulsation test with vortex generators can verify the reliability of the steady and unsteady numerical simulation results,respectively.By changing the radial size of the vortex generators inlet vortices of different intensities are induced to change the turbulence of the flow field at the inlet of the impeller.The inlet flow state is closely related to the external characteristics of the pump device,the worse of the inlet flow state,the more obvious the performance decline of the corresponding pump device,moreover,the larger the flow rate,the more obvious the influence of the inlet flow state on the performance of the pump device.The pressure pulsation induced by the inlet vortex is mainly low frequency,and there are pulsation components with the same frequency as the impeller,which indicates that the influent vortex and the impeller rotation interfere with each other.