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.

The demand for energy is increasingly covered through renewable energy sources. As a consequence, conventional power plants need to respond to power fluctuations in the grid much more frequently than in the past. Additionally, steam turbine components are expected to deal with high loads due to this new kind of energy management. Changes in steam temperature caused by rapid load changes or fast starts lead to high levels of thermal stress in the turbine components. Therefore, todays energy market requires highly efficient power plants which can be operated under flexible conditions. In order to meet the current and future market requirements, turbine components are optimized with respect to multi-dimensional target functions. The development of steam turbine components is a complex process involving different engineering disciplines and time-consuming calculations. Currently, optimization is used most frequently for subtasks within the individual discipline. For a holistic approach, highly efficient calculation methods, which are able to deal with high dimensional and multidisciplinary systems, are needed. One approach to solve this problem is the usage of surrogate models using mathematical methods e.g. polynomial regression or the more sophisticated Kriging. With proper training, these methods can deliver results which are nearly as accurate as the full model calculations themselves in a fraction of time. Surrogate models have to face different requirements: the underlying outputs can be, for example, highly non-linear, noisy or discontinuous. In addition, the surrogate models need to be constructed out of a large number of variables, where often only a few parameters are important. In order to achieve good prognosis quality only the most important parameters should be used to create the surrogate models. Unimportant parameters do not improve the prognosis quality but generate additional noise to the approximation result. Another challenge is to achieve good results with as little design information as possible. This is important because in practice the necessary information is usually only obtained by very time-consuming simulations. This paper presents an efficient optimization procedure using a self-developed hybrid surrogate model consisting of moving least squares and anisotropic Kriging. With its maximized prognosis quality, it is capable of handling the challenges mentioned above. This enables time-efficient optimization. Additionally, a preceding sensitivity analysis identifies the most important parameters regarding the objectives. This leads to a fast convergence of the optimization and a more accurate surrogate model. An example of this method is shown for the optimization of a labyrinth shaft seal used in steam turbines. Within the optimization the opposed objectives of minimizing leakage mass flow and decreasing total enthalpy increase due to friction are considered.

In this study a strategy for a 3D optimisation of the exhaust of a low pressure (LP) steam turbine is presented. The flow 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 flow. In the first part of the paper a numerical model and its validation is presented, which allows for a precise simulation of the diffuser flow including the actual 3D geometry. The second part describes the optimisation procedure and the necessary simplifications 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 finally scrutinzed with a flow simulation of the whole exhaust hood.

In steam turbine inlet valves used to adjust the power output of large steam turbines, the through-flow is reduced by lowering the valve plug and hence reducing the cross-sectional area between the plug and the seat. At throttled operation, a supersonic jet is formed between the plug and the seat. This jet bearing tremendous kinetic energy flows into the valve diffuser where it is dissipated. Depending on the dissipation process, a certain portion of the kinetic energy is converted to sound and subsequently to structural vibration, which can be harmful to the valve plug. The flow topology in the valve diffuser has a strong influence on the conversion of kinetic energy to sound and hence vibrations. Several studies show that an annular flow attached to the wall of the valve diffuser causes significantly less noise and vibrations than a detached flow in the core of the diffuser. The relation between the flow topology and the vibrations is already known, but the physics causing the transition from the undesired core flow to the desired annular flow and the dependency on the design are not fully understood. The paper presented here reveals the relation between the flow topology in the steam valve and the separation of underexpanded Coandă wall jets. The physics of the jet separations are clarified and a method to predict the flow separations with a low numerical effort is shown. Based on this, safe operational ranges free of separations can be predicted and improved design considerations can be made.

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.