Unsteady Flow Characteristics And Hysteresis Effect By Impeller Trimming Inside A Double-suction Centrifugal Pump

Double-suction centrifugal pumps have been extensively used in industry,irrigation and water diversion projects for the characteristics of high head and flowrate.Impeller trimming is a common method in engineering,used to modify pump performance without redesigning.However,operating stability will be destroyed account for the change of impeller geometry.Double-suction centrifugal pumps operate under off-design conditions frequently,with aggravated vibration and noise.Additionally,there would be hysteresis phenomenon under part-load conditions when the flowrate is changing continuously,which could threaten the safety of pump system.This thesis was supported by National Natural Science Foundation of China(Grant No.51879121)and Natural Science Foundation of Jiangsu Province(Grant No.BK20190851).The impeller of a double-suction pump was trimmed for 3%,6%and 9%,and both experimental and numerical methods were carried out to investigate in the unsteady flow pattern caused by trimming.Simultaneously,entropy theory was applied to probe into the mechanism of energy dissipation and efficiency decline.The discontinuity of head curve and hysteresis effect in this pump were discussed.The major work and innovation are shown as follows.(1)Pressure fluctuation characteristics of the double-suction centrifugal pump were experimentally investigated.The results indicated that the pressure fluctuation in suction chamber and volute case shared the same dominant frequency of blade passing frequency because of rotor-stator interaction.Under 0.6-0.8Q_d conditions,because the backflow around impeller inlet sections changed dramatically,fluctuation amplitude met a nonlinear variation.In the vicinity of pump outlet section,rotating frequency was more powerful under part-load conditions as the uniform flow in this area along with weak influence of rotor-stator interaction,which was extremely strong near the volute tongue.The dominant frequency changed to blade passing frequency under overload conditions.Separation and secondary flow also deteriorated and led to stronger low frequency fluctuation.(2)The external characteristics were investigated.It was found that the head curves met discontinuity when trimmed for 6%and 9%.Simultaneously,pump efficiency reduced aggressively with the increasing trimming size,especially under overload conditions,and the high efficiency region was narrowed in trimmed cases.Moreover,the best efficiency points in trimmed cases were the same with the origin.Trimming may keep geometric similarity,but the inner flows are not similar.Rotor-stator interaction was weakened and the fluid angle at volute inlet section increased,leading to uniform velocity distribution under part-load conditions.Correspondingly,separation and secondary flow in volute diffuser aggravated and vortexes blocked pump outlet section.The pressure fluctuation declined generally,but the intensity in diffuser increased,which could intensify the vibration of pump outlet pipe.(3)Hydraulic losses in all components were evaluated by the difference between inlet and outlet.The results indicated that the losses in suction and impeller met an extremely high value under part-load conditions,but the differences among trimming cases were tiny.The losses in volute had a valley around the best efficiency point and decreased when part-load after trimming while increased when overload.Due to the decline of input power,the proportion of losses in input power increased and resulted in efficiency drop.(4)It was an innovative method to calculate energy dissipation by entropy production theory in the stators of the double-suction pump to investigate in the effects of trimming on energy losses.The reason of high losses in trimmed cases when overload was revealed.It was found that flow separation and the mixing between secondary flow and dominant flow,which would be exacerbated after trimming,should be responsible for high energy losses and the severe drops.(5)Numerical investigation was conducted on the impeller for trimmed 6%to explore the characteristics that caused the discontinuity in head curve and hysteresis phenomenon.It was found that an abrupt change in the backflow near impeller shroud inlet led to the discontinuity in head curve and hysteresis phenomenon appears around the discontinuity point.Transient simulation was conducted to simulate the transient process under specific change rate.A typical characteristics of hysteresis phenomenon were noticed in head curve.This phenomenon was the results of vortexes in suction and the backflow close to impeller shroud at inlet section.The differences in these factors led to a gap in head curves and their sudden change caused the accidents.(6)It was the first time to apply continuous wavelet transform in analyzing the time-frequency domain characteristics of pressure fluctuation in hysteresis region.The discrepancy in different period of the hysteresis loop was reflected by low frequency fluctuation,which could be lower than rotating frequency.When decreasing the flowrate,these fluctuations increased instantaneously after passing the critical point,which would lag behind the change of flow pattern and head,but preceding when increasing the flowrate.