Evaluation possibility of the turbine disks using of converted aircraft engines of the NK family

. The article examines the possibility of using converted aircraft engines for gas pumping units using the example of the main parts of the GTE - the disks of the NK-8-2U aircraft engine and the NK-16ST and NK-16-18ST ground engines. For all disks of the second stage of low-pressure turbines of three engines, the stress-strain state calculation was performed using the boundary integral equations method (BEM) and using the FEM in the Ansys program. For three disk stress concentrators, the theoretical stress concentration coefficient was determined and the maximum equivalent stresses were calculated. During long-term operation, the main parameters of engine operation change due to degradation changes (wear, erosion and corrosion, development of seals, contamination and resizing of flow parts). As a result, the main parameters of engine operation change, in particular, the rotation speed of the rotors and the gas temperature in the turbine, which determine the stress-strain state, which must be taken into account when predicting durability. The compliance of the results of the calculated study of the stress-strain state of turbine disks with their real load was confirmed by the data of their metallurgical study after prolonged operation. Using the Manson formula for the disks of three engines, their cyclic durability was determined before the formation of a crack of low-cycle fatigue. Using the linear damage summation hypothesis, the total damage rates (under static loading and under fatigue loading) for the disks of three engines were calculated. It is established that after the conversion of aircraft engines that have spent their life in flight operation, it is advisable to continue the use of turbine disks on ground-based GTU.


Introduction
Currently, aviation gas turbine engines (GTE), which have spent their flight life, are widely used to drive gas pumping units (GTU), electric generators, gas jet installations, devices for cleaning quarries, snow plows, etc. The mass use of aircraft engines that have spent their flight life and have retained the ability for further use allows us to preserve the labor embodied in engines (and in general in high-tech products) and save expensive materials used in their creation, as well as achieve economic growth [1,2].
In the article, the prediction of the possibility of using converted aircraft engines on ground-based power plants will be investigated by the example of the main parts of the gas turbine engine (largely determining the weight, the possibility of obtaining high operating parameters, the resource and safety of engine operation. The destruction of the main parts of the turbine, both in flight and at gas pumping stations, leads, as a rule, to significant destruction inside the power plant. Therefore, the problem of accurately predicting the durability of the main parts of the turbine, minimizing the probability of destruction, has always been and remains relevant at all stages of the creation, refinement and operation of engines) -disks of gas generator turbines of two engines of the same family: serial singlecircuit two-shaft gas turbine ground engines NK-16ST and NK-16-18ST for the GTU, developed after the aircraft engines have worked out the resource in flight operation and based on the conversion of aircraft engines of the NK-8-2U family designed by N.D. Kuznetsov. The experience of creating drive units based on an aircraft engine shows that up to about 75% of the components and parts of the base engine can be preserved [1,2]. Changes are carried out in the nodes associated with the new purpose of the product, the change of the type of fuel used and the appearance of a power turbine in the converted engine. The power turbine is being created anew ( Fig. 1 shows the longitudinal sections of the NK-8-2U engines, as well as NK-16ST and NK-16-18ST). The turbine disks of the NK-8-2U, NK-16ST and NK-16-18ST engines are made of a deformable heat-resistant nickel-based alloy EI698-VD, belonging to the group of dispersion-hardened alloys. The particles of the hardening -phase have a spherical morphology and are coherently connected with the -matrix. The volume content of the strengthening -phase is about 20-30%. Grain boundaries have carbide hardening. The heat treatment to which the discs are subjected is two-stage and should form mainly spherical separations of the main hardening -phase in the alloy of two sizes (micro duplex).

Justification of the turbine disk choice for calculations
Details of turbines of aviation gas turbine engines are exposed to various types of loading during operation. For example, turbine stator parts (nozzle blades, housings, diaphragms, etc.) are affected by gas loads and temperature. In contrast, rotary parts (working blades, disks, shafts) experience, in addition to the above loads, also loads from centrifugal forces, so the loading of rotary parts is usually higher than that of stator parts.
From the point of view of the severity of the consequences of the destruction of turbine parts, the most severe consequences cause the destruction of disks, because when the disks are destroyed, their fragments have significant kinetic energy and, as a rule, are not held by the turbine housings. In this regard, the strength reliability of turbine disks largely determines the reliability of the engine, and sometimes limits its resource.
All the engines under study (NK-8-2U (18,000 hours in flight operation), NK-16ST and NK-16-18ST (150,000 hours)) are two-frame, i.e. they have high and low pressure turbines. The turbine of the NK-8-2U engine has three disks -one disk refers to a high-pressure turbine (HP) and two disks -to a low-pressure turbine (LP).
Since in the turbine of the NK-16ST and NK-16-18ST engines, the disk of the third stage (the most loaded in the NK-8-2U engine) of the ND rotor does not have a blade crown and its loading is significantly lower than that of the disk of the third stage of the NK-8-2U engine turbine, the disk was selected for comparative evaluation the second stage of the LP rotor ( Fig. 2). This disk, in comparison with the disk of the HP rotor, has a higher operating temperature and a higher temperature difference between the rim and the hub of the disk.

