Increasing resource ofthe aviation gas turbine blades by calculation methods

Аuthors

Magerramova L. A.

Central Institute of Aviation Motors, CIAM, 2, Aviamotornaya St., Moscow, 111116, Russia

e-mail: mag@ciam.ru

Abstract

The growing competitive requirements to improve parameters of gas turbine engines (GTE) lead to more accurate calculations, the use of new materials and designs. Besides the possibilities of mathematical modeling for design and strength calculations have considerably increased to take into account the particular load and behavior of materials.
The turbine wheels are among the main components of the gas turbine engine that largely determine engine basic parameters. The turbine parts operate at high temperatures at stationary and transient regimes under cyclic loading during aircraft evolutions. In addition, the turbine blades are exposed to aggressive environment, high temperature gradients, centrifugal and gas loads.
The calculation methods of turbine blade life time increase are based on:

• choice of alloy for blades and disk,
• optimization of blade airfoil according to gas-dynamic efficiency,
• choice of optimal cooling system of turbine blades,
• optimization places of gravity centers of blade sections,
• optimization of blade design taking into consideration the detuning from the «dangerous» resonances,
• design of blade damping system,
• detailed optimization of fillet radius, holes, partition thickness,
• optimization of designs of shrouds, platforms, shanks, blade root and disk-blades lock joint,
• choose of the optimum of the assembly oversize on tip shrouds,
• crystallographic orientation (CGO) optimization in single-crystal blades,
• rational choice of coatings,

For the more accurate determination of blade life time the calculation of the static capacity and cyclic durability, dynamic behavior of blades should be carried out by means of:

• three-dimensional modeling,
• accounting for anisotropy of material properties,
• accounting changes of material properties due to creep phenomena,
• accounting changes to stress strain state (SSS) during flight cycle

The various aspects of turbine blade life time designing of are discussed. The calculation methods are developed to improve the estimate reliability of blade static and cyclic durability. The investigations influence of various design features, alloy physical nonlinearity and operating conditions on blade strength. During turbine blade designing tip shrouds and root joints are especially important elements. The configuration of the shroud is determined by gas-dynamic and dynamic requirement as well as strength and oversize during operation that influences the above mentioned parameters. Based on the conducted investigations the method of shroud mounting oversize determination that provides blade strength and contact reliability during the whole life time has been developed. The recommendations of designing have been given based on the conducted investigations of the influence of design parameters of the root joint such as installation angle, availability of partitions and fillet radius on strength.
High temperature single crystal nickel alloys widely used in aviation turbine blades have anisotropic properties. The commercial computer complexes used in the world practice for complicated calculations by 3-dimensional element models allow to take into consideration only elastic properties of cube symmetry of single crystal blade. There are no material models that have cube symmetry properties of plasticity and creep in these complexes. Based on the conducted investigations a model of material behavior has been selected and a method of SSS calculations for single crystal turbine blades has been suggested. The investigations of crystallographic orientation influence on SSS and static strength of single crystal blades for high temperature gas turbines have been conducted to determine the optimal location of the single crystal in the blades. It is not possible to calculate turbine blade strength without taking into consideration stress relaxation during long-term operation at stationary regimes. It is necessary to know characteristics of alloy creep in the wide range of operating temperatures and time. However the volume of experimental data on creep resistance is limited. There is a set of equations to describe creep strain in the commercial computer complexes with 3-dementional element modeling. The algorithm has been developed to establish coefficients of the selected creep equation and (or) creep strain rate according to available experimental data. The investigations on stress relaxation using different methods have been conducted. The method of establishing blade allowable life time has been suggested.
It is expedient to conduct calculations of SSS using several flight cycles including transient regimes. The stress and strain rages can exceed the corresponding parameters of a zero-to-maximal cycle during loading, operation at different regimes, deceleration and shut-down.
One of the important trends of ensuring blade life time for advanced high temperature aviation gas turbine is the application of effective durable coatings. Specific problems connected with the ensuring of strength and durability for blades and coatings at thermal and mechanical loads.
The examples of numerical analysis of constructions are given, the actions on improvements that ensure durability are shown.
The recommendations on turbine blade life time designing are given.

Keywords:

gas turbines, blades, strength calculation, resource

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