- Email: [email protected]
Failures of multifunctional bulkhead caused by high gradient of temperature, pressure and speed of rotation
Engineering Failure Analysis 89 (2018) 100–117
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failures of multifunctional bulkhead caused by high gradient of temperature, pressure and speed of rotation
T
⁎
Nenad Kolarević , Milosav Ognjanović, Marko Miloš University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, Belgrade, Serbia
A R T IC LE I N F O
ABS TRA CT
Keywords: Gas turbine failures Thermal deformations Thermal wear
The multifunctional bulkhead is an assembly which divides the compressor and turbine space (cold and very hot area) in the gas-generator for turbo-jet and turbo-haft engines. The loads of this bulkhead present an interaction of mechanical, thermal, fluid stream and pressure effects which produce extreme operating conditions and specific kinds of failures. Additional unfavorable effects produce a high gradient and a variation of these loads. The article contains a numerical identification of bulkhead operating conditions and an innovative design of the bulkhead for this purpose. Design development is based on the specific failures which are analyzed together with causes and design possibilities for failure prevention. For this purpose, in the existing design approach “Property-based Design”, a specific procedure named Failure-based design – FBD is applied. The paper also includes CFD analysis, numerical analyses of temperature, stress and deflection distribution of the multifunctional bulkhead. The presentation of the test stand and results of gas-generator testing together with the design solution of the multifunctional bulkhead, are also part of the article.
1. Introduction The multifunctional bulkhead is an innovative design for a specific design of gas generator, developed for production of gas stream which can drive turbo-jet and turbo-shaft motor with free turbine as well as a tip-jet helicopter. The gas-generator consists of a radial compressor driven by a radial turbine appointed on the same shaft at a small distance, and its rotor is showed in Fig. 1. In comparison to a similar design of the gas-generator presented in article [1], the compressor and turbine disc are located at the opposite ends of the shaft. Higher level of efficiency provides a solution with free exhaust nozzle (Fig. 3), but it can be provided if the compressor and turbine disc are close together, divided by the stationary bulkhead. The function of the turbine is to drive the compressor impeller in order to compress the air for the combustion chamber (Fig. 3). This is the reason that the diameter of compressor impeller is higher in comparison to the turbine impeller. The compressor impeller is located in the area of cold air and the turbine disc in the area of the gas of high temperature. The speed of rotation is extremely high and the stationery bulkhead has to be contactless (Fig. 1b). The gradient of temperature, pressure and speed in this narrow space is very high. Since the multifunctional bulkhead is an innovative design structure, there are no exploitation or experimental results of operating conditions and failures of this structure. There are expectable specific kinds of failures caused by specific kinds of operating conditions. Similar and comparable to this are conditions and failures of components of the other mechanical systems which are also exposed to high heating, gas streaming and high fluid pressure. Turbine blades, turbine nozzle or piston head in the internal combustion engines are suitable components for condition and failure comparison to conditions and failures of the multifunctional
⁎
Corresponding author. E-mail address: [email protected] (N. Kolarević).
https://doi.org/10.1016/j.engfailanal.2018.02.022 Received 23 May 2017; Received in revised form 8 January 2018; Accepted 26 February 2018 Available online 27 February 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 1. a) Back-to-back relation of compressor and turbine impellers, b) Multifunctional bulkhead between cold and hot sections with potential operating problems.
bulkhead. The turbine blades are exposed to mechanical loads caused by gas flow and they operate in a nonhomogeneous temperature field. The subject of the analysis in article [2] is temperature distribution around the turbine blade and the results show that temperature distribution in 3D space is in relation to the blade shape. Failures caused by high level of heating can be reduced by ceramic coatings and by application of cooling channels in the blades. High temperature level leads to material degradation and accelerates fatigue cracks initiation. The paper [3] investigated necessary coating thickness in order to protect the material and increase the blade fatigue life. The research contains the stress analysis together with creep process of the blades. Article [4] contains research also in this area but it additionally underlines fatigue crack initiation in the cooling channels in the blades. Apart from these failures, gas streaming and high temperature also produce turbine blades cavitation and material erosion. Article [5] presents the late stage of turbine blades erosion which contains significant material degradation, loss of material and a large amount of initiated fatigue cracks. This is the result of overheating, whirling in the gas stream together with mechanical loads. A similar failure of the gas motor valve is the subject of article [6]. The research is carried out to introduce a metallography process of material degradation and to find the possibility of increasing the material resistance to this kind of failure including material creeping. The beginning of the blades wear caused by gas stream is analyzed in [7]. The results and conclusions show that the failure of the blade surface is complex and a combination of the pitting, cavitation, hot corrosion, erosion etc. Inter crystal corrosion can be important as the cause of turbine blades cracks initiation, discussed in papers [8,9]. The majority of surface wear and material degradation caused by fluid streaming initialize fatigue cracks in the blades or impellers and then fatigue failures [10,11]. Apart from turbine impellers and blades, the turbine nozzle is also exposed to thermal and gas streaming failures [12]. It is also important to mention intensive heating and wear of the vehicle brake discs [13] which are also similar to the failure of the multifunctional bulkhead which is the subject of analysis in this paper. Numerical simulation and analysis of the loads caused by the gas streaming – (CFD Computational Fluid Dynamics) is a methodology which is very suitable for identification of the loads acting on the multifunctional bulkhead, which is the subject of this work. Additionally, FEM (Finite Element Method) application provides the stresses and stress distribution caused by mechanical and thermal loads. Various design structures are analyzed by applying this combination of numerical methods. Using the CFD methodology, article [14] identifies and validates unsteady aerodynamic loads at wind turbine blades. By variation of the blade position and operating conditions, the air stream together with micro and macro vortexes is analyzed and transformed into the blade mechanical loads. Also in [15], the CFD methodology together with optimization technique provided a possibility to optimize design parameters of turbojet engine centrifugal compressor. Adhesive interaction between rotating surface and fluid flow, with the name “Tesla effect” is present in the space of the multifunctional bulkhead. For this reason, it is important to mention the procedure and results of the CFD analysis of the Tesla disc turbine, presented in papers [16–18]. The thermal stress distribution, caused by nonhomogeneous temperature field is also important for the bulkhead which is the subject of this paper. A more complex and similar analysis is carried out in article [19]. Turbine casing is exposed to non-uniform temperature field which produces thermal stresses which together with stress concentration can produce the cracks initiation. The objective of the paper was to harmonize the casing shape and temperature field including the use of cooling channels in the casing. A similar action is carried out for stator blades of the gas turbine [20], but with the aim to replace metal blades with ceramic. The indicators of design quality in this comparison were the deformations caused by the non-uniform temperature field for the two kinds of material. The thermal stress and deformation analysis caused by non-uniform temperature field and extremely high temperatures are also interesting for the pistons in engines with internal combustion. The investigation of the pistons is basically oriented to optimize the piston shape, to protect the piston with corresponding coatings etc. Some of this research, which is also in relation to bulkhead stress and deformation analysis, is presented in the articles [21–26]. 101
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Research in this paper is also based on experimental methods application. For this purpose the laboratory testing has to be carried out in a real gas generator as well as wind turbine blades tested in reference [14]. The test stand is equipped for operating conditions simulation and measurement of indicators of operating process. The test stand for long time testing of the power transmission system of the wind turbine is presented in [27]. The presented process of accelerated testing implies simulation of operating conditions during the entire service life. The combined approach of numerical analysis and testing of components, applied for bulkhead development, is also applied for turbine blades form development, stress reduction and failure prevention [28,29]. The development of the multifunctional bulkhead design in this paper is based on failures obtained by testing in a real gas generator and then on systematic analyses of the failure causes which provide information for design improvement in the next design iteration. This is the part of a general approach in Engineering design with the name “Property-based Design” [30]. The design property, which is the subject of discussion in this paper, is the failure of the bulkhead. In article [31], the subject of the analysis is the failure of the gear transmission units and for this purpose the model for Failure-based design – PBD is presented. Besides recognizing specific failures caused by specific and extreme operating conditions of the multifunctional bulkhead, the objective of this paper is also the development of numerical and experimental tools which will support the design process. A specific approach for the multifunctional bulkhead design has to be the contribution of the paper.
2. Failure types of the disks and bulkhead The extreme operating conditions in the presented assembly in Fig. 1 produce specific types of failure. These conditions identify a high gradient of temperature between a relatively cold zone of compressor space and a very warm zone of turbine space. This gradient is high in axial and in the radial direction. The next condition indicator is the pressure gradient in the bulkhead space. The pressure rapidly decreases toward rotation axe in radial direction. Both disks rotate with the same speed which can be up to 62,000 rpm. Peripheral speed increases in radial direction in the bulkhead space together with gas pressure. In these conditions the following types of failures are identified. Form deflection is the failure presented in Fig. 1b and Fig. 2a. The form of the bulkhead, which has to be a flat plate, is permanently changed. It has become convex, also undulated and it touched the rotating disk of the turbine (Fig. 1b). The reasons for it are non-uniform temperature field, high temperature level and disabled radial expansion of the plate. These conditions produce high residual stresses higher than plastic deformation limits. Additionally, high temperature reduces this limit of material and exceeding this limit for the stresses produces plastic deformations and form deflections of the bulkhead. Form deflections can be various in relation to the bulkhead design, supports and the gradient of service conditions parameters. More details are presented and discussed in Section 4. Surface scratching together with local welding of particles (blue areas in Fig. 2) are a result of high surface contact pressure and high sliding speed. The deflection of bulkhead brought contact of its stationary surfaces and rotating compressor and turbine disks. High sliding speed in the contact produces a few local effects. At the beginning when the contact starts, the wear produces local heating and later the higher increase of deflection produces higher local pressure in the contact. High speed with high local pressure causes extreme local heating and local welding. Local welds break off material particles, score the surfaces and they cause progressive wear. This process is progressive and can be stopped by braking caused by macro welding of sliding surfaces in contact or by bulkhead collapse. Fig. 2a shows one side of the bulkhead with relative small contacts after deflection and Fig. 2b shows the turbine disk progressively worn out. Blue color is evidence of high local heating and welding.
Fig. 2. Failures caused by high gradient of service conditions: a) bulkhead form deflection and wear, b) turbine disk failure by score and fracture.
102
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 3. Conceptual design of gas generator.
