Research On Unsteady Vortex Structure Characteristics In The Mixed-flow Pump And Vortex Dynamics Diagnosis

The mixed-flow pump is a kind of pump between the centrifugal pump and axial-flow pump.It has the advantages of both,in addition to high head,large flow rate and wide high-efficiency area,and has a compact structure and better improvement of the hump phenomenon.It has been more and more widely used in many fields.The internal flow of the mixed-flow pump is extremely complicated,and the generation and evolution of various vortex structures will seriously affect the internal flow field,external hydraulic performance and operating stability.Traditional velocity and pressure field analysis cannot fully reveal the true flow status of the flow field.Therefore,it is more important to use vortex dynamics to analyze the internal flow of the mixed-flow pump.Through the vortex recognition technology,the fine vortex structure inside the mixed-flow pump can be obtained,and the size,position and evolution of the vortex can be identified.Furthermore,the flow pattern inside the pump can be more clearly understood,which provides guidance and new ideas for the inner flow diagnosis and optimization of the mixed-flow pump.This paper was completed with the support of the open project of the Provincial and Ministry-level Discipline Platform of Xihua University,"Analysis of Internal Flow Field of the Mixed-Flow Pump with Guide Vanes under Different Working Conditions and Robust Optimization Design".On the basis of large eddy simulations,vortex dynamics analyses and diagnostic researches were carried out on a mixed-flow pump with guide vanes（HM350-600）produced by a pump company in Shanghai.The main research work and conclusion are as follows:1.The large-eddy simulation method was used to obtain the distribution law of the pressure field,velocity field and vorticity field in the mixed-flow pump.The applicability of the third-generation vortex identification method in the identification of the internal vortex structure of the mixed-flow pump was compared and analyzed.The research shows that compared with the first-generation vorticity criterion method,the second-generation vortex identification method represented by helicity method,Q criterion andλ₂ criterion can obtain more detailed vortex structure inside the mixed-flow pump.Among them,the Q criterion can remove most of the influence of the shear layer,and better capture the vortex structure inside the mixed-flow pump than other vortex identification methods.The displayed vortex size is more accurate,but the identification of the second generation vortex identification method is affected by the threshold larger and overly dependent on user experience.The third generation of vortex identification methods includingΩmethod and Liutex method can remove the influence of the wall shear layer in the identification of the vortex structure of the mixed flow pump,the identification result of the impeller component is poor,the identification result of the guide vane component is good,The value ofεin theΩmethod will greatly affect the result of eddy recognition,and it is also necessary to adjust the appropriate value manually.Therefore,combining multiple factors,the Q criterion is more suitable for vortex identification of the flow field in the mixed-flow pump.It has advantages in the display of vortex structure and vortex size,and can be used for analysis of the flow field in rotating machinery.2.Based on the BVF diagnosis and Q criterion vortex identification technology,the unsteady flow characteristics and vortex structure evolution law of the mixed flow pump are studied.The Q criterion does not consider the threshold problem,and only performs qualitative analysis.The analysis finds that,the peak value of BVF appears at the blade edge of the impeller and at the outlet edge and the inlet edge of the guide vane near the rim and the pressure surface near the hub.The positive and negative peaks of BVF appear alternately in the above area,the pressure gradient is large,and bad flow is easy to occur,which matches the vortex structure identified by the Q criterion.The impeller blade edge separation vortex is affected by the inlet vortex and the blade shunt.Within a period,the vortex structure first increases and then decreases.The vorticity gradually changes from negative to positive.The source of the change is the convergence and shedding of the boundary layer.The shedding of the impeller trailing edge vortex is inseparable from the separation of the surface layer.The evolution rule is that after 2/6T,the vorticity value does not change much after a split and merge.The vortex of the guide vane channel comes from the development of the vortex at the inlet and the separation of the boundary layer.The overall change of the positive vortex is not large.The negative vortex gradually merges first and then gradually breaks in a cycle.The influence of the positive vortex at the layer and the leaf margin gradually decreases during the cycle,the positive vortex continues to dissipate and disappear,and the negative vortex stretches and merges;the vortex structure in the draft tube shows a gradually decreasing trend during the cycle.3.Based on the quantitative analysis results of the Q criterion,an appropriate Q value is selected to compare and analyze the internal flow field and vortex structure of the mixed flow pump under the three operating conditions of 0.6Q_N,0.8Q_N,and 1.0Q_N.It is found that,when the flow rate is small,the three-dimensional flow line in the pump is more complicated,there are many places where vortices appear,the area of vortex clusters is large,which seriously blocks the flow channel and exacerbates the generation of bad flow and energy loss;The area and value of vorticity distribution in the vorticity field are too large.As the flow rate increases,the flow state in the flow field tends to be stable.The vortex structure identified by the Q criterion is closely related to the flow value.Under 0.6Q_N operating conditions,the vortex cluster mainly exists in the impeller rim hub,near the blade,and in the guide vane channel,There are also negative vortices.The intensity of the vortices is larger,the distribution is messier,and there is no regular shape and law;the flow rate continues to increase,and the number and intensity of vortex clusters continue to decrease.Under 1.0Q_N operating conditions,the vortex distribution is symmetrical and evenly distributed in each channel.