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.

Flowfields within turbines are generally three-dimensional and unsteady. Typically, the flow-fields in the interaction zone between the last stage and the diffuser are strongly inhomogeneous. This non-uniformity strongly limits the downstream diffusion process, so that significant improvement can be achieved by making the total pressure field in this zone more uniform. One way to achieve this is by using kinks in the endwall contours and, if necessary, one or two splitters. The use of balance-based space averaging and modelling procedures can help to characterize these flows, and to develop an optimum interaction zone and diffuser geometry. In the study described here, as an example the interaction zone of high-pressure/intermediate-pressure (HP/IP) and low-pressure (LP) diffusers of steam turbines were numerically and partly experimentally optimized for a fixed blading and exhaust. The operating conditions were also kept constant except that for low pressures the flowfield was studied for a range of back pressures (i.e. exhaust velocities). The optimization process starts with an initial flowfield in the interaction zone generated numerically or experimentally. Using these data a design procedure is applied that creates both a much more uniform total pressure field at the last stage exit and a diffuser geometry possibly with one or two splitters and proven for an earlier LP design experimentally. It was demonstrated that, depending on the inhomogeneity of the flow from the upstream stage, a performance improvement of several percentage points could be achieved.

On the basis of their inherent favourable aerodynamic properties coupled with past progress, 50 per cent reaction stages already achieve a high efficiency level. Developments aimed at further performance enhancement entail employment of advanced design features that require a deep understanding of the flow phenomena involved and their interactions. In addition, substantial on-going efforts are needed to improve the quality of the design tools. This paper focuses on the key design issues, including advanced quasi-three-dimensional and three-dimensional design aspects. It further describes developments by the authors' company during the last decade for the design of modern reaction blading and establishment of state-of-the-art design tools.

A numerical method for calculating three-dimensional, steady or unsteady, incompressible, viscous flow is described. The conservation equations for mass and momentum and the equations of the k–ε turbulence model are solved with a finite volume method on nonorthogonal boundary-fitted grids. The method employs cell-centered variable arrangement and Cartesian velocity components. The SIMPLE algorithm is used to calculate the pressure and to enforce mass conservation. The computer code is vectorizable as far as possible to achieve an optimal performance on modern vector computers. Results of steady flow calculations in the guide vane, the pump rotor, and the turbine rotor and of the unsteady interaction simulation of the pump and the turbine of a one-stage one-phase non-automotive hydrodynamic torque converter are presented.

A numerical method for calculating threedimensional, steady or unsteady, incompressible, viscous flow is described. The conservation equations for mass and momentum and the equations of the kε-turbulence model are solved with a finite volume method on nonorthogonal boundary-fitted grids. The method employs cell-centered variable arrangement and Cartesian velocity components. The SIMPLE-algorithm is used to calculate the pressure and to enforce mass conservation. The computer code is vectorizable as far as possible to achieve an optimal performance on modern vector computers. Results of steady flow calculations in the guide vane, the pump rotor and the turbine rotor and of the unsteady interaction simulation of the pump and the turbine of a one-stage one-phase non-automotive hydrodynamic torque converter are presented.