NUMERICAL STABILITY ANALYSIS OF OIL COLLECTOR CASE SELF-EXCITED VIBRATIONS

The investigation subject is an Oil Collector Case which is a thin axisymmetric stator part with console binding to the turbine rear frame. It had been suffering from high level vibrations and needed fixing measures to apply. Self-excited vibrations supposed as the main reason for defects to occur. The problem was examined in the engine and test rig conditions. The energetic method which is usually applied to blade flutter problems was used for aeroelastic stability analysis. Radial surface displacements were set according to the harmonic oscillation function that corresponds to the oil collector's first bending eigenmode with two nodal diameters. 3D aerodynamic model represents a 180 degrees sector, thus ensures that surface displacements and gas parameters on periodic surfaces are equal. A set of simulations was carried out and calculated; the aerodynamic damping coefficient values showed aeroelastic instability predisposition in both the engine and test rig conditions for most test cases. Influence of such model parameters as seal radial clearance, pressure ratio, inlet air temperature, wave propagation direction and rotor speed was investigated. A detailed analysis showed that for instability case positive aerodynamic work region lies inside the oil collector cavern downstream labyrinth seal where pressure wave and surface displacement speed wave are close to synchrony. Probable excitation mechanisms were described and discussed. It was demonstrated that aeroelastic instability of the same type as it was in the engine conditions can be reproduced in the test rig conditions.