Analysis of turbine disk loading conditions in flight operation and on a ground installation
During operation, the discs are cyclically exposed to temperature and mechanical loads. At the same time, in the most loaded sections of the disks, the acting stresses may exceed the limit of the proportionality of the material. In this case, plastic deformation of the material occurs with the accumulation of plastic deformations from cycle to cycle. The mechanism of material damage under these conditions is called low-cycle fatigue (LCF). The number of cycles before destruction, typical for LCF, is usually 10 3 ... 10 5 cycles.
Unlike an aircraft engine, the loading cycle of GTU is simpler [3]. The main differences between the loading cycles of an aircraft engine and a gas turbine drive are as follows: 1. The rotational speed of the rotors of aircraft gas turbine engines when the engine is operating in take-off mode is higher than when the gas turbine drive is operating in maximum continuous mode.
2. The operating time of the aircraft engine in take-off mode is from 1.5 to 5 minutes, and the operating time of the gas turbine drive in the maximum continuous mode can be from 1000 to 1500 hours, as the operating time between preventive inspections.
3. The average duration of the flight cycle of an aircraft engine is usually 2-4 hours, and the average duration of the loading cycle of a gas turbine drive, as mentioned earlier, is 1000-1500 hours.
Thus, if an aircraft engine has an assigned resource of 18,000 hours and, depending on the average duration of the flight (loading cycle), the assigned resource in cycles from 9,000 to 4,500 cycles, the gas turbine drive, even with its characteristic resources from 50,000 to 150,000 hours, will have no more than 100 loading cycles.
It follows from this that for the operation of ground-based GTU, unlike aircraft engines, cyclic loading is not decisive. The experience of operating aviation gas turbine engines has shown that for the disks of these engines, it was the LCF that often took place, limiting the resource of the disk, and sometimes the engine as a whole.
The destruction of disks caused by LCF was observed both in compressors and in turbines. LCF defects are resource-based, i.e., these defects are increasingly manifested with an increase in work resources and, accordingly, with an increase in the number of loading cycles.

Evaluation of the calculation of disks for strength and cyclic durability 4.1 Calculation of disks for strength
Stresses in the disks arise from the inertia forces of the disk itself and the blades connected to it, from uneven heating of the disk along the radius, from the tension created by pressing the disk onto the shaft if the disk is connected to the shaft in this way.
A significant complication in the calculation of disks is the presence of high temperature and its change along the radius of the disks. In this regard, there is a need to develop such calculation methods, which take into account the change in the radius of temperature, elastic constants and strength characteristics of the material. In many cases, it became necessary to determine stresses taking into account plastic deformations of the material.
The calculation of the stress-strain state of the disks of all the engines under study was performed using an effective engineering method -the boundary integral equations method (BEM) [4,5], based on the numerical solution of integral equations by using successive approximations of the difference of radial and circumferential (tangential) stresses, and using FEM in the Ansys program [6]. The tasks in Ansys were solved in an axisymmetric elastoplastic formulation. Isoparametric finite elements (FE) with linear and quadratic approximations of the displacement field within the FE were used to discretize the disk region.
The disks of the second stage of the LP turbine of the NK-8-2U, NK-16ST and NK-16-18ST engines differ in the distribution of temperature fields over them and the rotation frequencies of the ND rotor (Table 1).  Fig. 3 shows the results of calculating the disks of the second stage of LP turbines of the NK-16ST and NK-16-18ST engines (for the disk of the NK-8-2U engine, the picture of the stress intensity distribution is similar to GTU) using the FEM in Ansys. Table 2 shows the results of calculations using BEM and FEM in the Ansys program.   [7,8].
The paper analyzed the results of a statistical study of the data of metallurgical control of mechanical properties (short-term at t=20 0 C: tensile strength σB, conditional yield strength σ0.2, elongation δ, relative contraction ψ, impact strength aK, hardness ; long-term at t=650 0 C, σ=720 MPa: heat resistance (τ, h) turbine disks of NK-16ST and NK-16-18ST serial engines for a gas pumping unit made of EI698-VD alloy for a period of operation up to 80,000 hours [9].
The effect of operational operating time on the mechanical properties of turbine discs was determined by comparing the initial values of the characteristics of the mechanical properties of the disc material according to the results of acceptance control and the test results of samples cut from turbine discs after operation.
According to the results of the statistical analysis, the following was found: -the characteristics of the mechanical properties of σB, ψ, and noticeably decrease with an increase in the duration of operation, and the characteristics of σ0.2, δ, aK and lgτ change little with an increase in operating time; -the operation of disks up to 80,000 hours does not affect the numerical characteristics of the scattering of mechanical properties, but affects the average values; -with an increase in the duration of operation, the characteristics of σB, ψ, and decrease, but remain within the limits of the norms for the source material.
Using regression analysis methods, approximating empirical dependences of the average values of the characteristics of mechanical properties on the operating time were obtained.
During the long-term operation of the NK-8-2U aircraft engine, there is an increase in the parameters of the gas turbine engine, for example, an increment of the average value of the turbine rotor speed, an increment of the average gas temperature behind the turbine, etc. For the NK-16ST and NK-16-18ST engines, an inter-regulatory operating time of 3000 hours of operation with diagnostic measures is introduced -diagnostics of parts and flushing of the flow part with a special composition, adjustment of engine parameters. This allows you to restore the engine operation parameters to their original values.
The compliance of the results of the calculated study of the stress-strain state of turbine disks with their real loading is confirmed by the data of their metallurgical research after prolonged operation [9].

