Thomas Mokulys

A Three-Dimensional Diffuser Design for the Retrofit of a Low Pressure Turbine Using In-House Exhaust Design System

The performance of the last stage of a Low Pressure (LP) steam turbine is strongly coupled with the downstream exhaust hood performance. In particular, the effect of the diffuser within the exhaust hood on the pressure recovery is very important in retrofitting existing machines, which dictate many geometric constraints. Alstom’s in-house Exhaust Design System (EDS) simulates the three-dimensional flow in the exhaust hood by coupling the last stage blades and the exhaust hood. This EDS system can be used to design an LP diffuser in the exhaust hood and to achieve the required performance targets. In the first part of this paper, the EDS system is validated against measurements within model turbines, which represent both a standard machine as well as a retrofit machine. In the second part of this paper, an LP diffuser was redesigned to improve the performance using the EDS method. To begin with, an axi-symmetric diffuser was designed using numerical simulations of a passage in the last stage turbine as well as a slice of the diffuser and the exhaust hood. By carefully controlling the profile of the diffuser casing, the flow separation at the original casing walls was reduced significantly and this, in turn, improved the performance of the turbine substantially. Then, the full geometry of the exhaust hood was modeled in order to investigate the effect of the three-dimensional flow features. Based on the examined flow features, an asymmetric change was introduced to the diffuser casing to improve the three-dimensional flow structure. This new asymmetric diffuser was found to maximize the exhaust performance.Copyright

Validation of Conjugate Heat Transfer Predictions on Labyrinth Seals and Novel Designs for Improved Component Lifetime

Cyclic lifetime assessment of steam turbine components has become increasingly important for several reasons. In the last years and decades the nominal steam temperatures and pressures were further increased to improve cycle efficiency. In addition, the market constantly demands increased flexibility and reliability for given lifetime exploiting the limits of the existing materials. A number of components in a steam turbine are critical in the focus of lifetime predictions such as the rotor and front stage blades, the inner casing and the area of labyrinth seals connected to the life steam. For this reason, it becomes extremely important to rely on accurate predictions of local temperatures and heat-transfer-coefficients of components in the steam path. The content of this paper aims on the validation of the numerical tools based on CHT (conjugate heat transfer) approach against experimental data of a labyrinth seal regarding discharge coefficients and measured heat transfer coefficients. Furthermore, a real steam turbine application has been optimized in design and operation to improve lifetime. The improved prediction of temperature and heat transfer allowed novel designs of labyrinth seals of a single flow high-pressure turbine and a combined intermediate and low-pressure turbine, which helped to strongly increase the component lifetime of a steam turbine rotor by more than 100%.Copyright

Experimental and Numerical Investigation Into the Aerodynamics of a Novel Steam Turbine Valve and its Field Application

Control valves are one of the key steam turbine components that guarantee operational safety in a power plant.There are two aerodynamic aspects, which are the current focus for the development of Alstom’s valves. One is the reduction of the aerodynamic loss to increase the efficiency of the power plant. The other is operational flexibility, which is increasingly demanded to react faster to load requirements from the electric grid. This is becoming more important as power generation becomes increasingly decentralized, with a growing contribution from renewable energy sources. For this reason, a fast control loop is required for valve operation, which depends on an accurate linearization of the valve characteristic.In this paper the flow fields in an existing steam control valve have been analysed and subsequently optimized using CFD techniques. The approach specifically designed for drilled strainers is further illustrated. Following the validation of the baseline design with model testing, an improved diffuser has been designed using CFD analysis and the resulting performance benefit has been confirmed with further testing.The required grid frequency support requires control valve throttling. For this reason, an accurate prediction of the linearization table is extremely important to support the required response time limits. Further numerical work has been carried out with various opening positions of the valve, leading to an improved valve linearization characteristic. It is demonstrated that the numerical prediction of the linearization curve agrees very well with data obtained from operating power plants.Copyright

An Analysis of the Merits of CFD for the Performance Prediction of a Low Pressure Steam Turbine Radial Diffuser

The performance of the last stage and the diffuser of a low pressure steam turbine module are inherently linked. Design alterations in one part must take into consideration the subsequent effects imposed upon the other. Through a linked design system, including both the last stage and the exhaust, the last stage performance can be predicted more accurately and hence further performance improvements are possible. The importance of a valid prediction for the effects of the diffuser upon last stage power and efficiency become even more important when considering retrofit applications. In a retrofit project, a new flow path including modernised blades are installed in an existing older steam turbine casing. In these cases, the exhaust geometry does not lend itself to allowing the last stage to perform to its originally predicted level, but an optimization has to be established for a diffuser design, that links the last stage blade to the exhaust. This paper presents the coupled CFD design system, used by Alstom Power, to highlight the importance of including the 3D exhaust geometry, coupled to the last stage, when conducting last stage performance calculations. The test case is of a typical retrofit application. A comparison with measured test data shows the significance of the fidelity of any CFD system claiming to predict last stage performance, especially when used during the optimisation process of the diffuser.Copyright

Comparison Between Hot Wire and 5-Hole Pressure Probe Traverse Data in a Variable Density Two-Stages Air Turbine

This paper presents the comparison between hot wire and 5-hole pressure radial traverses carried out in a variable density two stages air turbine, available at the Alstom Power test facility in Rugby, UK. In this centre accurate and innovative measurement techniques are employed in support of the product validation process. Hot wire anemometry has been extensively used in environments were the flow density is constant, such as in open circuit wind tunnels or water tunnels. One of the intents in this series of tests was to derive a suitable hot wire calibration in a flow where the density could be two to four times higher than that at standard ambient conditions. This works analyses the traverse data performed at turbine inlet and exit planes by means of the two measuring techniques. The mean velocity profiles obtained with the 5-hole pressure probe are compared with those derived from a single hot wire. This is done by means of both two-dimensional and contour plots. The traverse data are also compared with the CFD results determined during the design of the blade being tested.Copyright