NUMERICAL COMPUTATION OF FORCES ACTING ON BLADES AS A FUNCTION OF AXIAL CLEARANCE AND MINIMUM THROAT AREA

It’s well known that in any stage of steam turbine the working fluid comes through the nozzles, which direct the flow towards the blades causing loads on the blade surfaces to move the rotor and produce useful work. These loads are oscillating in time in a harmonic pattern and could be computed by knowing the pressure around the blades in every moment. The variation in the loads are due because the interaction between the nozzle wakes with the rotating blades. In this paper, a 2D numerical computation of forces acting on blades as a function of the axial clearance and minimum throat area is presented. The pressure field in a Curtis stage of a 300 MW steam turbine was numerically computed. The Navier Stokes equations were resolved in 2D using a commercial program based on the finite volume method. The sliding mesh technique was used to take into account the interaction between the nozzle wakes and the blade motion. The forces acting on the blades were computed for several axial clearances and throat area variations. It is showed how these forces are affected by the variability of these distances. Dependence of the forces from the pressure field variation in time in the axial clearance is investigated. These forces, which cause forced vibrations on blades, are expressed as Fourier series in order to investigate the changes in these forces.