Study On The Influence Of The Pressure Chamber Form Of The Mixed Flow Pump On Its Internal Unsteady Flow Characteristics

Although the development of mixed flow pump is later than that of centrifugal pump and axial flow pump,mixed flow pump has a series of advantages,such as large flow rate,wide high efficiency area,wide range of head and not easy to cavitation,which makes it develop rapidly in engineering application.In particular,the mixed flow pump with medium-to-high specific speed（400～800）has been widely used in urban flood control and large-scale water transfer projects.According to the type of pressure chamber,the mixed flow pump can be divided into guide vane mixed flow pump and volute mixed flow pump.the guide vane mixed flow pump has produced a number of excellent hydraulic models in the construction of the east line of South-to-North Water transfer Project,and the engineering application is relatively mature.At present,the overall efficiency of the medium-to-high specific speed volute mixed flow pump is low,and there is a lack of excellent hydraulic model,but compared with the guide vane mixed flow pump,it has the advantage of shorter axial size and has been well applied in some civil fields,there is an urgent need for a large number of development and research to meet the engineering needs of the construction of major pumping stations.A large number of engineering examples show that the efficiency of the medium-to-high specific speed volute mixed flow pump is prone to a steep drop under large flow conditions,while the guide vane mixed flow pump can still maintain high efficiency under high flow conditions.Therefore,through an in-depth study of the internal unsteady flow characteristics of the medium-to-high specific speed guide vane mixed flow pump and the volute mixed flow pump of the same impeller model,this paper makes a comparative analysis of the efficiency of the guide vane and the volute to recover the rotational kinetic energy in order to find out the reasons for the low overall efficiency of the medium-to-high specific speed volute mixed flow pump and the steep decline of hydraulic performance under large flow conditions,in order to improve the efficiency of the medium-to-high specific speed volute mixed flow pump.Broaden its high efficiency area to provide theoretical basis.The main research work and conclusions of this paper are as follows:（1）The three-dimensional modeling of guide vane mixed flow pump and volute mixed flow pump is completed by using Creo software,and the structural grid of the calculation domain is divided by using ANSYS ICEM and Turbogrid software.the numerical simulation of the two model pumps under multi-flow conditions is carried out by ANSYS CFX.The comparison error between the numerical simulation results and the experimental results is less than 5%,which has a certain accuracy.Through the comparative analysis of the numerical simulation results of the external characteristics of the guide vane mixed flow pump and the volute mixed flow pump,it is found that the highest efficiency point of the guide vane mixed flow pump is higher than that of the volute mixed flow pump,and the high efficiency area is also wider than that of the volute mixed flow pump.the head and efficiency of the guide vane mixed flow pump are slightly lower than those of the volute mixed flow pump.Under the condition of high flow rate of 1600m³/h,the efficiency of volute mixed flow pump is lower than 60%,while that of guide vane mixed flow pump is still about 80%.（2）Through the comparison of λ₂ criterion,Q criterion andΩmethod under multivariable conditions,it is found that Q criterion andλ₂ criterion are very sensitive to the selection of threshold and are not as effective as theΩmethod when strong vortices and weak vortices coexist,while theΩmethod can objectively reflect the vortex distribution in the mixed flow pump.Based on theΩmethod,the vortex distribution of the guide vane mixed flow pump and the volute mixed flow pump under different flow conditions is compared and analyzed,and it is found that the flow patterns in the guide vane and volute are disordered and there are more vortices in the small flow conditions,and the strong vortices in the guide vane disappear and the vortices greatly reduce in the large flow conditions.the flow pattern in the diffusion section of the volute is disordered and the vortex intensity is the highest.（3）From the analysis of the characteristics of pressure fluctuation in time domain and frequency domain,it is found that the pressure curves of the internal monitoring points of guide vane mixed flow pump and volute mixed flow pump fluctuate periodically,and the main frequency of pressure pulsation is blade frequency.Under various flow conditions,the amplitude of the main frequency of the guide vane mixed flow pump decreases gradually from the impeller outlet to the guide vane outlet,while the volute mixed flow pump does not have the same regularity.this is mainly due to the sharp increase of the amplitude of pressure fluctuation near the separator tongue of the volute mixed flow pump with the increase of flow rate.In terms of pressure fluctuation intensity,the high intensity area of pressure fluctuation in the guide vane is mainly distributed at the entrance,and the pressure fluctuation intensity is the highest under the condition of low flow rate,which decreases significantly with the increase of flow rate.The distribution of the high intensity area of pressure fluctuation in the volute is inconsistent under different flow conditions,the pressure fluctuation intensity is lower under the design flow condition,and the pressure fluctuation intensity near the tongue increases significantly under the large flow condition,which is one of the important reasons for the internal unstable flow of the volute under large flow conditions.In terms of radial force,the radial force acting on the impeller of the guide vane mixed flow pump is much smaller than that of the volute mixed flow pump,and the radial force amplitude and fluctuation amplitude of the volute mixed flow pump are the highest under 1600m³/h high flow conditions.（4）The internal flow loss of guide vane mixed flow pump and volute mixed flow pump is analyzed based on the entropy production theory of the second law of thermodynamics.In the flow losses in the two kinds of mixed flow pumps,turbulent dissipation plays a dominant role,and the direct dissipation is negligible.The turbulent dissipation rate on the back of the impeller blade of the two kinds of mixed flow pumps is higher than that of the working face,and the overall turbulent dissipation has the same trend with the increase of flow rate.Under the condition of small flow rate,the backflow occurs in the flow channel of the guide vane,and the flow pattern at the tongue of the volute is disordered,so the turbulent dissipation rates of the guide vane and the volute are higher.Under large flow conditions,the turbulent dissipation rate in the guide vane is very low and the flow loss is small;due to the large-scale backflow in the volute diffusion section,the turbulent dissipation rate in the diffusion section is significantly higher than that in other flow conditions.therefore,the optimization of the volute diffusion section is one of the important factors to improve the hydraulic performance of the volute mixed flow pump under high flow conditions.