• Energy Research

Wall-detached flow inside an ultra-supercritical steam turbine control valve was comprehensively investigated with detached-eddy simulation, proper orthogonal decomposition (POD), and flow reconstruction. The dependency of the wall-detached flow on the control valve’s opening ratio and pressure ratio was established first. Scattered wall-detached-flow, merged wall-detached-flow, and intersected wall-detached-flow were then identified by distinguishing the detachment scale of the wall-detached jet. Subsequently, flow analysis was conducted in terms of the statistical flow quantities, i.e., velocity fluctuation, turbulent kinetic energy, pressure loss, and pressure fluctuation. The statistical results demonstrated that the merged wall-detached-flow facilitated the most intensive velocity and pressure fluctuations inside the steam turbine control valve. The intersected wall-detached-flow encountered significant shock-wave reflections along the downstream pipe. By conducting POD analysis and flow reconstruction on the instantaneous flow snapshots, the dominant vortex structures and energetic pressure fluctuation modes were extracted to illustrate the wall-detached flow’s unsteady behavior. The results showed that the instabilities of the scattered wall-detached-flow were primarily represented by the horizontal flapping motion of the wall-detached jet. However, for the merged wall-detached-flow, both the vertical out-phase oscillation and the horizontal flapping motion of the wall-detached jet intensified, yielding essential axial pressure fluctuation modes. As for the intersected wall-detached-flow, due to the complex wave reflections and propagations, essential regions with velocity discontinuities and diagonal crosslines with intensive pressure fluctuations formed inside the valve pipe. These findings are of great practical significance for the operation and optimization of steam turbine control valves in thermal power plants.

Abstract The unsteady flow behavior of a steam turbine control valve in the choked condition, which occurs during the warming-up process of a steam turbine, was studied through complementary techniques, including field measurements of the valve spindle’s vibration behavior, detached eddy simulation (DES) of the unsteady flow field, and acoustic modal analysis of the valve chamber. Three peak frequencies at St = 0.044, St = 0.17, and St = 0.59 were identified from field measurements of the vibration behavior. Subsequently, the unsteady flow fields in the control valve were determined from DES and then analyzed using the state-of-the-art data-driven proper orthogonal decomposition (POD) method and cross-correlation analysis, which extracted the dominant unsteady flow behavior in relation to the valve spindle’s vibrations. The findings demonstrated that the valve spindle’s lateral force fluctuations at St = 0.019 occurred due to the alternating oscillations of the annular wall-attached jet, which resulted from the first two POD modes occupying 25% of the turbulent fluctuation energy. The valve spindle’s axial force fluctuations at St = 0.043 were attributed to the synchronous oscillations of the annular wall-attached jet, which resulted from the third, fourth, and fifth POD modes occupying 15% of the turbulent fluctuation energy. Finally, through acoustic modal analysis that compared the pressure fluctuations extracted from the DES results, the axial acoustic mode in the valve’s cavity at St = 0.19 was found to be associated with the valve spindle’s axial force fluctuations at St = 0.174, while the first circumferential acoustic mode of the valve diffuser at St = 0.61 was found to be associated with the valve spindle’s lateral force fluctuations at St = 0.62. This confirmed the potential coupling between the acoustic mode pattern and pressure fluctuation pattern. These complementary techniques were demonstrated to be effective methodologies for the flow-induced vibration behavior and intensive acoustics in valves.

Due to the practical space limitation, the control valve in industrial utilities is usually immediately followed by a short flow passage, which would introduce considerable complexity into highly unsteady flow behaviors, along with the flow noise and structure vibration. In the present study, the unsteady behaviors of the steam flow inside a control valve with a T-junction discharge, when the valve operates under the choked condition, are numerically simulated. Toward this end, the detached eddy simulation (DES) is used to capture the spatiotemporally varying flow field in the serpentine flow passage. The results show periodic fluctuations of the aerodynamic forces on the valve spindle and periodic fluctuations of the pressure and flow rate at the two discharge outlets. Subsequently, proper orthogonal decomposition (POD) analysis is conducted using the velocity field and pressure field, identifying, respectively, the dominant coherent structures and energetic pressure fluctuation modes. Finally, the extended-POD method is used to delineate the coupling between the pressure fluctuations with the dominant flow structures superimposed in the highly unsteady flow field. The fourth velocity mode at St = 0.1, which corresponds to the alternating oscillations of the annular wall-attached jet, is determined to cause the periodic flow imbalance at the two discharge outlets, whereas signatures of the first three modes are found to be dissipated in the spherical chamber. Such findings could serve as facts for vibration prediction and optimization design. Particularly, the POD and extended-POD techniques were demonstrated to be effective methodologies for analyzing the highly turbulent flows in engineering fluid mechanics.

