Introduction
Axial flow compressors are generally operated in multiple stages, where each stage consists of a rotor and a stator, allowing the working fluid to flow parallel to the axis of rotation. The stationary blades in the stator decelerate the fluid, converting the circumferential component of the velocity into pressure. Axial flow compressors produces a continuous flow of compressed air, and are highly efficient with large mass flow rates. Therefore, axial flow compressors are generally employed in large gas turbines such as jet engines [
1], high speed ship engines, and small scale power stations requiring high mass flow rates. Mikoyan-Gurevich (MIG)-23, is an interceptor fighter aircraft containing an axial flow compressor with two spools and eleven stages, where the first five stages consists of low pressure compression and the next six stages contains high pressure compression. A picture of a MIG-23 engine fitted with compressor of R-35-300 type, after separating from a shutdown aircraft is shown in Fig. 1. The assembly of an axial flow compressor is complete with compressor’s front and rear cases, an inlet spacer, compressor stage and an air bleeding assembly, see Fig. 1. Front stages of the R-35-300 axial flow compressor are transonic [
2]. Dejen Aviation Industry supplies the MIG-23 fighter aircrafts to the Ethiopian army. However, the compressor blades of the supplied MIG-23 aircraft engines are observed to be frequently failing [
3] due to stone impacts.
Due to heavy import duties to replace the failed blades, we made an attempt here to identify the causes of failure and hence recommend alternate designs and materials meeting the specified requirements. Therefore, in this paper, we report a numerical analysis of one of the recently failed MIG-23 gas turbine compressor blade considering three different Aluminum based alloys. The objective of the present work is to develop a computational methodology to estimate the relevant stresses and hence suggest the suitable materials. A three-dimensional finite element analysis model of an aero-engine compressor blade is carried out in Refs. [
4,
5], to estimate the frequency response of the fan blades and the displacement, stress contours. Fatigue crack growth due to the presence of initial notches at the blade hub-stem junction on the leading edge side are investigated in Ref. [
6]. In general, gravel stones are found to impact the compressor blades of an aircraft engine while landing and takeoff. The gravel stones exists due to heavy civil works in the airport area. Therefore, we considered the impact of a granite stone in our analysis. In order to verify the impact strength of the rotor blades, a finite element analysis of the rotor blades hit by a granite stone is also performed. The numerical results are correlated to the practical observations.
Figure 2 shows a picture of the MIG-23 axial flow multistage compressor in two different views. Rotor and stator blades in different stages can be seen in the picture apart from the turbine shaft connected to the compressor shaft. A three-dimensional design of blades in the turbo-machine is presented in Ref. [
7]. A close up of the front view of the compressor disc showing the bases of fitted rotor blades is shown in Fig. 3(a). Isolated rotor and stator blades of the compressor, each with a mass of 1.3 and 0.65 kg are shown in Figs. 3(b) and 3(c), respectively. The rotor blades of the R-35-300 axial flow compressor in the present MIG-23 gas turbine engine are made up of Aluminum alloys, which can operate at high pressures and temperatures up to 500°C. During the operation of the aircraft, especially during takeoff and landing foreign objects are likely to enter and impact the compressor blades. As a result, the compressor blades will lose the airfoil geometry and hence, the performance and life of the compressor will be reduced. Such a damage has been observed in the MIG-23 aircraft compressor blades, see Fig. 4. Generally, the damage of blades can be categorized as safe, repairable and reject types. The aircraft can be operated when the damage is safe or with minor repairs when the damage is repairable [
8]. However, when the damage is of reject type, the blades need to be replaced before flying the aircraft. Replacing the damaged blades with new blades is an expensive process, as mentioned in the first paragraph. On the other hand, exchanging the blades from another aircraft involves several other issues such as removal and fitting problems.
Bird or stone impact is one of the factors causing severe damage to an aircraft engine. The impacts of an aircraft engine can be divided into three categories: elastic, plastic and hydrodynamic impact. The elastic impacts are typically low speed impacts, and the stresses generated in the elastic impact are generally lower than the material yield stress [
9,
10]. Therefore, the elastic impacts doesn’t cause plastic deformation and hence the engine can be operated without any problem. When the impact speeds are higher, the produced stresses can cause a plastic deformation of the target material. Therefore, the geometry of the blade is permanently deformed and hence will influence the engine performance. This is known as plastic impact. However, depending on the degree of deformation, plastic impact can be repaired when the deformations are small. Hydrodynamic impact is due to a fluid slug against a wall or turbine blade. Bird impact can lead to a hydrodynamic impact of aircraft engine. Bird impact poses serious threats to military and civilian aircrafts as they lead to fatal structural damage to critical aircraft components. However, bird strike experiments are very expensive and hence explicit numerical modeling techniques have grown importance. The initial degradation and failure of individual compressor blades struck by a bird were investigated in Refs. [
11–
13], followed by subsequent damage to other fan blades and engine components is estimated with an experimental verification in Ref. [
11].
