| The ultra-supercritical(USC)power generation is the most advanced technology in modern coal-fired power plant,due to its safety operation,unit durability and high efficiency.As the important control system of the ultra-supercritical steam turbines,the steam turbine control valves are commonly used to regulate the mainstream steam flow for the thermal power plants.The control valves are generally integrated by a main valve and a throttle valve in sequence.The main valve is used to keep the safety of the whole steam turbine system by rapidly cutoff the steam supply in emergency;and the throttle valve has to work under wide conditions to adjust to the variant output power of thermal plants.In practice,the mainstream steam flow with high pressure and high temperature could bring additional thermal stresses to the control valves.In addition,the compact configuration of the main valve and throttle valve would creat a complicated serpentine flow passage,may induce a highly unstable steam flow to the control valves.Even worse,the steam flow pattern of control valves can be various with the valve's open ratio and pressure ratio by considering the wide range of operation conditions.Previous studies had classified the valve's flow patterns into two categories,i.e.the attachment flow and detachment flow;which were always accompanied with the unstable flow structures such as the rotating pressure fluctuations,the impingement of shear flow,flow separations and reattachments.The unsteady flow behaviors of these flow structures can also be coupled with valve's acoustic modes,inducing to intensified noise;coupled with valve's structural modes,inducing to structural vibrations or even operational risk.Consequently,further understanding of the unsteady flow behaviors inside the control valves is of great practical significance to the effective operation management.This paper focused on the unstable steam flow of the control valves in a domestic ultrasupercritical steam turbine unit with second reheat.The time-averaged steam flow patterns and time-variant steam flow behaviors inside the control valves are captured with the Shear Stress Transport(SST)and Detached Eddy Simulation(DES)turbulence model,respectively.Based on the advanced analysis methods,which contain with the Proper Orthogonal Decomposition(POD),spatial and temporal cross-correlation analysis and acoustic modal analysis,the huge amounts of numerical data were deep excavated and analyzed,revealing the vortex dynamics and the acoustic response of the unsteady attachment and detachment flow inside the control valves.Towards this end,four research points were conductued in present study,i.e.influence of strainer on valve's time-averaged flow patterns,classification of valve's time-averaged flow patterns,unsteady flow behaviors of valve's attachment and detachment flow,validation on the flow-induced valve's vibrations at the steam turbine's warmup condition.To reveal the influence of the circular strainer on the flow patterns of steam control valves,a water flow test rig was built to validate the strainer porous model,which established the dependencies of the pressure drop through the strainer on the magnitude and direction of the mainstream flow's velocity.As the benchmark configuration,a valve without a strainer was used for comparison.The turbulent steam flow in the complex serpentine channel was simulated with the implementation of the proposed porous model for the strainer.The numerical results demonstrated that placing the strainer in the main valve resulted in dramatic changes of the flow patterns in the main valve's chamber and its diffuser,and even in the downstream throttle valve.The complex steam flow in the main valve was efficiently managed by the circular strainer especially at the fully opening of throttle valve;this is attributed to attenuated oscillation of the annular flow around the main valve's seat.However,the influence of strainer on the valve's flow pattern became weak at the small opening of throttle valve.Therefore,the configuration of strainer or even the main valve was removed to save the numerical cost in the subsequent simulations of throttle valve at small opening ratios.To classify the time-averaged flow patterns of the control valve corresponding with variant opening ratio and pressure ratio,a high pressure valve test rig was built to validate the ability of SST model to compute the valve's flow characteristic and pressure profile.Subsequently,the valve's attachment flow was identified with higher back pressure;while the detachment flow was captured when the back pressure was smaller than the critical detachment pressure.Furthermore,deep understanding showed that the transition from valve's attachment flow to detachment flow was closely related with the Coand? effect and the jet's expansionrecompression processes.Following the identifications of the time-averaged flow patterns,the DES simulations were performed to capture the unsteady flow behaviors corresponding with the attachment and detachment flow,respectively.Another high pressure valve test rig was especially built to evaluate the ability of DES model to capture the amplitudes and frequencies of the pressure fluctuations.Additionally,the POD analysis,cross-correlation analysis and acoustic modal analysis were performed on the numerical flow fields,indicating that: the alternating oscillations of wall-attached-jet or wall-detached-jet dominated the fluctuated flow fields of valve,resulting in the axial pressure modes and lateral pressure modes,which can act on the valve's spindle and induce the valve's axial vibrations and lateral vibrations.Moreover,the axial acoustic modes and the first circumferential acoustic mode were confirmed coupling with the unstable flow fields,which could no doubt lead to extensive noise.Finally,the flow-induced valve's vibrations at the wamup condition during the steam turbine's cold state startup,were comprehensively analyzed by using the combinatory method which contains with DES simulation,POD decomposition method,cross-correlation method and acoustic modal analysis.The fluctuation frequencies of valve's aerodynamic forces were extracted from the DES resolved unsteady steam flow.By comparison with vibration frequencies of valve's field measurements in power plant,it was concluded that the valve's vibrations were directly induced by the fluctuated forces.The results demonstrated that the control valve's axial vibration at St=0.174 was related with the axial acoustic mode in valve's cavity,and the cavity shear layer oscillations were found as the flow excitation source;however,the lateral vibration at St=0.62 was associated with the first circumferential mode of control valve,and the periodical pressure fluctuations generated by the turbulent mixing process were confirmed as the flow excitation source.By POD analysis,the first two POD modes occupied about 25% energy of the turbulent fluctuation,corresponding to valve's lateral vibration at St=0.019,were found resulting from the alternating oscillations of the annular wall-attachedjet.Simultaneously,the modes 3,4 and 5 occupied about 15% energy of the turbulent fluctuation,corresponding to valve's axial vibration at St=0.043,were found resulting from the synchronous oscillations of the annular wall-attached-jet. |
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