Research On The Transient Characteristics Of Startup Process And Cavitation Under Partial Load In A Centrifugal Pump

Centrifugal pump plays an extremely important role in our society.It is widely used in our daily life,industrial and advanced technology,and even the artificial heart pump which can be used to promote the blood circulation.With the development of the industry,the hydraulic performance of the centrifugal pump is putted forward the strict requirements.The unsteady characteristics of centrifugal pump is attracted widespread attention,which have great impact on the steady operation.And in the present work,the transient startup process and the unsteady characteristics of cavitation under partial load condition are investigated.Firstly,the external performance and internal flow field of the centrifugal pump startup process under different startup times are investigated by numerical method.Then,the cavitation flow field under partial load condition is conducted to reveal the unsteady cavitation evolution characteristics and cavitation-vortex-pressure fluctuation interaction.The main research works of this paper are listed as follows:1.Study on transient characteristics of centrifugal pump startup process.In order to reveal the influence of startup time on the transient characteristics in a centrifugal pump,the startup processes with time of 1s,0.5s and 0.1s are simulated by CEL language of ANSYS CFX.Due to the fluid inertia,it makes flowrate and head increase behind the growth of impeller rotation speed,and the lag phenomenon is more obvious with the reduce of startup time.In the initial stage of startup process,the combination of the low flow rate and sudden rise of fluid pressure leads to the maximum value of dimensionless head in this stage.For the flow field,the vortex area in the impeller flow passages increases rapidly with the reduce of the startup time.As the time is reduced to 0.1s,the large area of vortex structure also occurs in the volute.When the rotation speed reaches to the maximum value,there still a large area of vortex structure distributes in the impeller passage.The generation and disappearance of vortex structure induce the consume of energy,therefore the increases of head and flowrate lag the rotation speed.2.Variation of bubble rotation modified Zwart-Gerber-Belamri(ZGB)cavitation model.Considering the influence of the turbulent flow and the shear force on the bubble size,the bubble radius of ZGB cavitation model is modified as a function which has a relationship with impeller speed and geometric feature.In order to verify the rationality of the bubble rotation modified ZGB(BRZGB)cavitation model for the cavitation flow in the centrifugal pump,cavitation performance curves are calculated by ZGB and BRZGB cavitation models,respectively.The result shows that the cavitation performance curve calculated by BRZGB cavitation model has a good agreement with experiment measurement result.3.Study on the spatial-temporal evolution of cavitation under low flow condition in the centrifugal pump.In order to reveal the unsteady characteristics of cavitation flow in the centrifugal pump,the transient simulations are conducted by BRZGB cavitation model.It is found that the cavitation shapes show periodicity evolution under severe cavitation(NPSHA=1.01m),and the evolution frequency is 4fi(fi is impeller rotation frequency).The typical cavitation evolution process can be divided into three stages:developing stage,shedding stage and collapsing stage.The periodicity evolution mainly occurs on the rear part of cavity,and the pair of vortexes with counter-rotating direction play the main role in the evolution.4.Study on the cavitation-vortex-pressure fluctuation interaction in the centrifugal pump passage.In order to reveal interaction mechanism of cavitation-vortex and cavitation-pressure fluctuation,the transient cavitation flow under the non-cavitation,inception cavitation,critical cavitation and severe cavitation conditions are simulated by BRZGB cavitation model.The results show that the high vorticity magnitude region mainly occurs in the region near liquid-vapor interface at cavity front part,and expands to downstream with the development of cavitation.The high magnitude region also occurs in the rear part of cavity under the severe cavitation condition,and it has a good agreement with shedding location.The relative vorticity transport equation is introduced to further reveal the relationship between cavitation and vortex.It is found that the combination of the relative vortex stretching term and the relative vortex dilation term is dominant in the cavitation-vortex interaction.The pressure fluctuation intensity is defined by root-mean-square method,the contour of pressure fluctuation intensity under inception cavitation is similar to the non-cavitation condition,and it is relatively uniform in each passage.With the development of cavitation,the distribution of pressure fluctuation intensity is no longer even in different passages,and the high-level region above the middle chord.The asymmetrical distribution of pressure fluctuation intensity will lead to the imbalance of radial force and then deteriorate pump operational stability.