Cracks initiation is the third phase of the failure process. When the local stress exceeds the material resistance limit, a crack arises. The level of local stresses is a result of interaction of high level of local temperature and local rigidity of the turbine. In Fig. 2b it is visible that the crack appeared between the turbine blades where the local rigidity is less and deformation is higher. Also between the blades material mass is less and local temperature is higher. This difference produces high local stress and crack. Presented types of failures occurred at around 40,000 rpm of engine run, in the course of testing process at the test stand. Several kinds of design prototypes are varied and tested until an acceptable design solution of bulkhead is developed. 3. Bulkhead operating conditions and effects analysis 3.1. Operating conditions description In Fig. 3 a gas-generator is presented which produces the gas flow which can be used for various applications. Some of them are propulsion of gas-turbine in turbo-shaft engine or simple generating thrust force in turbo-jet variant. This gas-generator consists of intake, compressor, diffuser, combustion chamber, turbine stator, turbine rotor, nozzle and multifunctional bulkhead sections. This bulkhead has a very important role, to divide areas with high difference in temperature and pressure in the small space limited by the parts with extremely high speed of rotation. The role can be identified by the set of functions. These functions are the heat shield, fluid sealing, turbine cooling and reducing the resulting axial force occurred in the engine rotor system. The first function is the protection of the compressor from heat radiation that comes from the turbine (Fig. 4a). The turbine is the part that operates at high temperatures, around 700 °C, so it is made from a special alloy Inconel. The compressor is, on the other hand, a part made from titanium and aluminum alloy Ti-6Al-4V, and it does not allow temperatures that are even close to those values. In that purpose there is a need for some very effective heat shield to protect the compressor. Back-to-back concept of engine means an overhang shaft, so there is a need for the light rotors and as small as possible overhang. This implies a stationary heat shield
Fig. 4. Functions of multifunctional bulkhead: a) heat shield, b) fluid sealing.
103
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 5. Functions of multifunctional bulkhead: a) reducing the resultant axial force toward left on rotor assembly, b) turbine cooling.
with the smallest width dimension. The second function is the sealing which must prevent the leakage of operating fluid from higher pressure cold compressor zone to lower pressure hot turbine zone (Fig. 4b). The air gaps, between the rotor and stator parts, must exist because of radial elongation of certain rotors, so there is no way to apply the sealing between these parts. Based on this, labyrinth seal on multifunction bulkhead is adopted as an acceptable solution. This implies great rigidity of bulkhead structure to achieve desired labyrinth clearance, maximum around 0.1–0.2 mm, for effective operation of the seal and consequently higher engine performance. The third function is reducing the resultant axial force on the rotor assembly, which directly governs the bearings life. As compressor operates on higher static pressure than the turbine, and also has the bigger disk diameter, a significant axial force toward the engine intake is generated due to pressure distributions along these surfaces. In order to reduce this force, a “cold” high pressure air, behind the diffuser, needs to be introduced through the bulkhead and directed along the turbine disk (Fig. 5a). Therefore, there is an increase of the force component in opposite direction, i.e. there is a reduction in the resultant axial force. The fourth function is turbine cooling in order to increase its operation life. As turbine's high values of stress are in zone of its root, this is the most convenient zone for cooling with cold air. As turbine life is directly governed by stresses, and by temperature as in Lancer-Miller diagrams, we attempt to prolong its life by reducing the temperature level in this zone. Relatively cold air should be brought from the zone behind the diffuser, transmitted through the bulkhead and then directed to the turbine disk root areas (Fig. 5b).
3.2. Experimental measurement of condition indicators In order to identify indicators of operating conditions and to provide data for numerical calculations, the testing process is followed by permanent measurement of these indicators. In Fig. 6 there is a schematic presentation of all of them and Fig. 7 shows gas-generator test stand together with some of the used sensors for indicator measurement. The acquisition system for this testing consists of eight input signals. Fuel flow measurement in ignition and fuel supply system is the task of the flow meter. This sensor is installed on the fuel pump and mainly used to monitor and control the operation regime. Detection of damages in tested structure or resonances is the task of the accelerometer. Processing of the acceleration signal provides possibility to identify these events. The thermocouple probe is placed on the more loaded bearing to monitor its temperature during gas-generator operation. The second thermocouple probe measures static temperature of fluid mixture of combustion products. This probe is placed near the exit surface of the engine nozzle. The pressure transducer measures the static pressure behind the diffuser in order to determine the compression ratio of the gas-generator, i.e. maximum achieved overpressure in the gas-generator. The second pressure transducer, with special installation, measures the total pressure of combustion product on the exit of gas-generator nozzle. Speed of rotation measurement is
Fig. 6. Indicators of bulkhead operating conditions.
104
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 7. Test stand (testing set up) of gas-generator.
the task of the inductive sensor placed inside of the gas-generator, in the zone between the bearings. This measuring probe of the shaft revolution per minute (RPM) identification is seated on the intake and the measuring is done over the toothed distant bush on the rotor assembly. Measurement of gas-generator thrust force is the task of the load cell, with appropriate lever system on the test bench. Some of the measured indicators of gas-generator operating conditions are presented in Figs. 8 and 9. All diagrams contain measured gas-generator speed of rotation in RPM and duration of testing in seconds. In relation to these indicators we presented the value of fuel flow, temperature inside the nozzle, overpressure after the compressor and thrust which produces gas flow at the nozzle. At the beginning of the gas-generator testing, the fuel and electromotor starter are turned on at the same time in order to determine the rpm of the generator on which the combustion chamber ignities. On this test the ignition proces occured at the time below 80 s and this situation rapidly increases the rpm of the generator and consequently the pressure inside the system, Fig. 9. The ignition of the combustion chamber produces high temperature in the hot part of the generator (temperature rise on nozzle in Fig. 8) and creates impact on the structure which can be seen from the force sensor in Fig. 9. These two diagrams show the lagging in time due to response time of force and temperature sensors. The sudden drop in fuel mass flow at this point in time is the erroneus state of the flow meter sensor. When the pressure in the generator increases due to the increase of rotor rpm, the resistance which the fuel pump must overcome in order to drive the fuel through the injectors is also increasing and it leads to reduction in fuel mass flow. The flowmeter cannot provide continous flow indication on small values of fuel flow (below 7 g/s) and it creates the erroneous state of flow of around 2 g/s at this point in time, Fig. 8. There is a need to manualy increase the throttle of the pump to compensate this process. When the limit value of 7 g/s of fuel mass flow is passed at the time of around 90 s, the flowmeter contines to show proper flow value. At the moment of 168 s the failure of the bulkhead occures and slows down the generator. This is represented with a sudden drop of generator rpm from 44,000 rpm to 5000 rpm. At this point in time with the drop of the rpm, the pressure inside the generator also suddenly decreases. As the pump was left at the same throttle, while pressure in generator drops, it has much lower resistance and gives much higher mass flow of fuel untill the pump throttle was manualy decreased after 200 s. 3.3. Numerical analysis The volume in the area around the bulkhead is very narrow, and there is no room for measurement of operating conditions separately for the bulkhead. Measured indicators refer to overall conditions of the complete gas-generator. This is the reason that there is no way to create a model that will simulate just the flow in the bulkhead section. The only way is to create a numerical model that will simulate the flow through the entire gas-generator, so it can be validated with the measured parameters in certain points, described in the previous section. If the values of certain indicators of operating conditions, obtained by the numerical model and experimental measurements, are not deviated enough, it can be assumed that the model simulates truthfully enough the real flow through the gas-generator in the observed test. Then the flow aroud the bulkhead can be isolated and studied in order to determine its behavior and loads on the bulkhead structure. Because of limited hardware resorces of computers for simulating the entire volume, the model is created for a representative part 105
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 8. Measured values for the fuel mass flow and the temperature of the gas (combustion products) on the nozzle.
of the 40° cut-out (one ninth of the gas-generator). The CFD model contains the space of gas flow which constists of three main segments: cold part (intake of the air, compression and diffuser section space – blue color), multifunctional bulkhead space section – yellow color, and hot part (turbine stator, rotor and nozzle section space – red color), see Fig. 10. For operating fluid in the cold part of gas-generator, an air ideal gas was adopted as an acceptable approximation. An ideal gas with changed parameters at the temperature of 850 °C, Cp = 1,110Jkg−1 K−1 and molar mass of M = 29.32, is used for the mixture of combustion products in the hot part of the engine. Therefore, the complete model represents a two-component flow. Steady state solution was computed with included viscous work using the SST turbulence model with High Speed (compressible) Wall Heat Transfer model. The influence of the combustion chamber, the link between the diffuser outlet and the turbine stator inlet, is taken into account as total pressure drop of 3%, while the operating fluid is changed as described above. This means that the total pressure on the turbine stator inlet was adopted as the total pressure of the diffuser outlet reduced by 3%. As the total pressure drop through annular chambers is usually from 1% to 5%, the used value of 3% represents an acceptable approximation for rotor operation at 44,000 rpm, which is around 70% of its full operating condition. The numerical model consists of 7 domains, two inlets, two outlets, periodic interfaces and other boundaries conditions. Periodic interfaces are used to make the gas-generator cut-out geometry consistent with the entire gas-generator volume. The main advantage of cut-out geometry of 40° is that it can include a more detailed mesh for some areas of interest, around blades and around the multifunctional bulkhead with the same hardware possibilities. For this model a mesh of 7.5 million elements is created and used. Meshes of main domains are shown in Fig. 10. Results of CFD numerical calculation show that pressure distribution on the compressor and turbine disk surfaces is not uniform as previously thought. Fig. 11 shows pressure distribution on the stationary surface of the multifunctional bulkhead on sides toward the compressor and turbine. Also from CFD results it could be seen that due to narrow space and the influence of boundary layer, particles of flow rotate together with the compressor and turbine, Fig. 12. This means that there is a great influence of the Tesla Turbine effect on the pressure distrubution in this narrow cavity around the multifunctional bulkhead. Also the simulation shows that the bulkhead operates at higher temperatures than expected, Fig. 13. CFD temperature distribution shows the extremely warm red zone in the 106
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 9. Measured values for the static pressure behind the compressor and the effective thrust.
proximity of the axial labyrint due to the small gap and high tangential velocity of moving particles. But as mass flow of the operating fluid is very small around the bulkhead (due to narrow space and the presence of labyrint seals) the convection coefficients of temperature are very small. Total thermal model of the structure of the bulkhead gives a much different real state of body temperature (Fig. 14), because it consists of, besides the convection coefficients obtained from CFD, the cooling channels and the turbine rotor radiation influence. The structural analysis of the multifunction bulkhead structure is introduced through two numerical models, thermal calculation followed by static structural calculation. In the first thermal model, temperature distribution is achieved with convection heat transfer coefficient from CFD model applied on surfaces of the multifunctional bulkhead structure, which are in contact with the flow of the operating fluid, heat radiation from the turbine which is calculated separately and introduced in the model by heat flow, temperature in the cooling channels and some temperature boundary conditions. The results of thermal analysis of the bulkhead structure are shown in Fig. 14. It can be seen that there is high local temperature on the side toward the turbine, where it reaches the maximum value of 500 °C. In the areas of radial cooling channel the temperature of the structure is significantly lower. In triangular space between these channels the temperature rises, especially at the zone of turbine disk perimeter where maximum temperature occurs due to large heat from turbine radiation. The multifunctional bulkhead is made from stainless steel X5CrNi18-10 (1.4301) which has a very high melting point, around 1450 °C. But this material has very poor mechanical characteristics, yield strength 110 N/mm2 at temperatures of 300–500 °C given by the previous thermal numerical model. The presented temperature state is imported in the static structural model together with pressure distribution from CFD calculation. Pressure load in cooling channels is introduced separately with an assumption of no significant pressure drop along channel length. As air for cooling is taken right after the diffuser, its static pressure is used as the load on the multifunctional bulkhead in these areas of cooling channels. This represents the actual load state of the structure during operation. The graphical results are shown in Fig. 15 (stress state) and 16 (deformation). It can be seen that there are some erroneous values of stress on areas close to constrains 107
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 10. Geometry and mesh of CFD numerical model: a) Geometry (40° cut-out) of the cold, bulkhead and hot flow space with several details that show the enlarged calculation mesh in these zones, b) Location of the cold section on the compressor and diffuser, c) Location of the middle section around the multifunctional bulkhead, d) Location of the hot section on turbine stator and rotor.
