The design of bigger and more efficient steam turbines in the last decades led to a development of bigger blades even in high pressure steam turbines and therefore to increased problems with windage effects at certain operation points and during the shut down and particularly during trips of entire steam turbines. When the steam mass flow is low or even interrupted, it cannot cool down the rotating blades of the turbine. The kinetic energy of the rotor blades is thus transformed into thermal energy of the enclosed steam. The temperature increase in the steam leads to an increase in blade temperature which can lead to serious blade damage and therefore must be prevented. To better understand the aerodynamic characteristics and the flow mechanisms at part-load, investigations of the flow field at low Mach numbers have been undertaken in a four-stage research air turbine. Temperature, pressure, velocity and flow angles were measured in 6 different planes along the turbine annulus for different rotational speeds and different relative mass flows. CFD-simulations with the flow solver TRACE have been carried out and compared to the experimental data at partial load. The results show that the highest temperatures are developed in the last third of the turbine, which corresponds to experiences with observed damage in real turbines. The calculation results help to predict the critical operating conditions which lead to the highest thermal stresses and to quantify the rise in temperature. Such computational results for real multi-stage high-pressure turbines will help to develop rules for steam turbine operation and can help to prevent serious damage.

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