Thomas Zierer

Numerical Simulation of the Unsteady Flow Field in an Axial Gas Turbine Rim Seal Configuration

The fluid flow in gas turbine rim seals and the sealing effectiveness are influenced by the interaction of the rotor and the stator disk and by the external flow in the hot gas annulus. The resulting flow structure is fully 3-dimensional and time-dependant. The requirements to a sufficiently accurate numerical prediction for front and back cavity flows are discussed in this paper. The results of different numerical approaches are presented for an axial seal configuration. This covers a full simulation of the time-dependant flow field in a 1.5 stage experimental turbine including the main annulus and both rim cavities. This configuration is simplified in subsequent steps in order to identify a method providing the best compromise between a sufficient level of accuracy and the least computational effort. A comparison of the computed cavity pressures and the sealing effectiveness with rig test data shows the suitability of each numerical method. The numerical resolution of a large scale rotating structure that is found in the front cavity is a special focus of this study. The existence of this flow pattern was detected first by unsteady pressure measurements in test rig. It persists within a certain range of cooling air massflows and significantly affects the sealing behaviour and the cavity pressure distribution. This phenomenon is captured with an unsteady calculation using a 360 deg. computational domain. The description of the flow pattern is given together with a comparison to the measurements.Copyright

Simple Particle Separator for the Secondary Air System of Gas Turbines

In heavy-duty gas turbines as well as in aero-engines, air is extracted from the compressor and led to the hot parts of the combustor and the turbine in order to cool them. Despite active design solutions such as material selection, and inclusion of compressor inlet filters, dust holes, and so on, the cooling air can be charged with solid particles, which can block the cooling holes. Therefore prediction of the particle behaviour within the secondary air system remains crucial for the design of a robust and efficient cooling system for the hot parts.For this study a particle separator prototype was designed by Alstom and its particle separation efficiency together with its total pressure losses were measured at the Institute of Thermal Turbomachinery (ITS) at the Karlsruher Institute of Technology (KIT) for two geometrical configurations and numerous flow conditions. The test rig design was optimized to provide accurate boundary conditions for the simulations. In addition, the influence of the particle shape, size, and density on the separation efficiency was studied.The experimental results were used to validate the predicted flow field and to evaluate standard methods available in a commercial CFD-solver, to simulate the interaction of solid particles with turbulent flows and the containing walls. Comparisons between the measured and calculated separation efficiencies were performed for spherical and flat particles with different Stokes numbers. In particular, the way in which a simple modelling approach used for the prediction of sphere trajectories can be transferred to flat particles was investigated. Finally this study delivers generic data for improved modelling of solid particles, like spheres and flat particles, in turbulent flows.© 2012 ASME

Optimization of Anti-Icing Limits for Alstom Gas Turbines Based on Theory of Ice Formation

Over the past years, Alstom gas turbines have been protected against icing based on a set of ambient temperature and relative humidity limits. These limits were derived mainly from operational and fleet experience. In recent times, the potential for optimizing these limits arose as they were observed to be too conservative. It is recognized that lowering the icing limits by a better understanding of the formation of condensate ice offers an opportunity for engine performance optimization while simultaneously ensuring adequate protection of the engine hardware. However, the level to which the original limits could be extended has not been known and this necessitated the setting up of a dedicated project to address the issue. This paper presents part of the results of the work done within this project and addresses how the new limits have been derived based on the thermodynamics of ice accretion at stationary and rotating surfaces of the compressor. The theory of ice accretion on the variable inlet guide vane (VIGV) and compressor blade surfaces as the intake air is expanded through the GT inlet system presented in this paper covers the process of condensation of moist air, the solidification of the condensate and the accumulation of the sub-cooled water condensate on surfaces with temperatures below 0°C. Using a state-of-the-art gas turbine modelling environment, relevant thermodynamic quantities including static and velocity components up to the first rotating plane of the compressor have been used to quantify the amount of condensate in the intake air at the first compressor rotating plane at various ambient conditions of temperature and humidity and at various engine operation modes (base load and part load operation). Empirical in-house relations for surface temperatures have been used to estimate the VIGV and the surface temperature of the first blade of the compressor. The theoretical results obtained have been validated on a heavy-duty gas turbine engine. Based on the confirmation of the theoretical results with engine data, the presented method can accurately be used to determine the anti-icing limits for a gas turbine. The approach is a generic one and is therefore applicable to all compressor designs for stationary gas turbines.Copyright