(Fig. 15c), but the values of 500–600 N/mm2 can easily occur in real behaviour. The temperature field in the space where the disk is seated (Fig. 14) is non-uniform and much higher in the middle of the bulkhead disk. Heating of the disk in this area causes material expansion. Since the outside and inside contours of the disk are strangulated, because these areas have significantly lower temperatures, deformation goes along the axial direction. The disk gets bended, with local undulation (Fig. 16b), with the stress distribution presented in Fig. 15. The maximum axial deformation from finite element numerical model was lower than the clearance between the bulkhead and turbine, but the stress on areas of high temperature was several times larger than yield strength of applied material. Thus obtained value of deformation corresponds to elastic linear behaviour, the Hook law, but in reality the material is far away in plasticity. This means the bulkhead can easily nullify clearance of 1.3 mm (Fig. 1b). Therefore, contact occurs between the stationary part - bulkhead and the rotary part - turbine. This causes damage on both parts, because of the friction with large relative speed. In the contacts, high local pressure arises and with high 108
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 11. Pressure distribution on stationary surfaces of 40° cut-out of multifunctional bulkhead.
Fig. 12. Vector presentation of velocity in the flow around the multifunctional bulkhead which indicates the presence of Tesla turbine effect in this zone.
sliding speed also high local temperature and surface scratching.
4. Failure based design of multifunctional bulkhead Failure-based Design - FBD belongs to the engineering design approach Property-based design. This design approach is oriented to improve certain properties of the design structure, such as reduction of noise and vibration, increase of operating reliability, etc. Bulkhead specific failures are recognized here as one of the bulkhead properties and FBD is oriented to failure elimination by the set of design activities and testing in the form of iterative process. The bulkhead design process has to provide an innovative design structure which can carry out the presented functions and resist extreme operating conditions without failures. Four design solutions were developed and tested, while only the fourth was satisfactory. 109
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 13. Temperature distribution on stationary surfaces of 40° cut-out of multifunctional bulkhead.
Fig. 14. Temperature distribution on multifunctional bulkhead, front side toward compressor and rear side toward turbine.
4.1. Design version 1 In the first design the multifunctional bulkhead was made from two circular shit-metal plates, one of 0.5 mm and the other one of 1.5 mm thickness, laser welded together at several points. One plate, thicker, has radial cooling channels to transport air from the cold high pressure zone from behind the diffuser to the root zone of the turbine disk. Radial channels are formed by cutting in depth of 1 mm and they direct this cold high pressure air on the turbine root for its cooling and to raise the pressure on the turbine disk in order to reduce axial force on the bearings. The labyrinth is pressed inside the ring which is placed and welded inside the circular contour (central hole) of the plates i.e. in the center of the multifunctional bulkhead. The outside of the circular contour of the plates is used for the definition of the bulkhead central position. This contour is clamped to the rest of the stationary structure. The model of such multifunctional bulkhead and its cross section are presented in Fig. 17. 110
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 15. Stress state of multifunctional bulkhead: a) front side toward compressor, b) rear side toward turbine, c) erroneous values of stress in elongated hole due to presents of constrain (2460 N/mm2) and real expected values as 552 N/mm2 and lower.
The test of the gas-generator with the bulkhead, design version 1 (Fig. 17) is carried out until 40,000 rpm. Before this speed is reached, unexpected behaviour of the generator was detected. This behaviour was detected with acquisition system as unexpected vibrations and oscillations of rotation speed. After the disassembly it was determined that the bulkhead was damaged, together with the turbine disk. Failures of these parts are presented in Fig. 18. The heating of the multifunctional bulkhead structure causes material expansion. Since the outside contour of the plates is strangulated due to bulkhead centring, and inside contours because of higher rigidity due to the presence of the ring and labyrinth, deformation goes in axial direction. The bulkhead takes convex form toward turbine side with maximum deformation larger than minimum clearance of 1.3 mm between it and the turbine. There is higher fluid pressure on the compressor side, on the diameter corresponding to the largest turbine diameter, so the pressure difference bends the bulkhead toward the turbine. 111
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 16. Visually enlarged deformation of multifunctional bulkhead: a) total deformation which goes in radial direction permitted by system of elongated holes and pin support, b) directional deformation in the axis of engine crucial for establishing the contact.
Fig. 17. Multifunctional bulkhead version 1, 3D model and its cross section.
In relation to the set of specific failures caused by high gradient of temperature, pressure and speed of rotation, presented in section 2, design version 1 of the multifunctional bulkhead (Fig. 18) contains failures of the first two mentioned groups. The significant form of deflection is the result of high level of temperature and temperature gradient in the bulkhead space, together with the disabled peripheral thermal elongation in radial direction. The bulkhead form of the flat plate has a cone form with concave shape toward the compressor (Fig. 18a) and convex form toward the turbine side (Fig. 18b). Higher air pressure from the compressor side also contributed to this form of deformation. Inhomogeneous temperature field and limited thermal dilatation in circular direction caused one more deformation of the plate form. This is form undulation in circular direction. In the narrow space, bulges at the convex side of the plate got in touch with the turbine disc and caused the second kind of failure in the form of surface screeching with welding of particles. This failure is distributed at the convex side of the bulkhead (Fig. 18b) and at the turbine disc (Fig. 18c and d). In these contacts, high speed friction produces extremely high local heating, bulkhead expansion and progress of this failures. This shows that design solution of version 1 is not successful and has to be improved. Redesign has to provide free expansion of the plate 112
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 18. Failures of bulkhead version 1: a) front side toward compressor, b) backward side toward turbine, c) and d)failure of turbine disc.
and better temperature distribution in the bulkhead space.
4.2. Design version 2 Design improvement of version 2 in comparison with version 1 is provided by outside diameter reduction in order to provide radial clearance, so there is enough space for radial expansion of the structure mostly due to high temperature. The centring of the bulkhead in the assembly, instead of peripheral clamping is accomplished with four elongated holes, cut in its structure, and four pins placed on the backside of the diffuser (Fig. 19a). Vertical elongated holes permit the expansion of the bulkhead in vertical direction and horizontal holes permit the expansion in that direction. In this way the multifunctional bulkhead is centred in space, in order to maintain the labyrinth radial gap in narrow tolerances. Except for the radial labyrinth, an additional axial sealing labyrinth at the turbine side is appointed in version 2. A few grooves of this labyrinth will make it more difficult for air to leak in the turbine section. This also has the consequent effect on increasing the pressure on the turbine side space in order to prevent plate (bulkhead) deformation in the cone form. The assembly of the second version of the bulkhead is shown in Fig. 19. The test of the gas-generator with this second design version of the multifunctional bulkhead showed that generator operation is the most stable and the most durable in comparison with the previous ones. Also, vibrations were significantly lower with no oscillations of rotation speed. Interesting additional phenomenon is noticed. There was increased resistance on the system acceleration and it requires more fuel consumption than expected. It can be explained that the axial labyrinth was in contact with the turbine disk and the friction significantly increased the rotation resistance. The test results presented in Fig. 20 show that design improvements in comparison to version 1 were not successful. Failures of design version 2 are much worse than failures of design version 1. Conical deflection of the bulkhead plate is not prevented. The reason for it is that the limitation for radial thermal deflection of the plate is not only the mechanical peripheral clamping. The nonhomogeneous temperature field leaves the peripheral region of the plate much colder, which limits the internal plate region for 113
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 19. Multifunctional bulkhead version 2: a) elongated holes for centring the bulkhead permitting the radial deformation, b) cross section of bulkhead with marked parts.
Fig. 20. Failures of multifunctional bulkhead version 2: a) front (compressor) side of the plate, b) rear (turbine) side of the plate, c) turbine disc with screeching failure and the cracks, d) bulkhead plate with undulated form.
radial spreading. Axial deflection and undulation of the plate brought the axial labyrinth ring in contact with the turbine disc. This contact was the support against further deflection of the plate and it creates the possibility for longer operation in comparison to design version 1. At the same time, friction in this contact caused higher rotation resistance and extremely high local temperature. Testing with higher speed of rotation in relation to version 1 made these effects stronger. The conclusion is that the axial labyrinth at the turbine side did not fulfil its foreseen function to provide hominization of the temperature field. On the contrary, the friction 114
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 21. Failures of multifunctional bulkhead, design version 3 on the left and turbine disc on the right.
produced extremely high local heating. It is visible at the turbine side of the plate (Fig. 20b) and at the turbine disc (Fig. 20c). The ribs at the opposite side of the turbine disc are the reason that the heating effect of the disc was not uniform. This produced thermal cracks in the turbine disc. One of the largest is visible in Fig. 20c. The undulated bulkhead plate is presented in Fig. 20d. The front (compressor) side of the plate which has the concave form was not in contact with the compressor disc. The heated areas of this side, which Fig. 20a shows, are the result of heating at the opposite (turbine) side.
