Flow fields in low-pressure (LP) steam turbines, starting expansion well above the saturation line, show four pronounced non-equilibrium processes (called relaxation processes). The first one above the saturation line is centred around heterogeneous nucleation/ condensation, the second one around subcooling and the subsequent homogeneous nucleation/ condensation ('Wilson point'), and the third and the fourth ones are characterized by thermodynamic and mechanical effects in the established droplet-loaded part of the flow field.All these relaxation processes are interacting downwards in the progressive expansion in the turbine. In a multistage LP turbine, the most important ones are the second -because it is mainly responsible for the number of droplets (sizes) -and the third -because it creates most of the dissipation. In addition, the first and the fourth ones can damage the flow guiding geometry by corrosion and erosion, respectively.More than 40 years ago, Gyarmathy formulated the first strictly physical basis for these processes. Subsequently, many researchers have contributed to increase the physical understanding using both experimental and numerical methods. The intuition that the one-dimensional treatment of the flow field in multistage turbines cannot explain the measured droplet sizes behind the blading has been particularly relevant. It became clear that especially the temperature fluctuation within the blading has to be included in homogeneous nucleation/condensation considerations. Also of some importance has been the improved understanding of the first relaxation process, which was initially underestimated.This article highlights the current situation seen by a turbomachinery company that has been contributing to and supporting this discipline for many decades. A wide literature survey and a critical appraisal of published Baumann factors are included. High quality experimental tools and procedures are introduced on the basis of two generations of split-shaft model LP turbines and Damköhler numbers. Further, an overview of the in-house numerical tools and processes developed for wet steam applications is given.More recent experimental results on the influence of impurities and conditioning agents in the relaxation processes and newer numerical results on the influence of phase transition in the flow field around a blade row are presented.

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.

In the past it was found experimentally and numerically that eddy shedding on trailing edges of turbine aerofoils has significant effect on the aerodynamic performance. Aerofoils of gas turbines usually have relatively thick trailing edges for reasons of mechanical integrity, and hence strong unsteady wakes are created. The current investigation considers trailing edges without cooling injection; hence the area of application is limited to the low-and medium-pressure stages of a gas turbine. In this work a numerical investigation is performed with the aim to weigh the loss production inside these unsteady wakes against the available kinetic energy of the unsteady fluid to formulate a so-called unsteady recovery factor. Furthermore, several design exercises are presented on a gas-turbine rotor profile, which is interacting with the unsteady wake of an upstream vane with eddy shedding. This shows that the stage performance is quite significantly influenced comparing frontloaded, midloaded, and aftloaded profile designs in contradiction to standard steady design approaches. It was found that a frontloaded rotor design under consideration of interaction with an unsteady wake can increase the stage performance of a two-dimensional midsection considerably, while under steady assumptions it has about the same or worse performance. This result shows the importance of unsteady methods in turbine design and also potential new ways for improved profile design.

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