Analysis Of Stall Characteristics Of Vertical Volute Centrifugal Pump Based On The SST-SAS At Different Guide Vane Openings

The vertical volute centrifugal pump is the core power equipment in long-distance water division projects,with the characteristics of high flow,high head and high power.It has good development prospects and urgent market demand in the construction of large-scale water conservancy projects,agricultural irrigation pumping stations and other fields.In the actual working process,the pump flow adjustment range is large,and the unit is usually required to have a wide stable operation area.But the stall phenomenon often occurs in the vertical volute centrifugal pump under small flow rate conditions,which destroys the uniformity of the internal flow.It will lead to the decline of the pump performance and affect the safe and stable operation of the unit.This paper relied on the Jiangsu Provincial Key Research and Development Program Project“Development of a 40MW Oversize High-Performance Volute Centrifugal Pump Unit”（Project No.BE2019089）.Numerical simulation method was used to analyze the generating mechanism and characteristics of the stall in vertical volute centrifugal pump at different guide vane openings,and to explore the influence of different guide vane openings on the stall characteristics inside the pump.which can provide theoretical guidance for the stable operation of the vertical volute centrifugal pump under multiple working conditions.The main research works carried out in this paper are as follows:（1）The standard k-ε,RNG k-ε,and SST turbulence models were used to carry out numerical calculations on the vertical volute centrifugal pump,and CFD uncertainty analysis was carried out based on ITTC relevant procedures.Through the verification and validation of the numerical simulation results,the SST turbulence model and the appropriate number of mesh were selected in the steady calculation.Aiming at the unsteady flow inside the pump under stall conditions,the SST-SAS turbulence model was selected for calculation.Comparison of simulation results and energy characteristic test results showed that the energy characteristic curve predicted by the turbulence model was similar to the physical model test results.The CFD performance prediction error was less than 5%,which can meet the calculation requirements.（2）Through the analysis of the change law of hydraulic loss in the vertical volute centrifugal pump under stall conditions,it was found that vortexes appeared near the wall of the draft tube outlet,which leaded to an increase in the hydraulic loss of the draft tube section.When the flow was reduced from Q/Q_des=0.8 to Q/Q_des=0.7,at the impeller blade height of 0.1span,the low-velocity area on the suction surface in the middle of the flow channel increased.The impeller outlet also appeared the same pattern at the blade height of 0.9span.But at the blade height of 0.5span,the low-velocity area at the trailing edge of the blade showed a decreasing tendency.Under critical stall condition(Q/Q_des=0.8),there were large-scale vortices in the vaneless area between the impeller and the vane diffuser,resulting in a large adverse pressure gradient between the guide and stay vanes.Part of the fluid was forced to flow into the other channels from the vaneless area between the diffuser vanes,which induced the hydraulic instability.（3）According to the three typical guide vane openings of small opening（19°）,optimal opening（25°）and large opening（31°）,the influence of the guide vane openings on the internal flow characteristics of the vertical volute centrifugal pump was explored.The numerical simulation results showed that the flow-head curve had a hump area and the stall occurred under different guide vane opening conditions,and the flow conditions corresponding to the stall characteristic operating points were the same at the three guide vane openings.However,the positive slope corresponding to the flow-head curve was the largest when the opening was small,and the smallest when the opening was optimal.By comparing the hydraulic loss generated by each watershed in the pump at different guide vane opening conditions,it can be found that the hydraulic loss of the draft tube and the guide vanes was the largest under the large opening condition,and the impeller hydraulic loss was the largest at the optimal opening.The loss of the volute was the largest at the small opening condition.According to the changing law of the meridional velocity in the vane diffuser,it can be found that from the leading edge to the trailing edge of the diffuser vane,the lower meridional velocity area inside the vane diffuser gradually extends from the hub to the shroud under the stall conditions.It resulted that the flow blockage from the leading edge to the trailing edge of the diffuser vane increased.（4）With the using of entropy production rate analysis method,the influence of the guide vane opening on the stall characteristics of the vertical volute centrifugal pump was discussed.The research results showed that the distribution of entropy production rate in the impeller presented different characteristics with the change of guide vane openings under the deep stall condition(Q/Q_des=0.76).As the guide vane opening increased gradually,the flow separation area near the impeller inlet continued to expend at the impeller blade height of 0.1span.And under the conditions of the optimal opening and large opening,a large velocity gradient appeared near the blade channel exit at the impeller blade height of 0.5span,which caused an increase in the local entropy production rate.Under the deep stall condition(Q/Q_des=0.76),due to the flow separation in the blade channels,the pressure difference between the suction surface and the pressure surface of the blade were small.The blade load decreased,and it lead to the weakening of the impeller working ability.（5）Based on fast Fourier transform and wavelet analysis methods,the transient pressure characteristics of vertical volute centrifugal pumps under stall conditions were discussed.The research results showed that the main frequency of pressure fluctuation under the non-stall conditions in the vane diffuser was the blade passing frequency 7f_n,and after entering the stall area the main frequency changed to 0.9f_n.Combined with the transient flow field in the vane diffuser,the pressure fluctuation under non-stall conditions was mainly affected by the rotor-stator interaction,and the pressure fluctuation under stall conditions was related to the periodic large-scale vortex in the vane diffuser.Under the deep stall condition(Q/Q_des=0.76),when the guide vane opening decreased,the main frequency amplitude of the low-frequency pressure fluctuation in the draft tube increased.When the guide vane opening changed from the large opening to the small opening,the maximum fluctuation amplitude of the transient pressure in the volute decreased to 27.8%of the large opening.From the inlet to the outlet of the guide vanes,the amplitude of the blade passing frequency 7f_n continuously decreased,and the amplitude of the low-frequency pressure fluctuation signal increased with the increase of the guide vane opening.The main frequency amplitude of the pressure fluctuation gradually decreased from the stay vanes inlet to the outlet.