Christian Musch

Numerical Investigation on the Vibration of Steam Turbine Inlet Valves and the Feedback to the Dynamic Flow Field

The power output of steam turbines is controlled by steam turbine inlet valves. These valves have a large flow capacity and dissipate in throttled operation a huge amount of energy. Due to that, high dynamic forces occur in the valve which can cause undesired valve vibrations.In this paper, the structural dynamics of a valve are analysed. The dynamic steam forces obtained by previous computational fluid dynamic (CFD) calculations at different operating points are impressed on the structural dynamic finite element model (FEM) of the valve. Due to frictional forces at the piston rings and contact effects at the bushings of the valve plug and the valve stem the structural dynamic FEM is highly nonlinear and has to be solved in the time domain.Prior to the actual investigation grid and time step studies are carried out. Also the effect of the temperature distribution within the valve stem is discussed and the influence of the valve actuator on the vibrations is analysed.In the first step, the vibrations generated by the fluid forces are investigated. The effects of the piston rings on the structural dynamics are discussed. It is found, that the piston rings are able to reduce the vibration significantly by frictional damping. In the second step, the effect of the moving valve plug on the dynamic flow in the valve is analysed. The time dependent displacement of the valve is transferred to CFD calculations using deformable meshes. With this one way coupling method the response of the flow to the vibrations is analysed.© 2015 ASME

A New Emergency Stop and Control Valve Design: Part 2 — Validation of Numerical Model and Transient Flow Physics

In order to enhance steam mass flow through a turbine it becomes necessary to reduce the flow resistance of the turbine inlet valves. Consequently, a replacement of the high pressure turbine inlet valves is required. The valve combination described in this paper consists of a control valve and an emergency stop valve, opposite to the control valve. Both valves share a common valve seat. The control valve is a single-seat valve with integral pilot disc. A pre-stoke is introduced to allow for moderate opening forces. The emergency stop valve closes in countercurrent with the steam mass flow.The flow through the valve is analyzed by steady state and transient computational flow simulations. In addition to the steam mass flow, the forces acting upon the valve are determined. Transient behavior will be investigated by means of analyzing pressure fluctuations. Therefore frequencies caused by the steam flow are determined in the range up to 2000Hz.It will be shown that neither steady state nor transient simulations with a simple eddy viscosity turbulence model are capable to correctly predict the complex flow inside the valve. More sophisticated turbulence modeling like Large-Eddy simulation is thus inevitable. Furthermore, the physical phenomena causing the transient behavior are discussed. All findings are verified by comparison of the CFD with the measurements.Copyright

Combined optimisation of the last stage and diffuser in a steam turbine using meta-models

In this study a strategy for a 3D optimisation of the exhaust of a low pressure (LP) steam turbine is presented. The ?ow domain utilized consists of both the last stage and the exhaust diffuser. The optimisation is done with the help of a hybrid surrogate model for the diffuser ?ow. In the ?rst part of the paper a numerical model and its validation is presented, which allows for a precise simulation of the diffuser ?ow including the actual 3D geometry. The second part describes the optimisation procedure and the necessary simpli?cations applied to this model in order to get a numerical setup that is fast enough to actually perform a design study with roughly 200 design variants within a feasible time. This model is used afterwards to create a surrogate model. Based on this meta model an optimisation is carried out and ?nally scrutinzed with a ?ow simulation of the whole exhaust hood.

Numerical Investigation on the Time-Variant Flow Field and Dynamic Forces Acting in Steam Turbine Inlet Valves

The unsteady flow in inlet valves for large steam turbines used in power stations was investigated using the method of computational fluid dynamics (CFD). As the topology of the flow depends on the stroke and the pressure ratio of the valve, the flow was investigated at several positions. Various turbulence models were applied to the valve to capture the unsteady flow field. Basic Reynolds-averaged Navier–Stokes (RANS) models, the scale adaptive simulation (SAS), and the scale adaptive simulation with zonal forcing (SAS-F, also called ZFLES) were evaluated. To clarify the cause of flow-induced valve vibrations, the investigation focused on the pressure field acting on the valve plug. It can be shown that acoustic modes are excited by the flow field. These modes cause unsteady forces that act on the valve plug. The influence of valve geometry on the acoustic eigenmodes was investigated to determine how to reduce the dynamic forces. Three major flow topologies that create different dynamic forces were identified.

Optimization Strategy for a Coupled Design of the Last Stage and the Successive Diffuser in a Low Pressure Steam Turbine

In this study, an effective yet numerically simple approach for a coupled design of the last stage running blade and diffuser is presented. The method applied uses a 2-dimensional streamline curvature code combined with a boundary layer solver for the prediction of flow separation within the diffuser. An accurate representation of the diffuser flow is vital for the assessment of the overall performance. Thus the major influences from the turbine stage on the diffuser flow, i.e. the tip leakage jet and the swirl of the flow, are taken into account. Secondary effects like blade wakes are neglected. The basic capability of the method to correctly represent the flow is demonstrated by a comparison with 3-dimensional CFD simulations of a sample configuration. Solid correlation can be found between both cases. For the optimization process, a genetic algorithm is used. Optimization parameters include the blade exit angle and the diffuser contour. The results of the optimization are again scrutinized with the assistance of 3-dimensional CFD simulations.Copyright

Prediction of the windage heating effect in steam turbine labyrinth seals

Today’s steam turbine power plants are designed for highest steam inlet temperatures up to 620°C to maximize thermal efficiency. This leads to elevated thermal stresses in rotors and casings of the turbines. Hence, temperature distributions of the components have to be predicted with highest accuracy at various load points in the design process to assure reliable operation and long life time. This paper describes the windage heating effect in full labyrinth seals used in steam turbines. An analytical approach is presented, based on CFD simulations, to predict the resulting steam temperatures. A broad application range from very low to highest Reynolds numbers representing different turbine operation conditions from partial to full load is addressed. The effect of varying Reynolds number on the flow friction behaviour is captured by using an analogy to the flow over a flat plate. Additionally, the impact of different labyrinth geometries on the friction coefficient is evaluated with the help of more than 100 CFD simulations. A meta-model is derived from the numerical results. Finally, the analytical windage heating model is validated against measurements. The presented approach is a fast and reliable method to find the best performing labyrinth geometries with lowest windage effects, i.e. lowest steam temperatures.

Performance Increase of Steam Turbine Condensers by CFD Analysis

An effective measure to increase the performance of turbine power plants is to minimize flow losses in the condenser resulting in a smaller terminal temperature difference (TTD) — the more the TTD of the condenser can be reduced in an optimization process of a given power plant configuration, the more the exhaust pressure will decrease. With an optimized condenser tube bundle design the TTD can be improved.This study presents the modification of a commercial CFD code to simulate the three-dimensional flow field around and within tube bundles. Additionally the temperature distribution of the cooling water is part of the numerical solution without modeling each individual condenser tube.To show the accuracy of the CFD code the flow in a large scale power plant condenser is simulated and compared to measurements of local heat transfer coefficients in the bundles. The comparison shows that the presented CFD tool is valid to predict the performance of such condensers.Based on the results of the study, areas with low cooling performance are identified and suggestions are made for the increase of the overall condenser efficiency.Copyright