Modern steam power plants must operate safely at extremely low loads, known as windage, in which the low pressure (LP) turbine runs with decreased or even zero flow. Windage is characterized by a strongly unsteady three-dimensional (3D) flow field leading to possible aerodynamic excitations. Extensive flow field measurements were performed in an LP steam turbine test rig during windage, using pneumatic probes in the last stage and a diffuser. The flow field of the whole turbine was also calculated with steady 3D computational fluid dynamics (ANSYS CFX). Good agreement is found between the simulations and the measurements of the flow field, and the characteristic vortex structures behind the last rotor row are captured. The numerically predicted trends of power output, pressure ratio, and temperature of the last turbine blade row closely match the experimental data. The complex vortex flow in the stage is interpreted using both numerical and experimental results.
The phenomenon of hot gas ingestion through turbine rim seals is experimentally and numerically investigated for a complete stage with nozzle guide vanes and uncooled helicopter turbine rotor blades. In the experimental part, two different geometrical rim seal configurations are examined: 1. a simple axial gap between rotor and stator disk and 2. an axial gap between the rotor disk and a rim seal lip at the periphery of the stator disk. The results obtained are compared to experiments carried out for the same geometry but without rotor blades. The influence of the presence of rotor blades on hot gas ingestion is examined for different parameters such as nondimensional seal flow rate, Reynolds number in the turbine annulus and rotational speed. For the determination of the sealing efficiency measurements of carbon dioxide gas concentration are carried out in the wheelspace. The static pressure distribution in the cavity is measured by means of pressure taps at the stator disk. It is shown that for configuration 1 the presence of rotor blades causes a considerable drop in sealing efficiency whereas for configuration 2 the sealing efficiency increases significantly. In the numerical part results of three-dimensional unsteady CFD calculations for configuration 2 are compared to steady calculations for the same configuration without blades. Predictions of hot gas ingestion and carbon dioxide gas concentration in the hub region and inside the cavity are presented. Special emphasis is put on unsteady effects arising from rotor movement. A local ingestion zone rotating at approximately half rotor speed is numerically predicted. As indicated by the experimental results the rotor blades have a positive influence on the predicted sealing efficiency.
An Eulerian-Eulerian multi-phase computational fluid dynamics model for predicting phase transition under non-equilibrium conditions in the low-pressure stages of a steam turbine is presented. In a novel way the model introduces dispersed liquid phases into the solution according to phase transition type and location in the turbine. Such an approach provides added information on the location and extent of phase transition regions, including associated droplet size, number, and wetness distributions. The model provides a mass weighted size distribution throughout the turbine with a resolution depending on the number of phases chosen. The study presents examples of the models usefulness in isolating phase transition activity and growth at design and off-design conditions. Comparison against results based on equilibrium phase transition in the turbine is provided for verification and discussion. Validation against measured droplet and flow data from the model turbine is also discussed.
The ingestion of hot gas at the rim seal of a turbine has been investigated for a complete stage with nozzle guide vanes and rotor blades for two types of geometry: 1. the simple axial gap between a flat rotor disk and a flat stator disk, commonly used for industrial gas turbines and 2. an axial lip of the rim seal on the stator combined with a flat rotor disk, often found in aero engine applications. The clearance of the axial gap has been varied for the second type. The efficiency of the rim seal has been examined for different seal flow rates, rotational Reynolds numbers and Mach numbers in the main flow. For the determination of the sealing effectiveness carbon dioxide gas concentration measurements have been carried out in the wheelspace. The distribution of the static pressure in the vicinity of the seal and inside the wheelspace has been measured by means of pressure taps at the stator disk. It is shown that the external flow Mach number in the main flow has a significant effect on the sealing efficiency. As Mach number increases sealing efficiency goes down. The rotational Reynolds number has a distinct effect on the rim seal efficiency depending on the examined configuration. Even for high seal flow rates the ingestion of hot gas can not be fully avoided. The experimental results were the motivation for a three-dimensional CFD approach neglecting the influence of the rotor blades. The results give further insight into aerodynamic features of the ingestion phenomenon.
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