4.3. Design version 3 In design version 2 the intention was to make the temperature field in the bulkhead space more homogeneous. Because the testing results show that it was not successful, the design version 3 is developed with the aim to provide a possibility for the plate thermal dilatation in nonhomogeneous temperature field without deflection. For this purpose the plate is cut by radial gutters (Fig. 21). The gutters have a double function, to provide circular thermal dilatation of the plate without deflection and air circulation from compressor periphery to the root area of the turbine disc. Radial and axial labyrinth rings are removed in order to provide freedom for thermal dilatations. However, the results of the test were much worse in comparison to the previous two versions. The temperature field remains non-homogeneous with the highest temperatures around the average diameter of the plate. Radial gutters were not enough to provide circular elongation without axial deflection of the plate (Fig. 21). The cold central area and partly peripheral plate area prevent radial elongation and deformation spread axially toward to turbine side. Air pressure from the compressor supported the deformation in this direction. The strong contact between the plate and the turbine disc under very high speed of rotation produced extremely high heating and scratching of both surfaces (Fig. 21) together with the cracks in the turbine disc.
4.4. Design version 4 The previous three design versions show that the complex function of the bulkhead is not possible to be carried out by one compact plate. This complex function contains thermal protection and thermal space sharing together with air sealing of the air under the pressure. Additional secondary functions are temperature field homogenisation, thermal dilatation enabling and air pressure acceptance. This complex function needs a more complex design of the bulkhead structure. This structure has to contain one plate which will share the space and another one which will participate in temperature field harmonisation and thermal dilatation enabling. Air sealing including air pressure distribution have to be the duty of the sealing rings. In Fig. 22 design version 4 of the multifunctional bulkhead is presented. The main function is to hold the main support plate together with the radial labyrinth ring in central hole and axial labyrinth ring placed at the compressor side. The plate is located by the outside cylindrical surface and has the freedom for radial thermal elongation. The secondary function of the main plate is to carry another thin plate. This thin plate is made from thermal resistant material (Inconel 718) and located at the turbine side. The periphery of this sheet metal plate is elastically attached to the standing structure in the area where the temperature is higher. Thermal elongation in radial and circular direction is not limited by the colder zone at the higher diameter. The thin plate is fastened to the main plate with several screws. The cooling of the area of the turbine root zone, including the inside periphery of the thin plate, is provided by the radial holes in the main plate and axial holes in the thin plate. The testing of the gas-generator up to the speed of rotation of 60,000 rpm and more, with multifunctional bulkhead, design version 4 (Fig. 22) was successful. After the testing was finished and the structure disassembled, it was found out that there were no damages at the parts of the bulkhead. Both sides of the multifunctional bulkhead after testing are presented in Fig. 23. The presented process of bulkhead development based on failures of previous design solutions can be identified as Failure based design method – FDM. 115
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 22. Multifunctional bulkhead version 4.
Fig. 23. Multifunctional bulkhead, version 4, after testing: a) turbine side, b) compressor side.
5. Conclusion The presented work is a part of gas-generator and multifunctional bulkhead development. A few failed tests of bulkhead designs were the base and challenge for systematic research of failures in order to identify the reasons of failures that will lead to the correct design solution. The research results are the following.
• Identification of extreme operating conditions in the space of the multifunctional bulkhead which is innovative design of the gas-
generator for turbo-jet and turbo-shaft motors application. These conditions contain extreme speed of rotation, extreme 116
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
• • • •
temperature level and pressure with high gradients. Identification of a few specific kinds of failures: thermal deflection of the design form, surface scratching with extreme local heating and particle welding, and thermal crack initiation. Numerical model and procedure for bulkhead condition analysis is developed. The model contains CFD analysis of the flow space, temperature field analysis, stress distribution and bulkhead form deflection analysis. Testing procedure and test stand for the gas-generator and multifunctional bulkhead testing. Application of specific Failure-based design – FBD methodology and causes analysis of various failures of design solutions of the multifunctional bulkhead. The innovative design of the bulkhead is developed by application of the FBD methodology.
Acknowledgments We are using this opportunity to express our gratitude to EDePro company, in which the testing of the multifunctional bulkhead assembly was done. References [1] E. Benini, S. Giacometti, Design, manufacturing and operation of a small turbojet-engine for research purposes, Appl. Energy 84 (2007) 1102–1116. [2] T. Sadowski, P. Golewski, Multidisciplinary analysis of the operational temperature increase of turbine blades in combustion engines by application of the ceramic thermal barrier coatings (TBC), Comput. Mater. Sci. 50 (2011) 1326–1335. [3] M.R. Reyhania, M. Alizadeh, A. Fathi, H. Khaledi, Turbine blade temperature calculation and life estimation–a sensitivity analysis, Propulsion Power Res. 2 (2) (2013) 148–161. [4] Z. Mazur, A. Luna-Ramirez, J.A. Juarez-Islas, A. Campos-Amezcua, Failure analysis of a gas turbine blade made of Inconel 738LC alloy, Eng. Fail. Anal. 12 (2005) 474–486. [5] M.T. Naeem, N. Rezamahdi, S.A. Jazayeri, Failure analysis of gas turbine blades, Int. J. Engine Res. Innov. 1 (1) (2009) 29–36. [6] M.I. Khan, M.A. Khan, A. Shakoor, A failure analysis of the exhaust valve from a heavy duty natural gas engine, Eng. Fail. Anal. 85 (2018) 77–88. [7] J.R. Laguna-Camacho, L.Y. Villagrán-Villegas, H. Martínez-García, G. Juárez-Morales, M.I. Cruz-Orduña, M. Vite-Torres, L. Ríos-Velasco, I. Hernández-Romero, A study of the wear damage on gas turbine blades, Eng. Fail. Anal. 61 (2016) 88–99. [8] Z. Khan, S. Fida, F. Nisar, N. Alam, Investigation of intergranular corrosion in 2nd stage gas turbine blades of an aircraft engine, Eng. Fail. Anal. 68 (2016) 197–209. [9] J.A. Segura, L. Castro, I. Rosales, J.A. Rodriguez, G. Urquiza, J.M. Rodriguez, Diagnostic and failure analysis in blades of a 300 MW steam turbine, Eng. Fail. Anal. 82 (2017) 631–641. [10] T.F. Azevedo, R.C. Cardoso, P.R.T. da Silva, A.S. Silva, S. Griza, Analysis of turbo impeller rotor failure, Eng. Fail. Anal. 63 (2016) 12–20. [11] M. Zhang, Y. Liu, W. Wang, P. Wang, J. Li, The fatigue of impellers and blades, Eng. Fail. Anal. 62 (2016) 208–231. [12] Z. Mazur, A. Hernandez-Rossette, R. Garcia-Illescas, A. Luna-Ramirez, Failure analysis of a gas turbine nozzle, Eng. Fail. Anal. 15 (2008) 913–921. [13] A. Belhocine, M. Bouchetara, Structural and thermal analysis of automotive disc brake rotor, Arch. Mech. Eng. 61 (2014) 89–113. [14] G. Wu, Z. Qin, L. Zhang, K. Yang, Strain response analysis of adhesively bonded extended composite wind turbine blade suffering unsteady aerodynamic loads, Eng. Fail. Anal. 85 (2018) 36–49. [15] S. Guo, F. Duan, H. Tang, S.C. Lim, M.S. Yip, Multi-objective optimization for centrifugal compressor of mini turbojet engine, Aerosp. Sci. Technol. 39 (2014) 414–425. [16] S. Sengupta, A. Guha, Analytical and computational solutions for three-dimensional flow-field and relative pathlines for the rotating flow in a Tesla disc turbine, Comput. Fluids 88 (2013) 344–353. [17] A. Guha, S. Sengupta, The fluid dynamics of the rotating flow in a tesla disc turbine, Eur. J. Mech. B. Fluids 37 (2013) 112–123. [18] A. Hasan, A. Benzamia, Investigating the impact of air temperature on the performance of a Tesla turbine - using CFD modeling, Int. J. Eng. Innov. Res. 3 (6) (2014) 794–802. [19] E. Poursaeidi, M. Taheri, A. Farhangi, Non-uniform temperature distribution of turbine casing and its effect on turbine casing distortion, Appl. Therm. Eng. 71 (2014) 433–444. [20] P. Rajeevalochanam, B.V.G. Banda, Mechanical design and analysis of ceramic stator blades for gas turbine stage, Mater. Today 4 (2017) 8613–8623. [21] R. Gehlot, B. Tripathi, Thermal analysis of holes created on ceramic coating for diesel engine piston, Case Stud. Therm. Eng. 8 (2016) 291–299. [22] M.R. Ayatollahi, F. Mohammadi, H.R. Chamani, Thermo-mechanical fatigue life assessment of a diesel engine piston, Int. J. Automot. Eng. 1 (4) (2011) 256–266. [23] M. Kumar, A finite element thermo-mechanical stress analysis of IC engine piston, Int. Res. J. Eng. Technol. 4 (6) (2017) 1976–1981. [24] D. Sinha, S. Sarkar, S.C. Mandal, Thermo mechanical analysis of a piston with different thermal barrier coating configuration, Int. J. Eng. Trends Technol. 48 (2017) 335–349. [25] X.F. Liu, Y. Wang, W.H. Liu, Finite element analysis of thermo-mechanical conditions inside the piston of a diesel engine, Appl. Therm. Eng. 119 (2017) 312–318. [26] M. Cerit, M. Coban, Temperature and thermal stress analyses of a ceramic-coated aluminum alloy piston used in a diesel engine, Int. J. Therm. Sci. 77 (2014) 11–18. [27] S. Vasin, M. Ognjanović, M. Miloš, Wind turbine with continual variation of transmission ratio – design and testing methodology, Proc. Romanian Acad. Ser. A 16 (2) (2015) 184–192. [28] R. Wang, K. Jiang, F. Jing, D. Hu, Thermomechanical fatigue failure investigation on a single crystal nickel superalloy turbine blade, Eng. Fail. Anal. 66 (2016) 284–295. [29] H. Kou, J. Lin, J. Zhang, X. Fu, Dynamic and fatigue compressor blade characteristics during fluid-structure interaction: part I-Blade modelling and vibration analysis, Eng. Fail. Anal. 76 (2017) 80–98. [30] H. Krehmer, H. Meerkamm, S. Wartzack, Monitoring a property based product development – from requirements to a mature product, Int. Conf. Eng. Des. 1 (2011) 1–11. [31] M. Ognjanović, N. Kolarević, M. Stanković, S. Vasin, Gear transmission failures and failure based design, 8th International Scientific Conference IRMES, 2017, pp. 21–26.
