| Centrifugal pump is widely used in many industries and its operation stability is crucial to the whole pump system.The performance of centrifugal pumps,namely the head and efficiency representing the work and loss in the process of energy transfer,largely depends on the flow pattern in the centrifugal impeller.However,in many operating conditions,the flow is complex due to factors such as unsteady or ununiform inflow during the start/stop phase and in multistage pumps with varying speed.The flow in the impeller may separate from the solid wall,leading to separation bubbles which block the channel,thus affecting the interaction between the flow in the impeller and the inlet pipe or diffuser.In this thesis,the influence of different inflow on the flow characteristics in centrifugal are studied.The main research contents and conclusions are as follows:The unsteady numerical simulation of the three-dimensional flow in the centrifugal impeller is carried out by introducing the pre-swirl inflow with the experimental verification of the numerical simulation.The general characteristics of the flow in the impeller are analyzed globally,and the velocity and pressure fields at different positions in the impeller,the flow characteristics on the blade surface,and the radial force distribution are studied locally under the condition of pre-swirl inflow.Based on the artificially constructed simple pre-swirl flow with different directions and magnitude,the three-dimensional flow characteristics in the centrifugal impeller is investigated.The distributions of various velocity components along the circumferential and axial directions,the pressure distribution and limiting streamlines on the surfaces of a blade are analyzed,and the influence of pre-swirl inflow on the flow in the impeller are discussed.It is found the radial and circumferential velocity at the outlet of the impeller has obvious fluctuations for the counting-rotating inflow.For the co-rotating inflow,the fluctuation is near the hub.The pre-swirl flow has a great influence on the suction surface boundary layer flow,and occasionally small scale separation bubbles appear on the suction surface near the leading edge of some blades,while the re-attachment of separation flow is reduced for the counter-rotating inflow.A system consisting of a centrifugal impeller,a inlet-pipe with inlet guide vane,a extended outflow section is selected as the research object,and the influence of the inlet guide vane with different rotating directions and angle in the centrifugal impeller.It is found that with the decrease of flow rate,the periodicality of the relative flow angle disappears,and the flow becomes unstable in the impeller passage,and vortices of different scales appear.The high momentum flow will gradually move to the side of the shroud,and the instability of the flow can be partially suppressed for the inlet guide vane with a small angle Under the condition of small flow,the circumferential and radial velocity of different circumferential position in the channel is greater influenced by the couterclockwise rotation of the inlet guide vane.More backflow on the pressure side of the blade for the clockwise rotation of the inlet guide vane.And under the design flow condition,the circumferential and radial velocity at different heights is greater influenced by the rotation angle of the inlet guide vane.The radial force distribution gradually becomes ununiform as the flow rate decreases.The counter-clockwise inlet guide vane with 90° and 180° angle has the greatest influence on the flow stability of the impeller under the design flow condition and the small flow condition,respectively. |
