| Centrifugal pump is widely used in many fields of national economy,and is an important equipment to realize energy conversion and fluid transportation.In actual operation,it often deviates from the designed operation condition,resulting in unsteady flow.Especially for the low specific speed centrifugal pump,when it operates in a small flow condition,return flow may occur at the inlet of the impeller,which will worsen the flow field,lead to energy loss,and seriously affect the stability of the centrifugal pump operation.This paper takes low specific speed centrifugal pump as the research object,adopts numerical simulation method to study the influence of inlet return flow on the hydraulic performance of centrifugal pump,analyzes the flow characteristics of return flow vortex,and further discusses the methods and measures to restrain inlet return flow on this basis.A method of adding annular jet near the inlet of impeller to inhibit inlet return flow is proposed,and the influence of annular jet designed by Coanda wall attachment effect on inlet return flow is studied,which provides a new idea for improving inlet flow of centrifugal pump and improving the operation stability of the pump.The main research contents and conclusions of this paper are as follows:The flow characteristics of centrifugal pump under low flow condition were numerically simulated.It was found that when return flow occurred,vortex of different strength and scale appeared near the pump inlet near the outer wall of the pipeline,the flow was more chaotic,the main stream was crowded out,and the return flow strength gradually increased with the decrease of flow rate.At the inlet of the impeller,it is evident that the axial velocity is opposite to the direction of the main stream,and the circular velocity is the same as that of the impeller turning.The farther away from the impeller inlet,the smaller the reverse axial velocity and circumferential velocity.Under the influence of return flow,the inlet ring quantity of the impeller deviates more from the zero ring quantity,which increases the circumferential average value of the relative liquid flow Angle,resulting in the increase of the impact Angle,and the formation of alternate stall vortex in the impeller in the flow passage.The static pressure value of the blade pressure surface resulting in stall decreases rapidly,and the static pressure value of the head of the suction surface decreases.In addition,the return flow makes the pressure pulsation frequency composition in the inlet tube more complicated.Based on the entropy production theory to analyse the flow of centrifugal pumps,it is found that under different working conditions,the entropy value added of the inlet tube decreases with the increase of flow rate,the generation of return flow leads to a wide range of high entropy production area at the impeller inlet,and the loss inside the inlet tube under small flow rate working conditions is significantly greater than the design working conditions and high flow rate working conditions.As the flow rate decreases,the entropy production value increases,and the location of the high entropy production value appears in line with the location of the return flow vortex nucleus,mainly concentrated in the near-wall area.In order to reduce the influence of inlet return flow on the internal flow of the impeller,different types of annular jets are added to the inlet pipe wall.It is found that the flow line distribution in the inlet pipe can be improved by using the effect of jet attachment,the axial velocity near the wall of the inlet pipe can be increased under the condition of low flow rate,the circumferential velocity can be reduced,and the return area in the inlet pipe can be reduced,thus restraining the return flow.Compared with the oblique jet,the Coanda annular jet has more obvious inhibition effect on the return flow in the inlet tube because of its good wall attachment flow effect.When return flow occurs,there are other frequencies at the inlet of the pump except blade frequency and its frequency doubling.The amplitude of low-frequency pulsation under low flow condition is positively correlated with the return flow intensity.Under the action of annular jet,the amplitude of pressure pulsation in the inlet pipe and impeller decreases.In addition,the structure of the Coanda circular jet is optimized by orthogonal experimental design,and the optimal horizontal combination of the structural parameters of the Coanda circular jet is determined.The numerical simulation results show that the Coanda annular jet under the optimal horizontal combination has a more obvious inhibition effect on the inlet return flow phenomenon of the low specific speed centrifugal pump. |
