Numerical and experimental investigation of a low-pressure steam turbine during windage

Abstract

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.

On the Prediction and Theory of the Temperature Increase of Low Pressure Last Stage Moving Blades During Low Volume Flow Conditions, and Limiting it Through Steam Extraction Methods

“…While most authors investigating flow effects during very low volume flow have tended to use single passage, multistage calculations [5,12], the authors are not aware of a reported study indicating the sensitivity of the blade temperature prediction to the CFD setup. To address this, a number of sensitivity studies were conducted in order to ascertain the most suitable CFD setup to predict the LSB temperature for the required degree of accuracy.…”

“…Figure 21 shows the measurement blade temperatures and the spanwise distribution of maximum blade temperatures found along each span line up the blade. It is clear there is little sensitivity to running a steady state or transient full annulus calculations (cases [4][5][6] or if the unsteady SAS turbulence model is used (case 6). This supports the results of the temperature deltas in Fig.…”

“…Mechanical work is done by or on the flow (or both) and heat is removed by the condenser. When operating at volume flows for which ventilation occurs, parts of the LSB do work on the flow, as the tip pressure ratio increases above unity [2,5]. The rate of heat transfer in the condenser is reduced as the vortices in the exhaust prevent a large amount of the flow (heat) from entering the condenser.…”

“…1. The evolution of the flow structure as the volume flow is reduced was described in detail by Sigg et al [5]. As the recirculation regions in the last stage become larger, the net power produced by the last stage becomes negative, i.e., the LSB is driven by the rotor, returning more energy to the flow than it consumes.…”

“…The second effect is the increased heating of the LSB [5,12]. During low volume flow operation, the LSB experiences a large temperature increase over the normal operating temperature.…”