Study On The Internal Flow Characteristics Of Centrifugal Pump Under Different Units Combination Schemes

Pump station project is a hydraulic project used for long-distance water diversion,irrigation and drainage,and urban and industrial water supply.Centrifugal pump is one of the main pumps commonly used in pumping stations,and its operation is of great significance for safe,stable and efficient operation of pumping stations.For centrifugal pumps,the change of inlet condition is one of the main reasons for the change of internal flow in centrifugal pumps.This paper takes a large-scale pump station with 7 centrifugal pump units as the research object,combines experimental research and numerical simulation,carries out in-depth research on hydraulic characteristics and pressure fluctuation characteristics of centrifugal pump system under different inlet conditions,explores operation performance of pump station under different start-up combinations,and reveals the interaction mechanism between different inlet conditions(different start-up combinations,flow rates)and internal and external characteristics of centrifugal pump.An optimum start-up combination scheme for pump station system is proposed.The main r esearch work and achievements are as follows:(1)An experimental study was carried out on the external characteristics and pressure fluctuation characteristics of centrifugal pumps under different inlet conditions.In view of the different flow conditions of a single unit during operation,an experimental study on the external characteristics and pressure fluctuation characteristics of the centrifugal pump is carried out.The hydraulic losses at the inlet section of the pump station system when 2 units,3 units,4 units,5 units and 6 units are opened at the same time are analyzed.The results show that,compared with the designed flow and large flow conditions,the operation performance of centrifugal pump under small flow conditions is significantly worse,the pressure fluctuation amplitude at the inlet section of centrifugal pump is lower,and the pressure fluctuation amplitude near the nose end is the highest.The optimum start-up combination of pump station system is initially determined when 2-6 units are opened at the same time.(2)The variation rule of flow characteristics in centrifugal pump under different flow conditions was revealed.The internal flow of centrifugal pump under four typical flow conditions(0.6Qopt、0.8Qopt、Qopt、1.2Qopt)was analyzed numerically.The results show that when deviating from design conditions,complex flow phenomena such as recirculation and vortices appear on suction surface of blade and in guide vane passage.When the flow rate is larger than the design flow rate,the return flow will occur at the outlet of the volute,resulting in increased energy loss.The energy loss of centrifugal pump under other flow conditions mainly occurs at the inlet of guide vane and near the nose end of spiral case.(3)The pressure fluctuation characteristics of each overflow component of centrifugal pump under different flow conditions are analyzed.By combining the test monitoring data with the numerical simulation data,it is found that the trend of the test value is basically the same as that of the simulation value.The pressure fluctuation of centrifugal pumps at each typical flow rate has strong periodicity,and the pressure fluctuation frequency is mainly manifested by blade frequency and its frequency multiplication.Low frequency pressure pulsation with multiple frequencies occurs under small flow conditions.Under each flow condition,the pressure fluctuation at the guide vane inlet is the largest and the pressure fluctuation at the intake section is the lowest.(4)The operation performance of the pump station with different units on is explored.Aiming at the pump unit operating under design conditions,the axial velocity uniformity,average section deviation angle and hydraulic loss of the pump inlet section are taken as evaluation indexes,and the internal flow characteristics of the pump under different combinations of startup conditions are analyzed by combining the entropic production and vorticity.It is found that the difference of energy loss of centrifugal pump under different start-up combinations is mainly manifested in impeller and guide vane flow paths.The optimum start-up combination mode of pump station under different start-up combinations is determined as follows:when two units are opened at the same time,unit 1 and 3 are started preferentially;When three units are opened at the same time,unit 1,3 and 5 should be started first.When four units are opened at the same time,unit 1,3,5 and 7 should be started preferentially.When five units are opened at the same time,units 1,2,3,5 and 7 should be started preferentially.When six units are opened at the same time,units 1,2,3,4,5 and 7 should be opened preferentially.