The paper deals with experimental research of the flow and dynamics of the blades in the last stage of a steam turbine with nominal output 34 MW and a connected axial exhaust hood. The experiments were carried out on a turbine with relatively low steam inlet parameters. There was the pressure of 64 bars and the temperature 445 °C at the inlet to the turbine in most operations. It was possible to change the operating regimes of the turbine during the measurement period so that significant ventilation would be achieved in the last stage up to the point when an aerodynamic blockage occurred in the last stage and in the output body. In other words the turbine output varied from about 2 to 35 MW. 2 MW is the island operation of the turbine in this case.

The experiments were carried out using static pressure taps and measure of temperatures at the root and tip limiting wall. These measuring points were doubled circumferentially and they were installed before and behind stator blades and behind rotor blades. The measurement was also carried out at the outlet from the axial exhaust hood. In addition to static pressure taps and temperature measurement, it was also possible to carry out probing by pneumatic probes with a diameter of 30 mm at a distance of about 100 mm behind the trailing edges of the last stage. Blade vibration monitoring the last stage blade was also installed. The blade tip-timing acquisition hardware was used to monitor blade tip deflection. 

Due to experimental data gained it was possible to verify the behaviour of the last stage and the connected exhaust hood for selected measured variants. The courses of pressures and temperatures along the length of the cold end were determined. Furthermore, basic parameters of the last stage were determined, i.e. reactions of the stage, Mach number and Reynolds number and values of pressure recovery coefficient. 

Based on experimental data the boundary conditions for CFD calculations were defined. In particular cases ventilation phenomena were so significant that they reached up to last two stages. Therefore it was necessary to set the boundary conditions at the inlet to the third stage from the end of the turbine based on measurements and internal design SW.  Experiments included also the probing of pressure profiles and velocities behind the last moving blade using a pneumatic disc seven-hole probe. The problem was in unstable behaviour of the pressure in the condenser over time. In some modes pressures in the extent of hundreds Pascal within a few minutes changed. Due to this fact it was impossible to compare measured and calculated profiles of flow parameters along the length of the blade, but it was only possible to determine boundary conditions of calculation based on measured data in certain time, for example when the probe head was in the middle of the length of the blade. Another phenomenon which occurred during the probing of the flow parameters, particularly in ventilation modes, was the inability to determine parameters of steam due to low values of measured dynamic pressure in the vortex area at the heel of the blade. The probe was able to detect dynamic pressure at the level from 60 Pa and more. In other words the transition point between back and forward flows was identified. This point was used for further analysis of ventilation character of the steam flow depending on the ventilation coefficient c2x/u.

Due to the fact that it was also possible to measure amplitudes of the vibrations of blades using the tip-timing method for a variety of modes, the relationships between pressure ratio over the tip of the last moving blade and vibration amplitude were also determined. This verified that the highest amplitude of blade tips occurred just when the compression of the medium on the blade tip was maximum. 


Experimental and Numerical Study of Flow and Dynamics on LSB at 34 MW Steam Turbine

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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