Unsteady Forces in LP Last Stage 380 MW Steam Turbine Rotating and Non-vibrating Rotor Blades with Exhaust Hood

Abstract

Cracks were found in the roots of the last stage rotor blades. Tip-timing measurements showed high vibration amplitudes in certain working conditions. The aim of this paper is to analyse unsteady forces acting in such working conditions on the last stage of rotor blades, taking into account unsteady flow in the exhaust hood. Using ANSYS CFX, this paper examines unsteady forces in a low pressure steam turbine last stage with an exhaust hood. A sliding mesh model was used for interaction between the stator-rotor blades and the rotor blade- exhaust hood. The study included a 3D transonic viscous flow. The CFD mesh included all the last stage stator, rotor blades. The rotor blades vibrated but do not rotate. Numerical static pressure and temperature values in selected exhaust hood crosssections were compared with experimental ones. Unsteady rotor blade forces and moments were analysed along the lengths of the blades. This study has shown that interaction with the exhaust hood causes the non-symmetrical pressure and temperature along and behind the rotor blades. The low frequency unsteady harmonic is higher towards the tip of the blade. The greatest low component of unsteady forces was 50 Hz, which is far below the natural frequency and therefore irrelevant to rotor blade vibration. The second harmonic, 100 Hz, is much more dangerous, because is relatively close the natural frequency of the blade.