Research On The Unsteady Wake Flow Characteristics Of A Centrifugal Pump

The transient evolution and excitation characteristics of the three-dimensional wake flow in a centrifugal pump are the key problems to find out the internal flow mechanism of the rotor-stator interaction and realize the low-noise internal flow control of the pump.The paper is supported by the national natural science fund project"Unsteady evolution of three-dimensional wake vortex and its excitation characteristics in the centrifugal pump"(51706086).A centrifugal pump is investigated in the paper based on the numerical calculation method and the LDA laser doppler velocity test.the unsteady flow and its evolution will be clarified to provide theoretical support for low noise centrifugal pump flow control and explain the flow mechanism of rotor-stator interaction.Main conclusions are as follows:1.The numerical simulation method is validated by the experiments carried out in the closed test loop.Several working conditions are simulated to investigate the internal flow.Results show that the back flow will enchance the jet-wake flow structure at low flow rates.At the nominal and high flow rates,high wake vortex at the blade trailing edge will induce the jet-wake flow structure.Meanwhile,the wake region will increase with the flow rate increasing.Based on the numerical simulation results,the wake flow evelotion is depicted in detail,including its shedding and interaction with the solid wall.2.Unsteady pressure pulsations are carried out to find the relation with the internal flow.Resutls show that at various flow rates,pressue signals show periodic characteristics.In pressure spectrum,the dominant component is the blade passing frequency and its harmonics.The RMS and blade passing frequency amplitudes are similar.Pressure amplitudes at different points show evident difference,and the maximum pressure amplitude is generated downstream the volute tongue.It means that at the the typical region,high interaction effect is generated.From combined analysis of pressure pulsation and flow structure,it is found that at high working conditions,pressure amplitude is related to the unsteady wake flow,while at low flow rates,pressure pulsation is affected by the flow separation.3.The laser doppler velocimetry(LDA)is used to measure the velocity singals within the diffusion section and the volute to obtain the velocity pulsation charactersitics at different flow rates.Results show that when the model pump works in the range of rotating stall region(0-0.4Q_d),large scale flow separation phenomenon in the impeller and diffusion leads to high blade passing frequency signal.From the diffusion inlet to outlet,turbulent pulsation energy increases rapidly due to the deterioration of flow field,which achieves maximum at the diffusion outlet.Low turbulent energy is generated for the pump within 0.5-0.9Q_d.When the pump works within the range of 1.0-1.4Q_d,caused by the wake-tongue interaction,high blade passing frequency amplitude and turbulent pulsation energy are increased.At high flow rates,amplitudes at the diffusion outlet is increased,and it means that the velocity pulsation is more intense.The energy of pressure fluctuation is mainly concentrated on the blade passing frequency and harmonic,when the model of the pump works within the range of 0-0.6Q_d,the pressure fluctuation characteristics are mainly affected by the flow structure in the diffusion section,The intensity of pressure fluctuations is lower in the range of 0.4-0.6Q_d.In the range of rotating stall region(0-0.4Q_d),the pressure amplitudes are increaseed significantly with the decreaseing of flow rate.When the model pump works above the design condition,the pressure fluctuation is directly affected by the flow structure of the impeller outlet.With the increasing of the flow rate,the amplitude of blade passing frequency and RMS value are significantly increased,and due to the wake interference,the intensity of pressure fluctuations are more severe at the region far away from the tongue.