Flow Characteristics And Instability Suppression Of Tip Leakage Flow In A Mixed-flow Pump Under Stall Condition

As the core power equipment in major strategic projects such as nuclear power generation,ship propulsion,and South-to-North Water Diversion,mixed-flow pumps have important stability,safety,and performance issues.When the mixed-flow pump is in operational condition,due to the pressure difference between the pressure surface and the suction surface,a tip leakage flow(TLF)will be generated.The TLF mixes with the main flow forming a tip leakage vortex(TLV).TLF interferes with the movement of the main flow and affects nearly 30% of the flow channel.It is not only the main factor causing blockage of flow and turbulent pulsation in the end wall area,but also closely related to the rotating stall phenomenon.Therefore,it is necessary to study the spatial and temporal characteristics of the flow field in the tip clearance of a mixed-flow pump.The important guiding and significance of this study are to understand the fluid dynamics mechanism of the development of TLV,the internal connection between the unsteady evolution,rotating stall of TLV,and optimize the hydraulic and structural design.This research is founded by the National Natural Science Foundation of China“Dynamics behavior and instability mechanism of tip leakage flow in mixed flow pump under near stall condition”(No.51679111).In this study,the internal flow structure of a mixed flow pump under stall conditions were studied through numerical simulation.Combining experimental research and interaction between the TLF and the main flow,the energy loss mechanism and stall improvement condition of the mixed-flow pump were explored.This research provides a reference for the design and optimization of the mixed-flow pump under non-design conditions.The main research contents and achievements are as follows:1.Based on the corrected SST k-ω turbulence model,the external characteristics and internal flow field of a mixed-flow pump under different tip clearances were studied.The influence of tip clearance on the “saddle zone” of a mixed-flow pump is mainly discussed.The study has shown that the tip clearances have a significant impact on the“saddle zone” of the mixed flow pump.The positive slope region is gradually decreasing with the increase of the tip clearance,and the change in the tip clearance is non-linear with the interval of the “saddle zone”.This is mainly related to the unstable vortex structure such as the stall vortex,the secondary vortex,and the squeezed “flat”TLV in the impeller,and it is also related to the swirl flow at the pump inlet under the stall condition.2.Based on the steady calculation,the unsteady flow of the mixed-flow pump under stall conditions was numerically calculated.The flow morphology and propagation characteristics of the rotating stall in the mixed flow pump were analyzed.The flow in averaged time and the blockage effect on the impeller rim were also analyzed.At the same time,the pressure pulsation around the TLF was compared.The results stated that the stall core propagates circumferentially in each impeller passage when the mixed flow pump enters stall.The stall core experiences the process of birth,development and collapse.TLF distortion is one of the main causes of pre-stall and blade tip initial stall.Under stall conditions,TLF,inlet swirl,backflow and stall vortex around the impeller lead to high turbulent energy.As the tip clearance increases,the region of high turbulence energy dissipation increases.As the tip clearance increases,the low-frequency components become more concentrated,but the maximum of lowfrequency components increases first and then decreases.3.The boundary vorticity flux(BVF),diagnosis theory,and entropy generation theory were used to study the energy loss in the vortex structure of a mixed-flow pump.The influence of the tip clearance size on the internal flow structure,and energy loss of the mixed-flow pump were also compared.The results stated that under stall conditions,the increase in fluid pressure and power around the impeller is greatly influenced by the rotating stall.The total pressure flow integral shows a significant reduction trend with uniform Pu distribution.As the tip clearance increases,the Pu distribution in the impeller and the corresponding total pressure flow integral was decreases first and then increases.Under stall conditions,the total entropy production of the mixed-flow pump increases,and the total entropy production around the impeller was also increases.As the tip clearance changes,the hydraulic losses around the impeller do not have a linear relationship with the tip clearance size.4.Experiments on the external characteristics and pressure pulsation of mixed flow pumps were also carried out.The pressure pulsation characteristics at the inlet,middle,and outlet of the impeller of the mixed flow pump under different tip clearance were compared and analyzed,through the wavelet energy,and average spectrum.The coherence between the pressure pulsation signals was also discussed.The results stated that with the increase of the tip clearance,the positive slope region of the head curve was gradually decreasing,and the accuracy of the numerical method was verified.The impeller inlet was influenced by the inlet swirling flow,while the outlet of the impeller is affected by rotor and stator interference(RSI)flow,and stalled flow.As the tip clearance increased,the concentration of low-frequency energy spectrum was decreased around the inlet and outlet of the impeller.The spectrum coherence map showed that the sensor distance is also one of the factors that affect the coherence.5.Based on the particle image velocity(PIV)measurement technology,the internal flow of the mixed-flow pump under stall conditions with different tip clearances was measured,and the results were compared with the time-averaged constant values.The results stated that the numerical calculation values have a high degree of possibility.Under stall condition,due to the influence of the stalled and RSI flow,a vortex structure was generated in the impeller.With the increase of the tip clearance,the vortex structure gradually increases,and the scope of influence was also gradually increased.The blockage guide vane channel of the inlet squeezed the return vortex at the guide vane inlet around the hub area.The increase of the clearance will also have an impact on the stalled flow around the rim area.As a result,the increase in the velocity of the fluid near the rim area of the impeller was increasing the turbulence flow in the line.6.The method to improve the performance of the “saddle zone” of the mixed-flow pump was explored,and the rotational stall phenomenon of the mixed-flow pump was suppressed by changing the geometric structure of the blade tip.The results stated that under design conditions,the Groove_Tip structure can increase the head of the mixed flow pump by approximately 1.8%,while the Bulge_Tip and Double_Rib_Tip structures reduce the head by 2.0% and 3.8%,respectively.Under stall conditions,the Bulge_Tip structure and Double_Rib_Tip structure shifted the critical and deep stall point to a low flow operating point.The concluding outcome improves the performance of the “saddle zone” of the mixed flow pump to a certain extent.