Keywords: CFD, steam turbine design, blade-path design, labyrinth seal, exhaust hood.

CFD techniques provide a platform for enhanced understanding of the complex fluid dynamic mechanisms in turbomachines. Some of the progress that has been achieved by using modern CFD methods has been reported in [1,2,3,4]. Within the present paper, the capabilities of modern commercially available CFD codes are demonstrated using three typical applications in the field of steam turbine engineering. It is shown that CFD techniques provide an in-depth physical insight into complex three-dimensional flow features and lead to enhanced understanding of the fluid dynamic mechanisms.

A short summary of the mathematical and physical background of CFD is given, hereby referring especially to [5,6,7]. Considering a four stage model turbine, good agreement between the measured performance chart and the numerical predictions is achieved. Furthermore, it is shown, that the interaction between stator and rotor blade rows and their individual response to the flow field leaving the upstream blade row must be accounted for within the numerical calculation. As a consequence, high efficiency bladepath designs using CFD methods requests the code's capability to model at least a complete turbine stage, if not the entire turbine.

A labyrinth seal configuration was analysed with different mesh types. The results computed on an unstructured mesh compared well to those obtained on a structured mesh. Basic labyrinth seal flow features are discussed and related to literature [8]. The CFD analysis includes a systematic study of the radial gap influence on the leakage flow rate of a 3-strip seal configuration. The dimensionless leakage flow rate was calculated from the CFD results according to a special leakage flow correlation used by Siemens PG. The results compared excellent to measured data taken from literature [9] for a seal with 30 strips operating in air. This indicates the reliability of the CFD results and the universal applicability of the Siemens PG leakage flow rate correlation even for extremely different conditions.

Turbine down exhausts are generally of complex 3D nature and therefore predestined for application of unstructured meshes. Comparisons between structured meshes and unstructured meshes show that the results regarding static pressure distribution were almost the same. In order to demonstrate the flexibility obtained with such unstructured meshes, some results are presented taken from an exhaust hood study done for a service project. The final exhaust hood design was defined based on this study in order to guarantee a stable operation and a high power output over a wide operating range.

It is the authors strong opinion that CFD has reached such a sophisticated state these days that it is much more than just complementary to experimental investigations. CFD not only drastically reduces the design cycle time as well as the number of cost-intensive and time consuming experiments, but it has started to play an indispensable role in the development of ever more advanced steam turbines and can now be readily employed on a day-to-day basis in the design process. In fact, it is fair to say that the new generation of advanced steam turbines will be based primarily upon computer simulations, being supported by only a few - but nevertheless invaluable - experiments to calibrate these computations, whilst in the past a large number of very expensive and time-consuming experiments were needed to be carried out to obtain this information.

1

M. Jansen, W. Ulm, "Modern Blade Design for Improving Steam Turbine Efficiency", VDI-Berichte Nr. 1185, 1995.

2

H. Oeynhausen, A. Drosdziok, M. Deckers, "Advanced Steam Turbines for Modern Power Plants", IMechE Paper No. C522/032, 1997.

3

J.I. Cofer, "Advances in Steam Path Technology", Power-Gen Europe, Amsterdam RAI, NL, 1995.

4

R.B. Scarlin, "Advanced Steam Turbine Technology for Improved Operating Efficiency", Power-Gen Europe, Amsterdam RAI, NL, 1995.

5

R.B. Byrd, W.E. Stewart, E.N. Lightfood, "Transport Phenomena", John Wiley & Sons, New York, 1960.

6

F.M. White, "Viscous fluid flow", Mc Graw Hill, 1991.

7

V.C. Patel, W. Rodi, G. Scheuerer, "Turbulence Models for Near-Wall and Low Reynolds Number Flows: A Review", AIAA Journal, 23, 1308-1319, 1985. doi:10.2514/3.9086

8

K. Trutnovsky, K. Komotori, "Berührungsfreie Dichtungen", VDI-Verlag GmbH, Düsseldorf, 1981.

9

D. Minutschehr, "Theoretische und experimentelle Untersuchungen über axiale Labyrinthdichtungen bei Turbomaschinen", MTZ 30(4), 137-142, 1969.

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