6.11. Elimination of blade vibrational breakages

Measures taken to eliminate blade vibrational breakages during operational development of the engine, are subdivided into three groups.

The first group provides dangerous resonance displacement out of limits of the engine operation ratings by change of blade natural vibrational properties (for example, blade normal mode frequency at the expense of its geometrical characteristics change (Fig. 6.10)) or change of exciting frequency (for example, by change of racks, stator vanes or fuel nozzles quantity, etc.).

Fig. 6.10. Turbine blade with shears at the end of a trailing edge for

vibrations elimination of vibrations of maximum forms

The second group provides reduction of resonance vibration stresses in blades through lowering exciting forces amplitude values, detuning resonance vibrations, application of constructional damping of the blade vibration, application of compressor stages controls in the following way:

- by increasing axial clearance;

- by improving racks and blades streamlining;

- by using a variable step in a lattice of stator vanes;

- by changing the number, configuration and location of air by-pass windows;

- by phase distributing of exciting forces in a radial direction by application of inclined racks or guide vanes. In   this case the vibration stresses in rotor blades can be reduced by 40...50 %, but the amplitudes of the maximum order harmonics are augmented;

- by using antivibrational caps, ring and other shrouds, solid lubricant in the pinned blade (for example, on the basis of bi-sulphide molybdenum – МоS₂);

- by using rotor blades elongated shanks;

- by application of doubled rotor blades, i.e. installation of two blades in one groove of the disc, and also using rotor blades hinged attachment with the help of pinned blade. The vibration stresses can be halved in comparison with blades attachment with the help of a “«dovetailed” blade;

- by using compressor control systems of air by-pass or control systems of stator variable vanes.

The third group provides blades strengthening in this way:

-  by application of stronger materials;

-  by microblobs surface deformation or by method of diamond smoothering, etc.;

-  by blade surface grinding and polishing;

-  by application of more smoothly varying transitions from airfoil to a blade root and antivibrational caps;

-  by perfecting heat treatment technique and deposition of protective coatings.

6.12. Concept of blades self-oscillations

Sometimes, except for forced blades oscillations, there are special self-energizing oscillations, or self-oscillations. They appear when there is no external visible reason for oscillations. The characteristic feature of blades self-oscillations is, first, absence of any pressure or speed pulsation in a flow and, second, connection of self-oscillations origin not with measured engine rotor frequency, but with reduced rotational speed, that is with conditions characterizing flow about blades.

It should be noted, that each self-oscillation has its mechanism. Some of them are studied, many are known in general, and many still remain untreated.

Power concept of self-oscillations consists in the following. In real conditions any oscillations always happen with energy being taken away. The oscillations stability proves that there is an intake of energy to a system. Otherwise oscillations would stop.

The steady-state oscillations can appear without periodic exciting force, but then for them to appear the periodic force effect, which is caused by a source contained in the oscillatory system, is necessary. In Fig. 6.11. the change of damping E_damping and excitation E_exciting energies from amplitude x is shown. The point B determines steady-state oscillations with amplitude x_B, at which the excitation energy equals damping energy.

In case of compressor blades self-oscillations there is a mechanism which allows a blade, which began to oscillate because of random deviation, to get energy from an airflow. So oscillation amplitude is set, which can become dangerous. Input and output energies depend on oscillation amplitude. The system kind of controls energies balance through the amplitude.

Fig 6.11. Exciting (E_exciting) and damping (E_damping) force energies diagram