Numerical Stability Analysis of Oil Collector Case Self-Excited Vibrations

One of the most important tasks in gas turbine engine engineering process is to ensure low vibration levels. There are many vibrations sources of different nature and self-excited oscillations are arguably the most difficult to predict. 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. Experimental tests with strain gauges analysis showed that vibration frequency isn’t proportional to neither low or high pressure rotor shaft speeds so self-excited vibrations supposed as the main reason for defects to occur.

The problem examined in engine and test rig conditions. 3D steady CFD simulations were performed to achieve equivalence between experimental data, hydraulic calculations and CFD model. Test rig conditions were set as a result.

Energetic method (Marshall and Imregun, 1996) which is usually applied to blade flutter problems was used for aeroelastic stability analysis. The chosen method is fast enough to analyze multiple test cases, robust and is acceptable to determine the source location of positive aerodynamic work on the surface of test object which leads to instability. On the other hand eigen forms don’t change during oscillations under unsteady aerodynamic forces and it can’t predict stress levels in the material volume.

Radial surface displacements were set according to a harmonic oscillation function than corresponds to oil collector’s first bending eigen form  with two nodal diameters. 3D aerodynamic model represents a 180° sector, thus ensures that surface displacements and gas parameters on periodic surfaces are equal.

A set of simulations were carried out and calculated aerodynamic damping coefficient values showed aeroelastic instability predisposition in both engine and test rig conditions for most test cases. The influence of model parameters like oscillation amplitude, frequency, seal radial clearance, inlet air temperature, rotor speed were investigated. It turned out that seal radial clearance and rotor speed values determine aeroelastic stability the most.

Detailed analysis showed that for instability case positive aerodynamic work region lies inside the oil collector cavern, pressure wave and surface displacement speed wave are close to synchrony. Probable excitation mechanisms were described and discussed. As a counter measure the changes in oil collector case geometry were proposed in order to split eigen forms with equal frequencies (sine and cosine parts of elastic travelling wave).