Mao et al. [
14] have carried out a finite element analysis a bird striking an engine fan blade in LS-DYNA. They modeled the bird as a fluid jet with a homogenized fluidic constitutive relation, using the Brockman hydrodynamic model. The geometric effects of a bird strike are studied in Ref. [
15] and a parametric study is carried out in Ref. [
16]. Aeroelastic analysis of a stiffened composite wing structure is performed in Ref. [
17], to estimate the flutter behaviour and hence the flutters speed of the wing structure of an unmanned aerial vehicle. Foreign object damage on low pressure compressor blades of an aero gas turbine engine has been investigate in Ref. [
18], to assess the extent of damage and its root cause. The potential of tandem rotor blades in improving the overall performance of transonic compressors has been evaluate using a numerical investigation in Ref. [
19]. The root causes of the failure of a set of blades made up of 718 nickel base alloy in a high pressure compressor of an aircraft engines is explored in Ref. [
20], to report that the failure is mainly attributed to the impact of sand and stones. The root causes for the gas turbine engine break down due to large vibrations and subsequent output power reduction are investigated in Ref. [
21], to confirm the main reason as the domestic object damage due to the impact of the liberated components of the turbine engine on the blades. All the above studies are based on experiments. In other words, computational stone impact studies on compressor blades of a gas turbine engine are rare.
On the other hand, several advanced techniques of modelling material failure, such as: continuum based techniques [
22–
48], molecular dynamics [
49–
51] and multiscale methods [
52]. Frequent failures due to foreign particle impacts are observed in compressor blades of the fighter MIG-23 aircraft engines in the Ethiopian air force/Dejen Aviation Industry. Since it is expensive to replace the engine, we made an attempt here to estimate the failure of the compressor blades by impact of gravel stones. This will help to estimate the tolerable stresses under impact loads and hence the relevant materials can be selected accordingly. Furthermore, the analysis is extended to recommend the suitable materials to withstand the foreign body object impacts. The analysis will help in the manufacture of the blades locally in Africa, which will reduce the import costs. With this motivation, we present an impact analysis of compressor rotor blade strike by a granite stone. The solid model of the blade created in SOLIDWORKS software is imported to the ANSYS software for meshing and stress analysis, which further exported to LS-DYNA software for an impact analysis. Note that the solid model was generated by reverse engineering with the help of a 3D scanner and coordinate measuring machine.
Numerical analysis
In this paper, we mainly focus on the first stage design of the axial flow compressor assembled to the gas turbine engine of a MIG-23 fighter aircraft. The compressor is of constant tip diameter type where air is the working fluid, designed for a mass flow rate of 105 kg/s, pressure ratio of 1.75 and constant axial velocity of 168.1 m/s. Therefore, the air flow enters the compressor in the axial direction. The flow enters a row of the moving rotor blades, where the kinetic energy of the fluid increases, followed by an entry into a row of stator blades, where the fluid decelerates and hence the kinetic energy will be converted to pressure raise. This process will be repeated in several stages until the required pressure and velocity of the fluid is reached before entering the combustion chamber. A typical gas turbine blade is convex on the suction side and concave on the pressure side, see Fig. 5. The line of symmetry of the blade is the known as the camber line.
Modal and stress analysis
In the present work, we considered blades made up of three different of aluminum alloys: 6061-T6, 7075-T6, and 2024-T4. The weight% composition of various elements in each of the above alloys are available in Aerospace Specification Metals (ASM) data base. The material properties of all the three alloys are listed in Table 1 [
53]. According to Table 1, alloy 7075-T6 possess the highest yield and ultimate strength among the three alloys considered in this paper. Note that the selected alloys significantly differ in their ultimate strength. Also, specific heat capacity, fracture toughness and fatigue strength are higher for 7075-T6 alloy compared to the other two alloys. On the other hand, density of the 7075-T6 alloy is slightly higher (maximum 4% compared to 6061-T6 alloy), which can lead to more weight for the given geometry. However, the increase in weight can be compensated by significantly higher (minimum 35% compared to 2024-T4 alloy) value of the yield strength.
The hub to house the blades (see Fig. 3(a)) is made up of Aluminum alloy with Young’s modulus of 70 GPa, Poisson’s ration 0.33 and density of 2770 kg/m3. The measured outer and inner radius of the rotor disc are observed to be 0.1735 and 0.126 m, respectively. Whereas, the maximum diameter of the compressor disc is 0.877 m with top width of the blade as 0.045 m. Further details of the compressor are listed in Table 2. In Table 2, the rotational speed of 8464 RPM and the Mach number corresponds to the operating speed of the turbine engine. The design data in Table 2 corresponds to the extreme specified operating conditions of the aircraft. In this study, stone speed is considered as the impact speed. During the takeoff and landing the aircraft engine runs almost at its full potential. Therefore, impact speeds at the time of landing and takeoff are high due to sudden jerks, hence the stresses caused due to impact will be plastic. The plastic deformation is evident in the grounded aircraft engines, see Fig. 4.
