Combined-cycle power plant '500 MW on a single shaft' (AG Turbo II). Project 4.2.2B: Active influencing of thermodynamic and kinematic relaxation processes in low-pressure steam turbines by intensification of the nucleation process Final report / GuD-Kraftwerk ' 500 MW auf einer Welle' (AG TURBO II). Teilprojekt 4.2.2B: Aktive Beeinflussung der thermodynamischen und kinematischen Relaxationsprozesse in Niederdruck-Dampfturbinen durch Intensivierung des Keimbildungsprozesses Abschlussbericht

In order to achieve an increase in the overall thermodynamic efficiency of power plants it is necessary to optimize all components and incorporated machines. Here, the steam turbine plays a key role. Especially in the low pressure steam turbine considerable losses arise due to thermodynamic and kinematic relaxation after crossing the saturation line. The potential of reducing these relaxation losses is considerable and shall be evaluated and exploited by this research project. The project focuses on the nucleation process that begins at the saturation line and ends right after the spontaneous condensation. In the present project know-how is acquired in order to optimize the nucleation process by active manipulation of the nucleation phenomenon. The influence of the expansion rate, the influence of the surface tension of the generated droplets and the influence of chemical substances (impurities) on the homogeneous/heterogeneous nucleation process are investigated. Measures are developed in order to minimize the relaxation losses. Conclusions can be drawn for a modification of the water-steam-cycle process and for the design of a new generation of LP-steam turbine bladings. To achieve this, available nucleation models are investigated and taken as a basis for the development and implementation of a new nucleation model incorporating as well the homogeneous as the heterogeneous process. To get a closer numerical representation of the 'real' machine conditions, unsteady effects, turbulence and compressibility are taken into account. The numerical results exhibit that an active manipulation of the nucleation process is feasible. Moreover, a strong dependence of the dispersion of the wetness from the position is shown by the simulations. On the experimental side of the project a miniaturized light-scattering probe has been developed, designed, manufactured, calibrated and employed for droplet sizing measurements in a 5-stage LP-steam turbine. The probe meets the demands of wet steam flows with very low droplet sizes at highest droplet number concentrations. By means of this probe droplet size spectra and liquid mass flow densities were measured in a test turbine of the industrial partner in three control surfaces at six different operating conditions. The gathered knowledge and experience allow for a better understanding concerning the physical phenomena related to the nucleation in steam turbines. Especially, the spatial and temporal evolution of the nucleation front is of highest interest. Moreover, conclusions may be drawn in order to modify water chemistry and/or the flow passage design to change the local expansion rate in the vicinity of the nucleation front. Due to the close co-operation with the industrial partner ALSTOM it is ensured, that the results can be implemented directly in the internal development processes. (orig.) / SIGLE / Available from TIB Hannover: F04B1375 / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische Informationsbibliothek / Bundesministerium fuer Bildung und Forschung (BMBF), Bonn (Germany) / DE / Germany