| As the most common hydraulic machine,centrifugal pumps have an extremely important role in today’s production and life.With the progress of the times and the rapid development of technology,the demand for centrifugal pumps in the actual working process of the operating range is gradually expanding.Although the centrifugal pump will not operate under stall conditions for a long time,the unstable flow under low flow conditions will often have a non-negligible impact on the strength of the fluid mechanical structure and its service life.In this paper,a low specific speed centrifugal pump for PIV tests in the Fluid Dynamics Laboratory at the University of Magdeburg,Germany,was used as the study object.The Large Eddy Simulation（LES）method and the well-adapted dynamic Smagorinsky subgrid-scale stress model were adopted for the high-precision numerical simulation of the flow field inside the impeller of the centrifugal pump.The internal and external characteristic parameters carried by the numerical simulation of the centrifugal pump were analyzed by various data processing methods.Finally,the energy transport theory from vortex dynamics was combined with the stall flow field for the theoretical study.The main works and results are as follows:（1）Based on the development of previous experiments and numerical computations in the field of fluid mechanical stall,this paper reveals the mechanism of hydromechanical flow stall and describes the energy conversion characteristics of the stall process.In addition,model building,meshing and pre-processing settings are carried out for the low-specific-speed centrifugal pump used in the calculation,and the control equations and subgrid-scale stress model used in the LES method are introduced.The selection of the number of grids was based on the GCI criterion of Richardson extrapolation.In order to verify the applicability and accuracy of the pre-processing,grid independence and time-step independence were verified.The computational error has been controlled to be less than 5%by comparing the external characteristic curves of the centrifugal pump and the visualized flow field against the PIV for specific operating conditions.（2）The high-precision numerical method and the processing of transient calculation results can provide a more intuitive analysis of the unsteady spatio-temporal evolution characteristics and characteristic parameters of the stall flow field.The existence of a relatively stable stall vortex in the flow channel for the stationary stall condition（0.7Qd）with a stall vortex characteristic frequency fstall=1.94Hz.The stall vortex for the rotating stall condition（0.3Qd）has a relative motion with the impeller in the circumferential direction between the channels,with the characteristic frequency of the stall vortex fstall=2.49Hz and the propagation frequency fcs=0.71Hz.（3）Based on the vortex dynamics theory,the energy transport equation is decomposed and simplified for incompressible viscous fluids.The final relationship between the rate of change of momentum with time and pressure propulsive power,Lamb vector divergence and enstrophy is obtained.The contribution of the above three items to the energy transport during stall conditions is obtained from the variable distribution curves from impeller inlet to outlet and the plane projection diagram of the impeller cross section. |
