| Large-scale volute pumps are mostly vertical single-stage single-suction centrifugal pumps with diffuser,which are often used in high-lift,high-power pumping stations in large-scale water diversion projects.With the continuous improvement of water conservancy infrastructure,the power of large-scale volute pumps continues to increase,which puts forward higher requirements for its comprehensive performance.In order to enhance the hydraulic stability of large-scale volute pumps,the unsteady flow characteristics of large-scale volute pumps are deeply analyzed in this paper by combining numerical calculations and experiments.And through the vortex identification method,frequency domain and time-frequency domain analysis method,the internal vortex structure evolution and pressure fluctuation characteristics are studied.At the same time,in order to further improve the hydraulic efficiency and enhance the matching characteristics of the impeller and diffuser,a multi-objective collaborative optimization of the impeller and diffuser profile is carried out based on the artificial neural network model and multi-objective genetic algorithm.In addition,the impact of three different cut-water type volutes on the performance of large-scale volute pump,namely small cut-water angle volute,partitioned volute and large cut-water angle volute,is also compared and analyzed.The main research content is as follows:(1)Three-dimensional modeling and mesh generation are carried out for the hydraulic model of the large-scale volute pump,and the SST k-ωtwo-equation eddy viscosity model is used for numerical calculation,and the calculated hydraulic performance curve is compared with the test results.Under the design condition,the relative errors of efficiency,head and power are all less than 5%,indicating that the numerical calculation can accurately reflect the flow conditions inside the model pump.TheΩvortex identification method is used to study the vortex structure evolution law of the impeller and diffuser in one rotation period under different operating conditions.It is found that under different operating conditions,the periodic change of the regional distribution of higherΩcan be clearly observed in the impeller,and the regional distribution area first decreases and then increases.The area with higherΩdiffuses along the flow direction in the diffuser channel in a horizontal strip shape with the rotation of the impeller under the design condition and the over-load condition,and continues to exist in the diffuser under the part-load condition inside the channel.In addition,the Fast Fourier Transform method is used to analyze the pressure pulsation at different monitoring points in the impeller and diffuser in the frequency domain.It is found that the pressure pulsation frequency of each monitoring point in the impeller is dominated by the shaft frequency,the diffuser vane frequency and its multiple frequency,and the pressure fluctuation peak value of each point gradually increases along the flow direction,and the maximum is at the outlet of the impeller.Combined time-frequency domain analysis is performed by Wavelet Transform.It is found that near the trailing edge of the pressure surface,the periodic fluctuation of the amplitude of the pressure pulsation at high frequency can be observed,and the high frequency periodic fluctuation of the pressure pulsation is stronger near the wall in the impeller.In the diffuser,the pressure pulsation frequency of each monitoring point is dominated by the impeller blade frequency,and the peak value of pressure pulsation at each point decreases gradually along the flow direction.On the whole,the pressure pulsation is the strongest in the vaneless area between the impeller and the diffuser,and the pressure pulsation in the vaneless area is the strongest near the volute cut-water.It shows that the pressure pulsation in the vaneless area will also be affected by the flow in the volute.(2)Taking the head and efficiency under the design condition as the optimization objectives,the eight coordinates of the blade profile control points as design variables.The artificial neural network model is used to fit the nonlinear mathematical relationship between the design variables and the optimization objectives,and the multi-objective genetic algorithm is used to carry out multi-objective collaborative optimization of the impeller and diffuser of a large-scale volute pump.The automatic numerical calculation program is written by using Python and DOS commands to realize automatic calculation of multiple cases.Linear regression analysis is carried out on the fitted model,in which the efficiency and head R~2 values are both greater than0.9,and the numerical calculation and verification of the Pareto frontier points are carried out,and the relative errors of the predicted values near the optimum point of efficiency are all less than 5%.It shows that the prediction accuracy of the artificial neural network model is excellent.After optimization,the efficiency of the model is improved in all operating conditions.The efficiency under the design condition has been increased by 1.21%,and under the 0.7Q_d,the efficiency is improved the most,which is 1.98%.At the same time,the head remains unchanged,while the power decreases,which indicates that the hydraulic performance of the model has been substantially improved.The entropy production theory is used to compare and analyze the energy loss in the impeller and diffuser before and after optimization,the large entropy production area in different sections is significantly reduced.(3)The pressure pulsation characteristics of each monitoring point in the volute under the design condition of the model are analyzed.It is found that among the monitoring points of the volute,the peak-to-peak value of the pressure coefficient within one rotation cycle under the design condition is the largest near the cut-water.This shows that under the premise of large-scale volute pumps with radial diffuser,the pressure pulsation in the volute is still the strongest near the cut-water.The main frequency of the pressure pulsation at the monitoring point near the cut-water is the blade frequency of the impeller under different operating conditions.The pressure pulsation near the diffusion section will weaken with the increase of the flow rate,and the opposite will be the case near the first cross section,which will increase with the increase of the flow rate.At the same time,the impact of three different cut-water type volutes on the large-scale volute pump,namely small cut-water angle volute,partitioned volute and large cut-water angle volute,is compared and analyzed.It is found that the influence of three different cut-water type volutes on the performance of large-scale volute pumps is mainly under the over-load condition(1.1Q_d).Under this condition,the efficiency of the small cut-water angle volute is the highest,and the efficiency of the partitioned volute is the lowest,with a difference of 1.5%.Under other operating conditions,the hydraulic performance of the three is similar.Under the design condition,the main frequency of the pressure pulsation of the three schemes is the impeller blade frequency,but the pressure pulsation of the partitioned volute deteriorates,and the pressure coefficient fluctuation and the main frequency amplitude increase sharply.Therefore,from the perspective of pressure pulsation,the partition volute lacks obvious advantages in the application of large-scale volute pumps,and the small cut-water angle volute is more suitable. |
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