Some design concepts of an aircraft wing structure, both composite and morphing airfoil with auxetic structures discussed in Ref. [
54]. A three dimensional solid model of the rotor blade is created in the SOLIDWORKS commercial software, see Fig. 6. The three dimensional model from SOLIDWORKS is exported to ANSYS commercial software for further analysis. Due to complex geometry of airfoil blade section, quadrilateral elements are used in the discretization, whereas, the base of the rotor blades is discretized with brick elements. In the first step, modal analysis of the blade is carried out considering the three aluminum alloys 7075-T6, 6061-T6, and 2024-T4. The analysis steps in ANSYS are as follows. The angular velocity used in the calculations of the centrifugal force is specified during the solutions stage as follows. The structural inertia loads in the solution tab has been selected, where the angular velocity (in rad/s) along the
z direction is specified in the global cartesian system. In the similar lines, the pressure loads are prescribed on the surface of the component in the component manager tab.
The first ten natural frequencies of the blade for the above three materials are mentioned in Table 3. From Table 3 and Fig. 7, no significant difference is observed in the first three natural frequencies. However, a slight increase is observed in the higher order modes. Deformed configurations at the first four natural frequencies are plotted in Fig. 8. The first four modes are observed to first bending, axial, torsion and second bending modes, respectively. In the next step, we performed the stress analysis at rotational speeds of 5000, 6000, and 8464 RPM. Maximum displacements and von Mises stresses at various speeds are mentioned in Table 4. From Table 4 the estimated maximum displacements and von Mises stresses are within the yield stresses mentioned in Table 1.
Impact analysis
Most of the times stone and bird impact are the primary causes of damage to the compressor blades. Therefore, we performed an analysis of a small granite stone having the following mechanical properties: Young’s modulus 70 MPa, Poisson’s ratio 0.3, ultimate compression stress 280 MPa, density 2700 kg/m
3 [
55].
Figure 9(a) shows the meshed rotor blade and granite stone. The meshed components are further transferred to LS-DYNA in the ANSYS framework for an impact analysis. LS-DYNA is triggered through a program (macro) developed using the ANSYS Parametric Design Language (APDL) in the ANSYS framework. In the macro all the inputs required to execute the analysis in the LS-DYNA are specified. We used the ANSYS AUTODYN option to perform the explicit dynamic analysis of the present impact problem. The stone is made to impact the blade at a velocity of 3670 m/s corresponding to a rotational speed of 8464 RPM. Figure 9(b) shows the displacement plot of the blade in the deformed configuration. The blade tip speed (utip) is calculated using the formula 2prtN, where N is the blade speed in rotations per minute, r and t are the radius and average thickness of the blade, respectively. The maximum displacement is found to be on the leading edge of the blade tip, see Fig. 9(b). Plastic deformation is noticed on the deformed blades, where the maximum deformation in the present analysis is observed to be 6.885 mm.
Distribution of the stresses during the stone impact is shown in Fig. 9(c), where the maximum stress is observed to be 537.79 MPa. A close up of the Fig. 9(c) highlighting the point of maximum stress is shown in Fig. 9(d). From Table 1 the yield and ultimate strengths of 6061-T6 and 2024-T4 alloys are 275 MPa, 310 MPa and 324 Mpa, 469 MPa, respectively. Therefore, alloys 6061-T6 and 2024-T4 can not withstand the stone impact at operational speed and hence will lead to blade fracture, see Fig. 4. Such fracture points are noticed at several locations along the length of many rotor blades, refer to Fig. 4. On the other hand, from Table 1 the yield and ultimate strength of 7075-T6 alloy is 503 and 572 MPa, respectively. Hence, the von Mises impact stresses are more than the yield strength but less than the ultimate tensile strength of alloy 7075-T6. Hence, 7075-T6 can withstand the stone impact without fracture, although the impact may lead to plastic deformation. Furthermore, the fracture toughness and the fatigue strength of 7075-T6 alloy is higher compared to the alloys 6061-T6 and 2024-T4 alloys. Therefore, based on our analysis we recommend alloy 7075-T6 as the blade material.
Conclusions
The compressor blades of the supplied MIG-23 aircraft engines are observed to be frequently failing due to foreign particle impacts. An attempt has been made in this paper, to identify the causes of failure and hence recommend alternate materials meeting the specified requirements. Therefore, we report a numerical analysis of one of the recently failed MIG-23 gas turbine compressor blade made up of three Aluminum based alloys: 6061-T6, 7075-T6, and 2024-T4. An impact analysis of the rotor blades hit by a granite stone is also performed to verify the impact strength of the considered aluminium alloys. The numerical results are correlated to the practical observations.
Based on the modal analysis of the rotor blades, all the three considered alloys posses similar natural frequencies. Observed von Mises stresses at speeds 5000, 6000, and 8464 RPM are within the yield stresses of the three considered alloys. From the results of the impact analysis, alloys 6061-T6 and 2024-T4 can not withstand the stone impact at operational speed and hence will lead to blade fracture. Such fracture points are noticed at several locations along the length of many rotor blades. On the other hand, alloy 7075-T6 can withstand the stone impact without fracture, although the impact may lead to plastic deformation. Furthermore, the fracture toughness and the fatigue strength of 7075-T6 alloy is higher compared to the alloys 6061-T6 and 2024-T4. Therefore, based on our analysis we recommend alloy 7075-T6 as the blade material.
Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature
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