117
Contents lists available at ScienceDirect
Engineering Failure Analysis journal homepage: www.elsevier.com/locate/engfailanal
Failures of multifunctional bulkhead caused by high gradient of temperature, pressure and speed of rotation
T
⁎
Nenad Kolarević , Milosav Ognjanović, Marko Miloš University of Belgrade, Faculty of Mechanical Engineering, Kraljice Marije 16, Belgrade, Serbia
A R T IC LE I N F O
ABS TRA CT
Keywords: Gas turbine failures Thermal deformations Thermal wear
The multifunctional bulkhead is an assembly which divides the compressor and turbine space (cold and very hot area) in the gas-generator for turbo-jet and turbo-haft engines. The loads of this bulkhead present an interaction of mechanical, thermal, fluid stream and pressure effects which produce extreme operating conditions and specific kinds of failures. Additional unfavorable effects produce a high gradient and a variation of these loads. The article contains a numerical identification of bulkhead operating conditions and an innovative design of the bulkhead for this purpose. Design development is based on the specific failures which are analyzed together with causes and design possibilities for failure prevention. For this purpose, in the existing design approach “Property-based Design”, a specific procedure named Failure-based design – FBD is applied. The paper also includes CFD analysis, numerical analyses of temperature, stress and deflection distribution of the multifunctional bulkhead. The presentation of the test stand and results of gas-generator testing together with the design solution of the multifunctional bulkhead, are also part of the article.
1. Introduction The multifunctional bulkhead is an innovative design for a specific design of gas generator, developed for production of gas stream which can drive turbo-jet and turbo-shaft motor with free turbine as well as a tip-jet helicopter. The gas-generator consists of a radial compressor driven by a radial turbine appointed on the same shaft at a small distance, and its rotor is showed in Fig. 1. In comparison to a similar design of the gas-generator presented in article [1], the compressor and turbine disc are located at the opposite ends of the shaft. Higher level of efficiency provides a solution with free exhaust nozzle (Fig. 3), but it can be provided if the compressor and turbine disc are close together, divided by the stationary bulkhead. The function of the turbine is to drive the compressor impeller in order to compress the air for the combustion chamber (Fig. 3). This is the reason that the diameter of compressor impeller is higher in comparison to the turbine impeller. The compressor impeller is located in the area of cold air and the turbine disc in the area of the gas of high temperature. The speed of rotation is extremely high and the stationery bulkhead has to be contactless (Fig. 1b). The gradient of temperature, pressure and speed in this narrow space is very high. Since the multifunctional bulkhead is an innovative design structure, there are no exploitation or experimental results of operating conditions and failures of this structure. There are expectable specific kinds of failures caused by specific kinds of operating conditions. Similar and comparable to this are conditions and failures of components of the other mechanical systems which are also exposed to high heating, gas streaming and high fluid pressure. Turbine blades, turbine nozzle or piston head in the internal combustion engines are suitable components for condition and failure comparison to conditions and failures of the multifunctional
⁎
Corresponding author. E-mail address: [email protected] (N. Kolarević).
https://doi.org/10.1016/j.engfailanal.2018.02.022 Received 23 May 2017; Received in revised form 8 January 2018; Accepted 26 February 2018 Available online 27 February 2018 1350-6307/ © 2018 Elsevier Ltd. All rights reserved.
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 1. a) Back-to-back relation of compressor and turbine impellers, b) Multifunctional bulkhead between cold and hot sections with potential operating problems.
bulkhead. The turbine blades are exposed to mechanical loads caused by gas flow and they operate in a nonhomogeneous temperature field. The subject of the analysis in article [2] is temperature distribution around the turbine blade and the results show that temperature distribution in 3D space is in relation to the blade shape. Failures caused by high level of heating can be reduced by ceramic coatings and by application of cooling channels in the blades. High temperature level leads to material degradation and accelerates fatigue cracks initiation. The paper [3] investigated necessary coating thickness in order to protect the material and increase the blade fatigue life. The research contains the stress analysis together with creep process of the blades. Article [4] contains research also in this area but it additionally underlines fatigue crack initiation in the cooling channels in the blades. Apart from these failures, gas streaming and high temperature also produce turbine blades cavitation and material erosion. Article [5] presents the late stage of turbine blades erosion which contains significant material degradation, loss of material and a large amount of initiated fatigue cracks. This is the result of overheating, whirling in the gas stream together with mechanical loads. A similar failure of the gas motor valve is the subject of article [6]. The research is carried out to introduce a metallography process of material degradation and to find the possibility of increasing the material resistance to this kind of failure including material creeping. The beginning of the blades wear caused by gas stream is analyzed in [7]. The results and conclusions show that the failure of the blade surface is complex and a combination of the pitting, cavitation, hot corrosion, erosion etc. Inter crystal corrosion can be important as the cause of turbine blades cracks initiation, discussed in papers [8,9]. The majority of surface wear and material degradation caused by fluid streaming initialize fatigue cracks in the blades or impellers and then fatigue failures [10,11]. Apart from turbine impellers and blades, the turbine nozzle is also exposed to thermal and gas streaming failures [12]. It is also important to mention intensive heating and wear of the vehicle brake discs [13] which are also similar to the failure of the multifunctional bulkhead which is the subject of analysis in this paper. Numerical simulation and analysis of the loads caused by the gas streaming – (CFD Computational Fluid Dynamics) is a methodology which is very suitable for identification of the loads acting on the multifunctional bulkhead, which is the subject of this work. Additionally, FEM (Finite Element Method) application provides the stresses and stress distribution caused by mechanical and thermal loads. Various design structures are analyzed by applying this combination of numerical methods. Using the CFD methodology, article [14] identifies and validates unsteady aerodynamic loads at wind turbine blades. By variation of the blade position and operating conditions, the air stream together with micro and macro vortexes is analyzed and transformed into the blade mechanical loads. Also in [15], the CFD methodology together with optimization technique provided a possibility to optimize design parameters of turbojet engine centrifugal compressor. Adhesive interaction between rotating surface and fluid flow, with the name “Tesla effect” is present in the space of the multifunctional bulkhead. For this reason, it is important to mention the procedure and results of the CFD analysis of the Tesla disc turbine, presented in papers [16–18]. The thermal stress distribution, caused by nonhomogeneous temperature field is also important for the bulkhead which is the subject of this paper. A more complex and similar analysis is carried out in article [19]. Turbine casing is exposed to non-uniform temperature field which produces thermal stresses which together with stress concentration can produce the cracks initiation. The objective of the paper was to harmonize the casing shape and temperature field including the use of cooling channels in the casing. A similar action is carried out for stator blades of the gas turbine [20], but with the aim to replace metal blades with ceramic. The indicators of design quality in this comparison were the deformations caused by the non-uniform temperature field for the two kinds of material. The thermal stress and deformation analysis caused by non-uniform temperature field and extremely high temperatures are also interesting for the pistons in engines with internal combustion. The investigation of the pistons is basically oriented to optimize the piston shape, to protect the piston with corresponding coatings etc. Some of this research, which is also in relation to bulkhead stress and deformation analysis, is presented in the articles [21–26]. 101
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Research in this paper is also based on experimental methods application. For this purpose the laboratory testing has to be carried out in a real gas generator as well as wind turbine blades tested in reference [14]. The test stand is equipped for operating conditions simulation and measurement of indicators of operating process. The test stand for long time testing of the power transmission system of the wind turbine is presented in [27]. The presented process of accelerated testing implies simulation of operating conditions during the entire service life. The combined approach of numerical analysis and testing of components, applied for bulkhead development, is also applied for turbine blades form development, stress reduction and failure prevention [28,29]. The development of the multifunctional bulkhead design in this paper is based on failures obtained by testing in a real gas generator and then on systematic analyses of the failure causes which provide information for design improvement in the next design iteration. This is the part of a general approach in Engineering design with the name “Property-based Design” [30]. The design property, which is the subject of discussion in this paper, is the failure of the bulkhead. In article [31], the subject of the analysis is the failure of the gear transmission units and for this purpose the model for Failure-based design – PBD is presented. Besides recognizing specific failures caused by specific and extreme operating conditions of the multifunctional bulkhead, the objective of this paper is also the development of numerical and experimental tools which will support the design process. A specific approach for the multifunctional bulkhead design has to be the contribution of the paper.
2. Failure types of the disks and bulkhead The extreme operating conditions in the presented assembly in Fig. 1 produce specific types of failure. These conditions identify a high gradient of temperature between a relatively cold zone of compressor space and a very warm zone of turbine space. This gradient is high in axial and in the radial direction. The next condition indicator is the pressure gradient in the bulkhead space. The pressure rapidly decreases toward rotation axe in radial direction. Both disks rotate with the same speed which can be up to 62,000 rpm. Peripheral speed increases in radial direction in the bulkhead space together with gas pressure. In these conditions the following types of failures are identified. Form deflection is the failure presented in Fig. 1b and Fig. 2a. The form of the bulkhead, which has to be a flat plate, is permanently changed. It has become convex, also undulated and it touched the rotating disk of the turbine (Fig. 1b). The reasons for it are non-uniform temperature field, high temperature level and disabled radial expansion of the plate. These conditions produce high residual stresses higher than plastic deformation limits. Additionally, high temperature reduces this limit of material and exceeding this limit for the stresses produces plastic deformations and form deflections of the bulkhead. Form deflections can be various in relation to the bulkhead design, supports and the gradient of service conditions parameters. More details are presented and discussed in Section 4. Surface scratching together with local welding of particles (blue areas in Fig. 2) are a result of high surface contact pressure and high sliding speed. The deflection of bulkhead brought contact of its stationary surfaces and rotating compressor and turbine disks. High sliding speed in the contact produces a few local effects. At the beginning when the contact starts, the wear produces local heating and later the higher increase of deflection produces higher local pressure in the contact. High speed with high local pressure causes extreme local heating and local welding. Local welds break off material particles, score the surfaces and they cause progressive wear. This process is progressive and can be stopped by braking caused by macro welding of sliding surfaces in contact or by bulkhead collapse. Fig. 2a shows one side of the bulkhead with relative small contacts after deflection and Fig. 2b shows the turbine disk progressively worn out. Blue color is evidence of high local heating and welding.
Fig. 2. Failures caused by high gradient of service conditions: a) bulkhead form deflection and wear, b) turbine disk failure by score and fracture.
102
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 3. Conceptual design of gas generator.
