The flow in nuclear steam turbines is mainly affected in normal operating conditions by the condenser pressure, which depends on the heat sink temperature variations. This has an effect not only on the performance, but can also lead to undesired flow regimes and become related to safety issues. The flow in the low pressure last stages is characterized by two main features: unsteadiness due to the rotation and asymmetry due to the exhaust hood. The effect of the condenser pressure on the stage flow is numerically analyzed with steady and full unsteady approaches. Three distinct flow regimes can be identified at nominal, low and high condenser pressures with different types of unsteady behavior. A detailed investigation of the steam flow and possible consequences for the turbine safety is carried out at nominal and critical operating points. Comparisons of steady/unsteady approaches and the effect of the hood design are also presented.

This paper is devoted to the measuring methods and instrumentation used in the wet-steam area for both aerodynamics measurements (pressure, temperature and velocity) taking into account the interaction between the vapour and the liquid phase, and the characterization of the liquid phase itself (dispersed phase and liquid film). The development of suitable instrumentation is an important part in understanding the formation and evolution of the liquid phase but is now more and more oriented towards practical industrial concerns, such as the direct determination of turbine performance by measuring exhaust wetness or chemical measurements relating to erosion in the transition zone inside the turbine. Emphasis has been given to techniques suitable for use in actual turbines.

In the process of developing tools for cavitation erosion prediction of prototypes from mode) tests, 4 on board aggressiveness evaluation methods were tested on a severely eroded blade of a 266 MW Francis turbine. These are pressure, pit counting, DECER electrochemical and vibration measurements. All methods provided coherent results on the blade mounted measurements. The test program provided understanding of the heterogeneous erosion distribution of the prototype blades and quantitative data for comparison in subsequent tests on the mode) of the machine. IntroductionThe prediction of cavitation erosion of a prototype turbine from mode! tests requires that measurement tools be available to characterize the aggressiveness of the cavitating flow in both scales. To develop such tools, IMHEF, Electricité de France and Hydro-Québec pooled their resources in an ambitious research program involving various measurement techniques bath on the prototype and on the mode) of a 266 MW Francis turbine with a well documented cavitation erosion history. Preliminary measurements on this prototype (1) had proven to be incomplete but also very encouraging. A more ambitious test program was conceived with improvements in the array of sensors utilized, sample mounting methods, data acquisition systems and hydraulic test conditions. The test program was divided in two parts, the first with pressure sensors mounted on the suction side of blade #4, a well eroded blade, ta identify the type of cavitation present and its aggressiveness in terms of pressure pulses. In the second, polished metallic samples were mounted in place of the sensors for pitting studies. This experimentation program took place in June 1995.

Summary:In the frame of a joint research programme between EDF, HydroQuébec and IMHEF, different methods are investigated to predict cavitation erosion on Francis turbines from mode!. They are based on measurement of pitting, pressure fluctuations and acceleration. The measurement techniques have been detailed in Part I and Part 2. The present article describes essentially the theoretical and practical aspects of the methods and discusses the results obtained until now from the mode! and prototype tests. The first analysis shows that the methods proposed are suitable to measure cavitation aggressiveness on mode! and on prototype, and that the level on the mode! is several orders of magnitude smaller than on the prototype. To adjust transposition laws, a more complete set of data is needed.

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