Research On Instability And Induced Vibration Of Leakage Flow In Blade Tip Clearance Of Steam Turbine

As a large rotary machine and one of major devices applied at a power station,steam turbine plays an important role in energy conversion and transmission.However,steam turbines are generally subject to the phenomenon of the large blade tip clearance during operations.As a result,a blade tip clearance leakage flow is generated,leading to blade tip leakage losses and decease in economic efficiency of the steam turbines.In addition,with the increase of unit capacity and increase of parameters,safe running of the steam turbine is also threatened seriously by the steam-induced vibration problem of blade tip clearance.At present,systematic studies have not been carried out on the vortex system structure and liquidity features of leakage flows of blade tip clearance.The ability of control technology to restrain tip clearance leakage needs to be developed and improved.The paper combines experimental research and numerical simulation to discuss unsteady flowing in the blade tip clearance,and study formation,development and interaction rules of vortex systems in blade tip seal cavities,mechanisms for clearance leakage losses and influences of blade tip steam structure parameters on clearance leakage flowing.Relations between instability of the blade tip clearance leakage flow vortex system and the pulsation of steam-induced excitation force are established.The purpose is to reveal the vortex system structure and flow characteristics of blade tip clearance leakage flows of a steam turbine as well as mechanisms concerning how clearance leakage vortexes induce generation of vibration.At first,steady numerical simulation of blade tip leakage flow under three different seal structures were studied.Details inside blade tip seal clearance vortex motion and interaction mechanisms of vortex systems were analyzed carefully.Development and changes of vortex systems in blade tip clearance were studied.Rules about influences of blade tip clearance structure changes on vortex motion and dissipation of blade tip leakage were explored.In addition,quantitative distribution of losses caused by the clearance leakage flow was given.As shown in the results,flowing in the blade tip seal cavity manifested a complicated vortex motion form with a circumferential spiral manner.Reduction of the blade tip leakage based on adjustment of the blade tip seal structure was mainly the control of circumferential speeds of the cavity leakage vortexes.Secondly,aiming at the unsteady flow field of the blade tip clearance formed after rotor eccentricity,comparative analysis was conducted on pressure pulsations in each cavity under the blade tip seal,which were obtained through experiments and numerical computation.Unsteadiness and changing rules of the blade tip seal cavity vortexes were analyzed.As shown in the results,in the eccentric unsteady flow field of one period,the steam flow speed of each seal cavity among the flat-teeth seal and high-low-teeth seal showed the time-based periodical changes: strong?weak?strong.However,in side-teeth seal cavities,influential scopes of large vortexes and shroud-wall vortexes showed periodical changes.This result indicates that the more complicated the vortexes in the blade tip end area are,the more obvious the instability of flow field will be.Then,steam-induced excitation force caused by rotor eccentricity was divided into steam-induced excitation force in the rotor passage and the steam-induced excitation force induced by the tip seal flow field.The steam-induced excitation force in the rotor passage was theoretically computed,so that a simple computation method applicable to the steam-induced excitation force in the rotor passage in engineering was obtained.Numerical computation was conducted on the steam flow excitation force induced by the tip seal flow field.The BVF was induced to closely associate the tip seal clearance flow field with force bearing on the shroud.Hence,flow field factors which cause changes in steam-induced excitation force of the tip seal were obtained to analyze changing rules of steam-induced excitation force induced by the uneven flow field of cavities with different steam seal structures.In the end,time analysis and frequency spectrum analysis were used to seek for correlations between evolutions of the blade tip clearance leakage flow vortex systems and the pulsation of steam-induced excitation force.This result was obtained: steam-induced excitation force was induced by unsteadiness of the seal clearance leakage flows: pulsation of the radial steam-induced excitation force on the blade tip was finally caused by the differences in pressure fluctuation ranges of large and small circumferential clearances of the blade tip seal flow field.The periodical change in distribution rules of the BVF of shroud,which was caused by the shroud-wall vortexes,was the major cause for fluctuations in the blade tip horizontal steam-induced excitation force.Research of the paper provides a theoretical foundation and a referential basis for revealing the mechanisms about how the blade tip seal clearance leakage flows induces the steam flow excitation force and further improving running safety and economic efficiency of turbines.