Cracks initiation is the third phase of the failure process. When the local stress exceeds the material resistance limit, a crack arises. The level of local stresses is a result of interaction of high level of local temperature and local rigidity of the turbine. In Fig. 2b it is visible that the crack appeared between the turbine blades where the local rigidity is less and deformation is higher. Also between the blades material mass is less and local temperature is higher. This difference produces high local stress and crack. Presented types of failures occurred at around 40,000 rpm of engine run, in the course of testing process at the test stand. Several kinds of design prototypes are varied and tested until an acceptable design solution of bulkhead is developed. 3. Bulkhead operating conditions and effects analysis 3.1. Operating conditions description In Fig. 3 a gas-generator is presented which produces the gas flow which can be used for various applications. Some of them are propulsion of gas-turbine in turbo-shaft engine or simple generating thrust force in turbo-jet variant. This gas-generator consists of intake, compressor, diffuser, combustion chamber, turbine stator, turbine rotor, nozzle and multifunctional bulkhead sections. This bulkhead has a very important role, to divide areas with high difference in temperature and pressure in the small space limited by the parts with extremely high speed of rotation. The role can be identified by the set of functions. These functions are the heat shield, fluid sealing, turbine cooling and reducing the resulting axial force occurred in the engine rotor system. The first function is the protection of the compressor from heat radiation that comes from the turbine (Fig. 4a). The turbine is the part that operates at high temperatures, around 700 °C, so it is made from a special alloy Inconel. The compressor is, on the other hand, a part made from titanium and aluminum alloy Ti-6Al-4V, and it does not allow temperatures that are even close to those values. In that purpose there is a need for some very effective heat shield to protect the compressor. Back-to-back concept of engine means an overhang shaft, so there is a need for the light rotors and as small as possible overhang. This implies a stationary heat shield
Fig. 4. Functions of multifunctional bulkhead: a) heat shield, b) fluid sealing.
103
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 5. Functions of multifunctional bulkhead: a) reducing the resultant axial force toward left on rotor assembly, b) turbine cooling.
with the smallest width dimension. The second function is the sealing which must prevent the leakage of operating fluid from higher pressure cold compressor zone to lower pressure hot turbine zone (Fig. 4b). The air gaps, between the rotor and stator parts, must exist because of radial elongation of certain rotors, so there is no way to apply the sealing between these parts. Based on this, labyrinth seal on multifunction bulkhead is adopted as an acceptable solution. This implies great rigidity of bulkhead structure to achieve desired labyrinth clearance, maximum around 0.1–0.2 mm, for effective operation of the seal and consequently higher engine performance. The third function is reducing the resultant axial force on the rotor assembly, which directly governs the bearings life. As compressor operates on higher static pressure than the turbine, and also has the bigger disk diameter, a significant axial force toward the engine intake is generated due to pressure distributions along these surfaces. In order to reduce this force, a “cold” high pressure air, behind the diffuser, needs to be introduced through the bulkhead and directed along the turbine disk (Fig. 5a). Therefore, there is an increase of the force component in opposite direction, i.e. there is a reduction in the resultant axial force. The fourth function is turbine cooling in order to increase its operation life. As turbine's high values of stress are in zone of its root, this is the most convenient zone for cooling with cold air. As turbine life is directly governed by stresses, and by temperature as in Lancer-Miller diagrams, we attempt to prolong its life by reducing the temperature level in this zone. Relatively cold air should be brought from the zone behind the diffuser, transmitted through the bulkhead and then directed to the turbine disk root areas (Fig. 5b).
3.2. Experimental measurement of condition indicators In order to identify indicators of operating conditions and to provide data for numerical calculations, the testing process is followed by permanent measurement of these indicators. In Fig. 6 there is a schematic presentation of all of them and Fig. 7 shows gas-generator test stand together with some of the used sensors for indicator measurement. The acquisition system for this testing consists of eight input signals. Fuel flow measurement in ignition and fuel supply system is the task of the flow meter. This sensor is installed on the fuel pump and mainly used to monitor and control the operation regime. Detection of damages in tested structure or resonances is the task of the accelerometer. Processing of the acceleration signal provides possibility to identify these events. The thermocouple probe is placed on the more loaded bearing to monitor its temperature during gas-generator operation. The second thermocouple probe measures static temperature of fluid mixture of combustion products. This probe is placed near the exit surface of the engine nozzle. The pressure transducer measures the static pressure behind the diffuser in order to determine the compression ratio of the gas-generator, i.e. maximum achieved overpressure in the gas-generator. The second pressure transducer, with special installation, measures the total pressure of combustion product on the exit of gas-generator nozzle. Speed of rotation measurement is
Fig. 6. Indicators of bulkhead operating conditions.
104
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 7. Test stand (testing set up) of gas-generator.
the task of the inductive sensor placed inside of the gas-generator, in the zone between the bearings. This measuring probe of the shaft revolution per minute (RPM) identification is seated on the intake and the measuring is done over the toothed distant bush on the rotor assembly. Measurement of gas-generator thrust force is the task of the load cell, with appropriate lever system on the test bench. Some of the measured indicators of gas-generator operating conditions are presented in Figs. 8 and 9. All diagrams contain measured gas-generator speed of rotation in RPM and duration of testing in seconds. In relation to these indicators we presented the value of fuel flow, temperature inside the nozzle, overpressure after the compressor and thrust which produces gas flow at the nozzle. At the beginning of the gas-generator testing, the fuel and electromotor starter are turned on at the same time in order to determine the rpm of the generator on which the combustion chamber ignities. On this test the ignition proces occured at the time below 80 s and this situation rapidly increases the rpm of the generator and consequently the pressure inside the system, Fig. 9. The ignition of the combustion chamber produces high temperature in the hot part of the generator (temperature rise on nozzle in Fig. 8) and creates impact on the structure which can be seen from the force sensor in Fig. 9. These two diagrams show the lagging in time due to response time of force and temperature sensors. The sudden drop in fuel mass flow at this point in time is the erroneus state of the flow meter sensor. When the pressure in the generator increases due to the increase of rotor rpm, the resistance which the fuel pump must overcome in order to drive the fuel through the injectors is also increasing and it leads to reduction in fuel mass flow. The flowmeter cannot provide continous flow indication on small values of fuel flow (below 7 g/s) and it creates the erroneous state of flow of around 2 g/s at this point in time, Fig. 8. There is a need to manualy increase the throttle of the pump to compensate this process. When the limit value of 7 g/s of fuel mass flow is passed at the time of around 90 s, the flowmeter contines to show proper flow value. At the moment of 168 s the failure of the bulkhead occures and slows down the generator. This is represented with a sudden drop of generator rpm from 44,000 rpm to 5000 rpm. At this point in time with the drop of the rpm, the pressure inside the generator also suddenly decreases. As the pump was left at the same throttle, while pressure in generator drops, it has much lower resistance and gives much higher mass flow of fuel untill the pump throttle was manualy decreased after 200 s. 3.3. Numerical analysis The volume in the area around the bulkhead is very narrow, and there is no room for measurement of operating conditions separately for the bulkhead. Measured indicators refer to overall conditions of the complete gas-generator. This is the reason that there is no way to create a model that will simulate just the flow in the bulkhead section. The only way is to create a numerical model that will simulate the flow through the entire gas-generator, so it can be validated with the measured parameters in certain points, described in the previous section. If the values of certain indicators of operating conditions, obtained by the numerical model and experimental measurements, are not deviated enough, it can be assumed that the model simulates truthfully enough the real flow through the gas-generator in the observed test. Then the flow aroud the bulkhead can be isolated and studied in order to determine its behavior and loads on the bulkhead structure. Because of limited hardware resorces of computers for simulating the entire volume, the model is created for a representative part 105
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 8. Measured values for the fuel mass flow and the temperature of the gas (combustion products) on the nozzle.
of the 40° cut-out (one ninth of the gas-generator). The CFD model contains the space of gas flow which constists of three main segments: cold part (intake of the air, compression and diffuser section space – blue color), multifunctional bulkhead space section – yellow color, and hot part (turbine stator, rotor and nozzle section space – red color), see Fig. 10. For operating fluid in the cold part of gas-generator, an air ideal gas was adopted as an acceptable approximation. An ideal gas with changed parameters at the temperature of 850 °C, Cp = 1,110Jkg−1 K−1 and molar mass of M = 29.32, is used for the mixture of combustion products in the hot part of the engine. Therefore, the complete model represents a two-component flow. Steady state solution was computed with included viscous work using the SST turbulence model with High Speed (compressible) Wall Heat Transfer model. The influence of the combustion chamber, the link between the diffuser outlet and the turbine stator inlet, is taken into account as total pressure drop of 3%, while the operating fluid is changed as described above. This means that the total pressure on the turbine stator inlet was adopted as the total pressure of the diffuser outlet reduced by 3%. As the total pressure drop through annular chambers is usually from 1% to 5%, the used value of 3% represents an acceptable approximation for rotor operation at 44,000 rpm, which is around 70% of its full operating condition. The numerical model consists of 7 domains, two inlets, two outlets, periodic interfaces and other boundaries conditions. Periodic interfaces are used to make the gas-generator cut-out geometry consistent with the entire gas-generator volume. The main advantage of cut-out geometry of 40° is that it can include a more detailed mesh for some areas of interest, around blades and around the multifunctional bulkhead with the same hardware possibilities. For this model a mesh of 7.5 million elements is created and used. Meshes of main domains are shown in Fig. 10. Results of CFD numerical calculation show that pressure distribution on the compressor and turbine disk surfaces is not uniform as previously thought. Fig. 11 shows pressure distribution on the stationary surface of the multifunctional bulkhead on sides toward the compressor and turbine. Also from CFD results it could be seen that due to narrow space and the influence of boundary layer, particles of flow rotate together with the compressor and turbine, Fig. 12. This means that there is a great influence of the Tesla Turbine effect on the pressure distrubution in this narrow cavity around the multifunctional bulkhead. Also the simulation shows that the bulkhead operates at higher temperatures than expected, Fig. 13. CFD temperature distribution shows the extremely warm red zone in the 106
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 9. Measured values for the static pressure behind the compressor and the effective thrust.
