4. The ways to eliminate dangerous resonance oscillations of rotor wheels

Since oscillations of discs and rotor blades are interdependent, it is necessary to consider disc and blades, arranged on them, as one unit, i.e. to talk about oscillations of rotor wheels, at disc oscillation calculations and researches.

If safety factor coefficient value is less than recommended by strength standards, the measures of dangerous oscillations elimination are put into practice.

The basic measures will be mentioned below. These measures were partly reviewed in paragraph 6.11 when dangerous oscillations of blade were considered.

1. Effect on the value and distribution of exciting forces. The main reasons for appearance of circumferential irregularity of flow are connected with interception of ring-type GTE air-gas channel by such engine elements, as racks of supports, guide vanes and nozzle diaphragms, etc. Their aerodynamic “wakes” deform field of flow velocities and pressures, causing appearance of powerful harmonics of excitation with the orders, equal to their number.

If, for example, the intensive resonance oscillations of compressor rotor wheel with frequency which is four times greater than the rotor rotational speed (i=4) are detected and the forward support of the engine is supported by four load-bearing racks arranged uniformly on a circumference, these racks are most probable excitation sources. The load-bearing racks create disturbance propagating more along the flow and less against the flow. The cases are known, when in engine jet nozzle wake there was a deformation of velocity and pressure field created by racks of a forward engine support.

Perfecting of rack aerodynamic form and reduction of flow velocity reduce the circumferential irregularity and the value of exciting forces. Using a variable step in circumferential arrangement of the racks is an example of effect on circumferential distribution of exciting forces. Hence, at practically the same depth and width of each aerodynamic “wakes” the powerful harmonics can be slacken off, however other harmonics will appear, which can result in dangerous resonance oscillations at other engine ratings. It should be taken into account when such measures are taken.

The excitation by compressor guide vanes and turbine nozzle diaphragms is similar. Orders of powerful harmonics are equal to their number. They predominantly excite high-frequency resonance oscillations of blades. The increase in axial clearance between stator and rotor lattices allows to reduce intensity of rotor blades excitation, specially when the initial gap was small.

Using optimum variable step in circumferential arrangement of stator vanes can also be to lower excitation intensity. There are cases, when the stator vanes are set slantly to radius. It affects radial distribution of exciting forces acting on a rotor blade. In such conditions the aerodynamic “wake” passes not simultaneously through tip and root parts of a blade. Hence, the component in radial distribution of exciting force amplitudes responsible for excitation under the first form of console blade bending oscillations is relaxed. However, at the same time the component responsible for oscillations under the second bending form is intensified. It can be dangerous on different engine ratings.

Let us note, that increase in the angle of incidence when the guide vane is streamlined can provoke stall, augmenting depth and width of aerodynamic “wakes”, and consequently, increases intensity of excitation at off-design operational ratings. Improvement of streamlining is an effective means to lower excitation intensity.

The circumferential irregularity of a flow is also generated by combustion chambers. Order of powerful harmonics here is equal to the number of main fuel nozzles. The multiinjector annular combustion chambers are most “quiet”. It is important to note, that the irregular operation of fuel nozzles can result in appearance of powerful and dangerous harmonics with orders smaller than their number. The other reasons for circumferential irregularity of flow force effect are: irregular on a circumference a compressor by-pass and air bleed; ovalization of casings caused by force and thermal loads action; buckling of combustion chamber flame tubes, etc. The elimination of these reasons allows to reduce intensity of excitation.

2. Frequency tuning out of dangerous resonant ratings. The purpose of such tuning out is elimination of dangerous resonant ratings from range of rotor operation rotational speed, or their displacement on those ratings, where the intensity of excitation is less or the operation time at them is small. The main means of tuning out is change of rotor wheel normal mode frequency from values that characterised by dangerous resonance oscillations. Tuning out is carried out by purposeful change of ratio of the geometrical sizes both blades and discs. It can be implemented both by increasing, and decreasing normal mode frequencies. Of great importance are theoretical methods of computation of normal mode frequency, when the required change of normal mode frequency is in compliance with computational optimum change of geometrical shapes of blades or discs. The experimental methods are also used, however, especially experimental way is associated with carrying out of costly actual tests of rotor wheels with changed geometrical sizes.

3. Enhanceing of damping properties of a design. The amplitude of oscillations at a steadied resonance is inversely proportional to logarithmic decrement, characterizing the capacity of a system to disperse the energy of oscillations. The energy dissipation goes through the following main channels: internal friction in a material of oscillating structural elements (damping in material); friction in places of details connections (structural damping); removal of oscillations energy in a gas flow (aerodynamic damping).