Abstract Transient thermal behaviors of ultra-supercritical steam turbine control valves during the cold start warm-up process of steam turbine systems were comprehensively studied using conjugate heat transfer (CHT) simulation. The geometrical configurations and boundary conditions used in simulation were identical to the field setup in a thermal power plant. The simulated temperature variations were first validated using measurements by the flush-mounted thermocouples inside the solid valve bodies. The CHT simulation implementing the shear stress transport (SST) turbulence model demonstrated good agreement with the field data, and the overall numerical errors were below 10%; however, the numerical errors of the simulation, which used empirical heat transfer coefficients at the fluid–solid interfaces, reached 40%. The determined temperature differences between the cold valve bodies with the hot steam flow decreased significantly. Specifically, the temperature differences along the inner wall surfaces of the valve bodies decreased to less than 50 °C. Further investigation of the transient heat flux distributions and Nusselt number distributions confirmed that the unsteady flow behaviors, such as the alternating oscillations of the annular wall-attached jet, the central reverse flow and the intermediate shear layer instabilities, enhanced the fluid–solid heat convection process and thus contributed to the warming up of the solid valve bodies.

Abstract The steam flow in a bell-shaped control valve, located before an intermediate pressure cylinder of an ultra-supercritical steam turbine unit, was numerically investigated. Toward this end, the Reynolds-averaged Navier–Stokes (RANS) simulation approach using a shear stress transport (SST) turbulence model was first validated against wall pressure measurements in tests using a scaled valve model. The dependency of the full-scale valve’s steam flow patterns on its open ratio and pressure ratio was then established. The annular attachment flow and central detachment flow were classified and analyzed in terms of flow quantities including velocity vectors, Mach number contour, pressure distribution and turbulent kinetic energy contour. The numerical results demonstrate that the attachment flow was produced mainly by the annular wall-attached jet and the central reverse flow, whereas the detachment flow was mainly produced by the central detached-jet expansion and the ambient reverse flow. Further analysis of the flow field indicates that the transition from attachment flow to detachment flow could be satisfactorily explained in terms of the Coanda effect and was influenced mainly by the ratio of the valve seat’s radius to the valve’s lift displacement. In addition, when decreasing the valve’s pressure ratio to the critical detachment pressure ratio, two phenomena were observed: discrete elliptical regions with jet’s expansion-and-recompression in the Mach number contour and a spatial variation at the pressure profile along with reducing magnitude, which was induced by the interaction between the expansion wave and the compression wave in a Prandtl–Mayer expansion process. These findings are of great practical significance for the valve’s operation and design optimization.

Abstract The influence of a circular strainer on unsteady flow behavior in steam turbine control valves, which are commonly placed between an intermediate-pressure turbine and a boiler in thermal power plants, was numerically studied. A porous-medium model, which established the dependencies of the pressure drop through the strainer on the magnitude and direction of the fluid flow’s velocity, was validated by experimental measurements in a water flow test rig. 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, significantly reducing the cross-sectional force on the main valve’s spindle; this is attributed to attenuated oscillation of the annular flow around the main valve’s seat. As for the downstream throttle valve, the pressure drop and the fluctuating lateral force on the spindle were intensified, which was shown to be closely related to the continuous impingement of the flow onto the throttle valve’s cavity wall. In comparison with the configuration without a strainer, the placement of the strainer in the main valve gave rise to a pair of intensified secondary vortices in the diffuser section behind the throttle valve.