proximity of the axial labyrint due to the small gap and high tangential velocity of moving particles. But as mass flow of the operating fluid is very small around the bulkhead (due to narrow space and the presence of labyrint seals) the convection coefficients of temperature are very small. Total thermal model of the structure of the bulkhead gives a much different real state of body temperature (Fig. 14), because it consists of, besides the convection coefficients obtained from CFD, the cooling channels and the turbine rotor radiation influence. The structural analysis of the multifunction bulkhead structure is introduced through two numerical models, thermal calculation followed by static structural calculation. In the first thermal model, temperature distribution is achieved with convection heat transfer coefficient from CFD model applied on surfaces of the multifunctional bulkhead structure, which are in contact with the flow of the operating fluid, heat radiation from the turbine which is calculated separately and introduced in the model by heat flow, temperature in the cooling channels and some temperature boundary conditions. The results of thermal analysis of the bulkhead structure are shown in Fig. 14. It can be seen that there is high local temperature on the side toward the turbine, where it reaches the maximum value of 500 °C. In the areas of radial cooling channel the temperature of the structure is significantly lower. In triangular space between these channels the temperature rises, especially at the zone of turbine disk perimeter where maximum temperature occurs due to large heat from turbine radiation. The multifunctional bulkhead is made from stainless steel X5CrNi18-10 (1.4301) which has a very high melting point, around 1450 °C. But this material has very poor mechanical characteristics, yield strength 110 N/mm2 at temperatures of 300–500 °C given by the previous thermal numerical model. The presented temperature state is imported in the static structural model together with pressure distribution from CFD calculation. Pressure load in cooling channels is introduced separately with an assumption of no significant pressure drop along channel length. As air for cooling is taken right after the diffuser, its static pressure is used as the load on the multifunctional bulkhead in these areas of cooling channels. This represents the actual load state of the structure during operation. The graphical results are shown in Fig. 15 (stress state) and 16 (deformation). It can be seen that there are some erroneous values of stress on areas close to constrains 107
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 10. Geometry and mesh of CFD numerical model: a) Geometry (40° cut-out) of the cold, bulkhead and hot flow space with several details that show the enlarged calculation mesh in these zones, b) Location of the cold section on the compressor and diffuser, c) Location of the middle section around the multifunctional bulkhead, d) Location of the hot section on turbine stator and rotor.
(Fig. 15c), but the values of 500–600 N/mm2 can easily occur in real behaviour. The temperature field in the space where the disk is seated (Fig. 14) is non-uniform and much higher in the middle of the bulkhead disk. Heating of the disk in this area causes material expansion. Since the outside and inside contours of the disk are strangulated, because these areas have significantly lower temperatures, deformation goes along the axial direction. The disk gets bended, with local undulation (Fig. 16b), with the stress distribution presented in Fig. 15. The maximum axial deformation from finite element numerical model was lower than the clearance between the bulkhead and turbine, but the stress on areas of high temperature was several times larger than yield strength of applied material. Thus obtained value of deformation corresponds to elastic linear behaviour, the Hook law, but in reality the material is far away in plasticity. This means the bulkhead can easily nullify clearance of 1.3 mm (Fig. 1b). Therefore, contact occurs between the stationary part - bulkhead and the rotary part - turbine. This causes damage on both parts, because of the friction with large relative speed. In the contacts, high local pressure arises and with high 108
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 11. Pressure distribution on stationary surfaces of 40° cut-out of multifunctional bulkhead.
Fig. 12. Vector presentation of velocity in the flow around the multifunctional bulkhead which indicates the presence of Tesla turbine effect in this zone.
sliding speed also high local temperature and surface scratching.
4. Failure based design of multifunctional bulkhead Failure-based Design - FBD belongs to the engineering design approach Property-based design. This design approach is oriented to improve certain properties of the design structure, such as reduction of noise and vibration, increase of operating reliability, etc. Bulkhead specific failures are recognized here as one of the bulkhead properties and FBD is oriented to failure elimination by the set of design activities and testing in the form of iterative process. The bulkhead design process has to provide an innovative design structure which can carry out the presented functions and resist extreme operating conditions without failures. Four design solutions were developed and tested, while only the fourth was satisfactory. 109
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 13. Temperature distribution on stationary surfaces of 40° cut-out of multifunctional bulkhead.
Fig. 14. Temperature distribution on multifunctional bulkhead, front side toward compressor and rear side toward turbine.
4.1. Design version 1 In the first design the multifunctional bulkhead was made from two circular shit-metal plates, one of 0.5 mm and the other one of 1.5 mm thickness, laser welded together at several points. One plate, thicker, has radial cooling channels to transport air from the cold high pressure zone from behind the diffuser to the root zone of the turbine disk. Radial channels are formed by cutting in depth of 1 mm and they direct this cold high pressure air on the turbine root for its cooling and to raise the pressure on the turbine disk in order to reduce axial force on the bearings. The labyrinth is pressed inside the ring which is placed and welded inside the circular contour (central hole) of the plates i.e. in the center of the multifunctional bulkhead. The outside of the circular contour of the plates is used for the definition of the bulkhead central position. This contour is clamped to the rest of the stationary structure. The model of such multifunctional bulkhead and its cross section are presented in Fig. 17. 110
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 15. Stress state of multifunctional bulkhead: a) front side toward compressor, b) rear side toward turbine, c) erroneous values of stress in elongated hole due to presents of constrain (2460 N/mm2) and real expected values as 552 N/mm2 and lower.
The test of the gas-generator with the bulkhead, design version 1 (Fig. 17) is carried out until 40,000 rpm. Before this speed is reached, unexpected behaviour of the generator was detected. This behaviour was detected with acquisition system as unexpected vibrations and oscillations of rotation speed. After the disassembly it was determined that the bulkhead was damaged, together with the turbine disk. Failures of these parts are presented in Fig. 18. The heating of the multifunctional bulkhead structure causes material expansion. Since the outside contour of the plates is strangulated due to bulkhead centring, and inside contours because of higher rigidity due to the presence of the ring and labyrinth, deformation goes in axial direction. The bulkhead takes convex form toward turbine side with maximum deformation larger than minimum clearance of 1.3 mm between it and the turbine. There is higher fluid pressure on the compressor side, on the diameter corresponding to the largest turbine diameter, so the pressure difference bends the bulkhead toward the turbine. 111
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 16. Visually enlarged deformation of multifunctional bulkhead: a) total deformation which goes in radial direction permitted by system of elongated holes and pin support, b) directional deformation in the axis of engine crucial for establishing the contact.
Fig. 17. Multifunctional bulkhead version 1, 3D model and its cross section.
In relation to the set of specific failures caused by high gradient of temperature, pressure and speed of rotation, presented in section 2, design version 1 of the multifunctional bulkhead (Fig. 18) contains failures of the first two mentioned groups. The significant form of deflection is the result of high level of temperature and temperature gradient in the bulkhead space, together with the disabled peripheral thermal elongation in radial direction. The bulkhead form of the flat plate has a cone form with concave shape toward the compressor (Fig. 18a) and convex form toward the turbine side (Fig. 18b). Higher air pressure from the compressor side also contributed to this form of deformation. Inhomogeneous temperature field and limited thermal dilatation in circular direction caused one more deformation of the plate form. This is form undulation in circular direction. In the narrow space, bulges at the convex side of the plate got in touch with the turbine disc and caused the second kind of failure in the form of surface screeching with welding of particles. This failure is distributed at the convex side of the bulkhead (Fig. 18b) and at the turbine disc (Fig. 18c and d). In these contacts, high speed friction produces extremely high local heating, bulkhead expansion and progress of this failures. This shows that design solution of version 1 is not successful and has to be improved. Redesign has to provide free expansion of the plate 112
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 18. Failures of bulkhead version 1: a) front side toward compressor, b) backward side toward turbine, c) and d)failure of turbine disc.
and better temperature distribution in the bulkhead space.
4.2. Design version 2 Design improvement of version 2 in comparison with version 1 is provided by outside diameter reduction in order to provide radial clearance, so there is enough space for radial expansion of the structure mostly due to high temperature. The centring of the bulkhead in the assembly, instead of peripheral clamping is accomplished with four elongated holes, cut in its structure, and four pins placed on the backside of the diffuser (Fig. 19a). Vertical elongated holes permit the expansion of the bulkhead in vertical direction and horizontal holes permit the expansion in that direction. In this way the multifunctional bulkhead is centred in space, in order to maintain the labyrinth radial gap in narrow tolerances. Except for the radial labyrinth, an additional axial sealing labyrinth at the turbine side is appointed in version 2. A few grooves of this labyrinth will make it more difficult for air to leak in the turbine section. This also has the consequent effect on increasing the pressure on the turbine side space in order to prevent plate (bulkhead) deformation in the cone form. The assembly of the second version of the bulkhead is shown in Fig. 19. The test of the gas-generator with this second design version of the multifunctional bulkhead showed that generator operation is the most stable and the most durable in comparison with the previous ones. Also, vibrations were significantly lower with no oscillations of rotation speed. Interesting additional phenomenon is noticed. There was increased resistance on the system acceleration and it requires more fuel consumption than expected. It can be explained that the axial labyrinth was in contact with the turbine disk and the friction significantly increased the rotation resistance. The test results presented in Fig. 20 show that design improvements in comparison to version 1 were not successful. Failures of design version 2 are much worse than failures of design version 1. Conical deflection of the bulkhead plate is not prevented. The reason for it is that the limitation for radial thermal deflection of the plate is not only the mechanical peripheral clamping. The nonhomogeneous temperature field leaves the peripheral region of the plate much colder, which limits the internal plate region for 113
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 19. Multifunctional bulkhead version 2: a) elongated holes for centring the bulkhead permitting the radial deformation, b) cross section of bulkhead with marked parts.
Fig. 20. Failures of multifunctional bulkhead version 2: a) front (compressor) side of the plate, b) rear (turbine) side of the plate, c) turbine disc with screeching failure and the cracks, d) bulkhead plate with undulated form.
radial spreading. Axial deflection and undulation of the plate brought the axial labyrinth ring in contact with the turbine disc. This contact was the support against further deflection of the plate and it creates the possibility for longer operation in comparison to design version 1. At the same time, friction in this contact caused higher rotation resistance and extremely high local temperature. Testing with higher speed of rotation in relation to version 1 made these effects stronger. The conclusion is that the axial labyrinth at the turbine side did not fulfil its foreseen function to provide hominization of the temperature field. On the contrary, the friction 114
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 21. Failures of multifunctional bulkhead, design version 3 on the left and turbine disc on the right.
produced extremely high local heating. It is visible at the turbine side of the plate (Fig. 20b) and at the turbine disc (Fig. 20c). The ribs at the opposite side of the turbine disc are the reason that the heating effect of the disc was not uniform. This produced thermal cracks in the turbine disc. One of the largest is visible in Fig. 20c. The undulated bulkhead plate is presented in Fig. 20d. The front (compressor) side of the plate which has the concave form was not in contact with the compressor disc. The heated areas of this side, which Fig. 20a shows, are the result of heating at the opposite (turbine) side.