Damping in structural materials used is, as a rule, insignificant. It is slightly increased with temperature rise in details, that is positive, especially for turbine rotor wheels. Such damping can be essential for high-frequency oscillations of blades, when in general balance of energy dissipation other kinds of damping become less considerable.

For console compressor blades, fastened mostly in discs by means of “dovetail”‑type locks, the aerodynamic damping has the main influence on their oscillations under the elementary forms. It is proportionate to amplitudes of movement of blade airfoils in a flow. However, under particular circumstances it can become negative, when favourable conditions of flow energy supply for development of oscillation process appear. It means that self-oscillation can appear.

Structural damping during oscillations occurs when there is a possibility of relative movements of friction surfaces of jointed details. If, for example, in “dovetail”‑type blade roots because of large friction forces (appearing under action of centrifugal forces) the movement of jointed surfaces is practically absent and damping in such roots is insignificant, in turbine blade “fir‑tree”‑type roots it can be considerable. It is reached due to selection of geometrical parameters of blade root and such distribution of efforts in teeth, when the upper pair of teeth is largely (but not completely) free from transmission of centrifugal forces, and movement on their joint surfaces is possible, accompanying by dissipation of energy when friction forces act.

The increase in joint movability of rotor blades and disc favours the possibility of damping increase. In this respect the hinged fittings have essential advantages. Twin blades also have increased damping in blade roots.

Ring-type shrouding of rotor wheel blades is an effective means, reducing their general tendency to dangerous oscillations both due to system rigidity increase, and as a result of essential damping in joints of shroud caps. The required optimum tightness between blade caps at rotor wheel assembly is reached by elastic twist of blades in the direction of their “natural” twist. The centrifugal forces, trying to untwist the “naturally” twisted blades, augment this tightness.

Blade antivibrational caps arrangement approximately on one-third of blade length from their end is widely applied in fan stages and in first compressor stages. Here their use (despite some efficiency lowering) is an important means to lower dangerous oscillations. Lately there has been the tendency not to use fan antivibrational caps and use wide-chord, hollow, compound, and shelled (with a filling material) rotor blades instead. It allows to increase the compressor stage efficiency at essential oscillation damping in compound blades.

Turbine blades, as a rule, are equipped with shroud caps. They are located on blade tip. Such shroud caps (damping dangerous oscillations) also promote increase (specially in turbine first stages with short rotor blades) in turbine efficiency due to losses reduction in radial clearance.

At the same time the guarantee of optimum tightness in blade shroud caps, which should be kept at any engine ratings and during all resource, is connected with technical difficulties, particularly, in first turbine stages with short and rigid blades. On the one hand, the necessary tightness (with rigid blades) is reached at a high level of assembly pressures (there are examples, when the cracks appeared in blades when rotor wheel was mounted, which is naturally inadmissible and demands tightness value lowering). On the other hand, at engine acceleration ratings the contact stresses considerably increase owing to temperature deformations of blade caps, that warm fast though blade airfoils and disc are not warm enough. It results in appearance of contact plastic deformations (at high temperature) and tightness reduction. The multiplicity of acceleration cycles can cause tightness loss and, owing to its initial value limitation, appearance of clearances at engine operational ratings. It is inadmissible because of the possibility of blade dynamic stress increase. Accumulation of creep deformations of blades and disc with increase in operation time can cause the same tightness loss. Sometimes, for these reasons we refuse from cap shrouding of first turbine stages, resorting to other means of dynamic stress lowering.

Reduction of tightness is also connected with wearing of conjugated surfaces, which is typical of compressor and turbine blade shroud caps. However, in lengthy and consequently rather low‑rigid blades reduction of tightness because of wearing is insignificant in comparison with initial tightness, which for lengthy blades can be large enough at a reasonable level of assembly stresses. Stabilization of tightness promotes maintaining optimum dynamic characteristics of rotor wheels. With the purpose of cap wearing reduction the covers from special alloys with high hardness and wear resistance are put on their contact surfaces. The tightness between compressor and turbine rotor blade shroud caps is inspected when used. The appearance of clearances is inadmissible.

Sometimes with the purpose of damping increase the special dampers are installed on rotor wheels.

4. Increase in fatigue resistance. Reduction of stress concentration and application of hardening detail techniques are the main means.

Reduction of stress concentration is reached by smoothly varying of blades and discs different surfaces conjugating, by elimination of scratch, etc.

Hardening methods allow essentially to increase fatigue resistance of details. These are methods, creating squeezing residual stresses in detail surface layer (tensile stresses instigate crack initiation and apex of crack is powerful stress concentration). They are air-operated and hydro‑shotblasting strain hardening, vibrotumbling, diamond burnishing, etc.