Determination of cyclic durability of turbine disks
The increase in the resource of aviation gas turbine engines operating under cyclic loading, the danger of the consequences of the destruction of rotor parts rotating at high circumferential speeds, especially disks, require improvement of computational methods for forecasting durability.
As a result of the action of large non-stationary loads in the disks during operation, irreversible deformations of plasticity and creep appear and damage accumulates, which can lead to the destruction of the structure. In this case, the destruction occurs at the number of loading cycles in the range N = 10 3 ... 10 5 cycles.
The destruction of parts due to LCF begins most often in places of stress concentration. To determine the cyclic durability (the number of cycles before destruction, the number of cycles before the formation of a crack of the LCF), it is first necessary to conduct a stressstrain state study in the areas of the projections of lock joints, holes for tie bolts and for cooling air, labyrinth projections and in places with abrupt changes in the configuration of parts, which was performed in paragraph 4.1.
The cyclic durability of Np before the formation of a crack of the LCF can be found both from the experimental curves of the LCF, and using the empirical dependence proposed by Manson [5,10]: To use the dependence, it is necessary to determine the characteristics of the elasticplastic deformation cycle at the design point of the part. Table 3 shows the results of calculating the cyclic durability of the disks of the second stage of the LP turbine of the NK-8-2U, NK-16ST and NK-16-18ST engines [5].
The cyclic durability of the disk of the second stage of the LP turbine of the NK-16ST engine is 4.57 times greater, and the NK-16-18-ST engine is 3.5 times larger than the disk of the NK-8-2U engine.

The principle of linear summation of damages
In real operation, aviation gas turbine engines operate with complex changes in loading modes during the operational cycle. For engine parts, each operating mode is characterized by its own level of temperature, static and vibration loads. Each mode contributes its share to the exhaustion of the resource, therefore, the resource assessment should be carried out taking into account the operating time in all modes of the engine. under appropriate loading conditions [12]. The linear hypothesis of damage summation does not take into account the mutual influence of transitions from one load level to another and alternating loads on durability, and therefore may introduce a certain error in the calculations. In order to take into account the mutual influence of different load levels within the framework of the linear hypothesis of damage summation, various methods are proposed to adjust the magnitude of the maximum accumulated damage during destruction, for example, in the case of fatigue loading. However, given the wide range of characteristics of long-term strength, low cycle durability and fatigue resistance, the accuracy of calculations using the linear hypothesis of damage summation is quite sufficient for engineering calculations, especially since the performance of structures is estimated taking into account the normalized stock coefficients.
Under static loading, the total damage to the NK-8-2U engines disk is 0.025, the total damage to the NK-16ST ground GTU disk is 0.5, and the NK-16-18ST ground GTU disk is 0.52.
Under fatigue loading, the total damage to the disk of the NK-8-2U engines is 0.065, the total damage to the disk of the ground GTU NK-16ST is 0.000244, and the disk of the ground GTU NK-16-18ST is 0.000318.
Thus, the total damage to the NK-8-2U disk under static loading after flight operation and when working on the ground GTU NK-16ST will be no more (and when working on the ground GTU NK-16-18ST will be no more), and during cyclic loading (and during cyclic loading on the ground GTU NK-16-18ST will be no more), which is significantly less than one.

Conclusion
In the article, using the example of the main parts of the GTD -disks of the second stage of low-pressure turbines of the NK-8-2U aircraft engine gas generators and NK-16ST and NK-16-18ST ground-based gas turbine units, the possibility of using converted aircraft engines on ground-based power plants was predicted.
For all disks of the second stage of low-pressure turbines of three engines, the stressstrain state was calculated using the the boundary integral equations method and using the finite element method in the Ansys program. For three disk stress concentrators, the theoretical stress concentration coefficient was determined and the maximum equivalent stresses were calculated.
The correspondence of the results of the calculated study of the stress-strain state of turbine disks to their actual loading was confirmed by the data of their metallurgical study after prolonged operation.
With the help of Manson's formula, for all disks of the second stage of the low-pressure turbine of three engines, the cyclic durability before the formation of a crack of low-cycle fatigue was determined.
Using the linear damage summation hypothesis, the total damage rates (under static loading and under fatigue loading) of the disks of the second stage of low-pressure turbines of three engines were calculated.
It is established that after the conversion of aircraft engines that have spent their life in flight operation, it is advisable to continue the use of turbine disks on ground-based GTU.