4.3. Design version 3 In design version 2 the intention was to make the temperature field in the bulkhead space more homogeneous. Because the testing results show that it was not successful, the design version 3 is developed with the aim to provide a possibility for the plate thermal dilatation in nonhomogeneous temperature field without deflection. For this purpose the plate is cut by radial gutters (Fig. 21). The gutters have a double function, to provide circular thermal dilatation of the plate without deflection and air circulation from compressor periphery to the root area of the turbine disc. Radial and axial labyrinth rings are removed in order to provide freedom for thermal dilatations. However, the results of the test were much worse in comparison to the previous two versions. The temperature field remains non-homogeneous with the highest temperatures around the average diameter of the plate. Radial gutters were not enough to provide circular elongation without axial deflection of the plate (Fig. 21). The cold central area and partly peripheral plate area prevent radial elongation and deformation spread axially toward to turbine side. Air pressure from the compressor supported the deformation in this direction. The strong contact between the plate and the turbine disc under very high speed of rotation produced extremely high heating and scratching of both surfaces (Fig. 21) together with the cracks in the turbine disc.
4.4. Design version 4 The previous three design versions show that the complex function of the bulkhead is not possible to be carried out by one compact plate. This complex function contains thermal protection and thermal space sharing together with air sealing of the air under the pressure. Additional secondary functions are temperature field homogenisation, thermal dilatation enabling and air pressure acceptance. This complex function needs a more complex design of the bulkhead structure. This structure has to contain one plate which will share the space and another one which will participate in temperature field harmonisation and thermal dilatation enabling. Air sealing including air pressure distribution have to be the duty of the sealing rings. In Fig. 22 design version 4 of the multifunctional bulkhead is presented. The main function is to hold the main support plate together with the radial labyrinth ring in central hole and axial labyrinth ring placed at the compressor side. The plate is located by the outside cylindrical surface and has the freedom for radial thermal elongation. The secondary function of the main plate is to carry another thin plate. This thin plate is made from thermal resistant material (Inconel 718) and located at the turbine side. The periphery of this sheet metal plate is elastically attached to the standing structure in the area where the temperature is higher. Thermal elongation in radial and circular direction is not limited by the colder zone at the higher diameter. The thin plate is fastened to the main plate with several screws. The cooling of the area of the turbine root zone, including the inside periphery of the thin plate, is provided by the radial holes in the main plate and axial holes in the thin plate. The testing of the gas-generator up to the speed of rotation of 60,000 rpm and more, with multifunctional bulkhead, design version 4 (Fig. 22) was successful. After the testing was finished and the structure disassembled, it was found out that there were no damages at the parts of the bulkhead. Both sides of the multifunctional bulkhead after testing are presented in Fig. 23. The presented process of bulkhead development based on failures of previous design solutions can be identified as Failure based design method – FDM. 115
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
Fig. 22. Multifunctional bulkhead version 4.
Fig. 23. Multifunctional bulkhead, version 4, after testing: a) turbine side, b) compressor side.
5. Conclusion The presented work is a part of gas-generator and multifunctional bulkhead development. A few failed tests of bulkhead designs were the base and challenge for systematic research of failures in order to identify the reasons of failures that will lead to the correct design solution. The research results are the following.
• Identification of extreme operating conditions in the space of the multifunctional bulkhead which is innovative design of the gas-
generator for turbo-jet and turbo-shaft motors application. These conditions contain extreme speed of rotation, extreme 116
Engineering Failure Analysis 89 (2018) 100–117
N. Kolarević et al.
• • • •
temperature level and pressure with high gradients. Identification of a few specific kinds of failures: thermal deflection of the design form, surface scratching with extreme local heating and particle welding, and thermal crack initiation. Numerical model and procedure for bulkhead condition analysis is developed. The model contains CFD analysis of the flow space, temperature field analysis, stress distribution and bulkhead form deflection analysis. Testing procedure and test stand for the gas-generator and multifunctional bulkhead testing. Application of specific Failure-based design – FBD methodology and causes analysis of various failures of design solutions of the multifunctional bulkhead. The innovative design of the bulkhead is developed by application of the FBD methodology.
Acknowledgments We are using this opportunity to express our gratitude to EDePro company, in which the testing of the multifunctional bulkhead assembly was done. References [1] E. Benini, S. Giacometti, Design, manufacturing and operation of a small turbojet-engine for research purposes, Appl. Energy 84 (2007) 1102–1116. [2] T. Sadowski, P. Golewski, Multidisciplinary analysis of the operational temperature increase of turbine blades in combustion engines by application of the ceramic thermal barrier coatings (TBC), Comput. Mater. Sci. 50 (2011) 1326–1335. [3] M.R. Reyhania, M. Alizadeh, A. Fathi, H. Khaledi, Turbine blade temperature calculation and life estimation–a sensitivity analysis, Propulsion Power Res. 2 (2) (2013) 148–161. [4] Z. Mazur, A. Luna-Ramirez, J.A. Juarez-Islas, A. Campos-Amezcua, Failure analysis of a gas turbine blade made of Inconel 738LC alloy, Eng. Fail. Anal. 12 (2005) 474–486. [5] M.T. Naeem, N. Rezamahdi, S.A. Jazayeri, Failure analysis of gas turbine blades, Int. J. Engine Res. Innov. 1 (1) (2009) 29–36. [6] M.I. Khan, M.A. Khan, A. Shakoor, A failure analysis of the exhaust valve from a heavy duty natural gas engine, Eng. Fail. Anal. 85 (2018) 77–88. [7] J.R. Laguna-Camacho, L.Y. Villagrán-Villegas, H. Martínez-García, G. Juárez-Morales, M.I. Cruz-Orduña, M. Vite-Torres, L. Ríos-Velasco, I. Hernández-Romero, A study of the wear damage on gas turbine blades, Eng. Fail. Anal. 61 (2016) 88–99. [8] Z. Khan, S. Fida, F. Nisar, N. Alam, Investigation of intergranular corrosion in 2nd stage gas turbine blades of an aircraft engine, Eng. Fail. Anal. 68 (2016) 197–209. [9] J.A. Segura, L. Castro, I. Rosales, J.A. Rodriguez, G. Urquiza, J.M. Rodriguez, Diagnostic and failure analysis in blades of a 300 MW steam turbine, Eng. Fail. Anal. 82 (2017) 631–641. [10] T.F. Azevedo, R.C. Cardoso, P.R.T. da Silva, A.S. Silva, S. Griza, Analysis of turbo impeller rotor failure, Eng. Fail. Anal. 63 (2016) 12–20. [11] M. Zhang, Y. Liu, W. Wang, P. Wang, J. Li, The fatigue of impellers and blades, Eng. Fail. Anal. 62 (2016) 208–231. [12] Z. Mazur, A. Hernandez-Rossette, R. Garcia-Illescas, A. Luna-Ramirez, Failure analysis of a gas turbine nozzle, Eng. Fail. Anal. 15 (2008) 913–921. [13] A. Belhocine, M. Bouchetara, Structural and thermal analysis of automotive disc brake rotor, Arch. Mech. Eng. 61 (2014) 89–113. [14] G. Wu, Z. Qin, L. Zhang, K. Yang, Strain response analysis of adhesively bonded extended composite wind turbine blade suffering unsteady aerodynamic loads, Eng. Fail. Anal. 85 (2018) 36–49. [15] S. Guo, F. Duan, H. Tang, S.C. Lim, M.S. Yip, Multi-objective optimization for centrifugal compressor of mini turbojet engine, Aerosp. Sci. Technol. 39 (2014) 414–425. [16] S. Sengupta, A. Guha, Analytical and computational solutions for three-dimensional flow-field and relative pathlines for the rotating flow in a Tesla disc turbine, Comput. Fluids 88 (2013) 344–353. [17] A. Guha, S. Sengupta, The fluid dynamics of the rotating flow in a tesla disc turbine, Eur. J. Mech. B. Fluids 37 (2013) 112–123. [18] A. Hasan, A. Benzamia, Investigating the impact of air temperature on the performance of a Tesla turbine - using CFD modeling, Int. J. Eng. Innov. Res. 3 (6) (2014) 794–802. [19] E. Poursaeidi, M. Taheri, A. Farhangi, Non-uniform temperature distribution of turbine casing and its effect on turbine casing distortion, Appl. Therm. Eng. 71 (2014) 433–444. [20] P. Rajeevalochanam, B.V.G. Banda, Mechanical design and analysis of ceramic stator blades for gas turbine stage, Mater. Today 4 (2017) 8613–8623. [21] R. Gehlot, B. Tripathi, Thermal analysis of holes created on ceramic coating for diesel engine piston, Case Stud. Therm. Eng. 8 (2016) 291–299. [22] M.R. Ayatollahi, F. Mohammadi, H.R. Chamani, Thermo-mechanical fatigue life assessment of a diesel engine piston, Int. J. Automot. Eng. 1 (4) (2011) 256–266. [23] M. Kumar, A finite element thermo-mechanical stress analysis of IC engine piston, Int. Res. J. Eng. Technol. 4 (6) (2017) 1976–1981. [24] D. Sinha, S. Sarkar, S.C. Mandal, Thermo mechanical analysis of a piston with different thermal barrier coating configuration, Int. J. Eng. Trends Technol. 48 (2017) 335–349. [25] X.F. Liu, Y. Wang, W.H. Liu, Finite element analysis of thermo-mechanical conditions inside the piston of a diesel engine, Appl. Therm. Eng. 119 (2017) 312–318. [26] M. Cerit, M. Coban, Temperature and thermal stress analyses of a ceramic-coated aluminum alloy piston used in a diesel engine, Int. J. Therm. Sci. 77 (2014) 11–18. [27] S. Vasin, M. Ognjanović, M. Miloš, Wind turbine with continual variation of transmission ratio – design and testing methodology, Proc. Romanian Acad. Ser. A 16 (2) (2015) 184–192. [28] R. Wang, K. Jiang, F. Jing, D. Hu, Thermomechanical fatigue failure investigation on a single crystal nickel superalloy turbine blade, Eng. Fail. Anal. 66 (2016) 284–295. [29] H. Kou, J. Lin, J. Zhang, X. Fu, Dynamic and fatigue compressor blade characteristics during fluid-structure interaction: part I-Blade modelling and vibration analysis, Eng. Fail. Anal. 76 (2017) 80–98. [30] H. Krehmer, H. Meerkamm, S. Wartzack, Monitoring a property based product development – from requirements to a mature product, Int. Conf. Eng. Des. 1 (2011) 1–11. [31] M. Ognjanović, N. Kolarević, M. Stanković, S. Vasin, Gear transmission failures and failure based design, 8th International Scientific Conference IRMES, 2017, pp. 21–26.
117