Thermomechanical Failure of Turbine Blades. Analysis and Recommendations


Bachelor Thesis, 2015
58 Pages, Grade: First class

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Table of Contents

Acknowledgment

Abstract

Introduction

1. Blade science
1.1 The Turbine Blade
1.2 Turbine Blade science
1.3 The Jet engine

2. Critical analysis
2.1 Data collection
2.2 Findings
2.3 Aircraft information Survey
Statement
Brief Description
General information
If there is a specific part that requires most attention
Methods of rectification of the problem
Recommendations on how this maintenance procedure can be improved

3. Maintenance
3.1 Turbine blades
3.1.1 Visual inspection
3.1.2 Liquid penetrating testing
3.1.3 Magnetic particle testing
3.1.4 Eddy current inspection

4. Improvements
4.1 Dynamic spin testing
4.1.1 Current methods for blade damping
4.2 New visual inspection method
4.2.1 The IPLEX video scope

5. Blade destruction analysis and damage to aircraft
5.1 Examples of turbine blade failure
5.2 Damage to aircraft
5.2.1 Contained
5.2.2 Uncontained failure

6. Cause of blade failure
6.1 Thermo - mechanical fatigue
6.1.1 Creep
6.1.2 Corrosion
6.1.3 Foreign object damage

7. Preventing blade failure
7.1 Material
7.2 Coating the blade
7.3 Existing cooling methods
7.3.1 Internal cooling
7.3.2 External cooling
7.3.3 Newer designs

8. ANSYS Thermo-mechanical analysis
8.1 First reading
8.2 Second reading
8.3 Third reading
8.4 Fourth reading

Conclusion

Recommendations

Bibliography

List of Figures

Appendices

Appendix A:

Appendix B:

Acknowledgment

The completion of this research paper needed time and effort put in by many parties, from my professors, instructors and engineers who offered there time to ensure all the completion of the paper.

I would like to first thank Sir Nasser Chakra for his support and guidance throughout the course in pointing me in the right direction and correcting and supporting me at the right time.

I would also like to thank Mr. Channe DaSilva for assisting me throughout the report, informing me about the right information and steps into completing the report.

I would also like to thank Mr. Challaka for giving me his valuable time and information for the completion of the report

Thank you everyone who supported me in this report.

Imal Liyanage

Abstract

This report paper explains the roll of a turbine blade and its analysis in today’s jet engines. The report first discusses all the relevant information about how the conclusion of the turbine blade was reached from all the other probable components in the engine, then it moves on the full analysis of the turbine blade.

Discussing about the Maintenance and Testing of the blade to see where the industry is and where its heading, then it moves on to the failure analysis. This shows the major pitfalls in the design of the blade and if a failure occurs, how the aircraft I built to react to such an incident.

Material inspection and possible causes of failure is also discussed next, deformation characteristics due to thermally induced stress and new methods to prevent such failures is looked into.

Introduction

Ever since the dawn the jet engine the turbine blades were a main focus, because developing material and coming up with revolutionary methods to increase internal temperature as well as cooling the blades was a hurdle the industry had to get over in order to move on. General Electric’s latest turbo fan engines can produce up to 115,000 foot pounds of thrust, and later this year they will be launching a new more fuel efficient model to tackle the short hauling flights.

It’s all possible because of better efficiency in the burning and harnessing power of the turbine blades. As will be mentioned in the report, increasing the exhaust gas temperature and having a richer more uniformed flow will increase the power output of the engine at lower fuel consumption levels, and this is possible because the turbine blades can survive in this environment and thrive in it. Operating close to 1600˚C in some conditions the blades spin around at 15,000 rpm to transfer the energy to the fan and compressor assemble in the front, and all this is achieved by ground breaking super alloys and revolutionary new methods in turbine blade cooling.

1. Blade science

1.1 The Turbine Blade

The turbine blade, able to withstand temperatures of over 1500˚C and still retain its integrity to harness the energy from the hot expending gas, this truly is one of the most refined parts of any modern jet engine.

From the time of its conception in the early 1930’s the idea of propelling aircrafts beyond the operational speeds of existing propeller driven aircrafts (beyond 600km/h) was un heard of. Materials able to withstand the extreme temperatures reached in the combustion and turbine section simply did not exist at the time. The turbine blade is the key link that can Figure 1 Frank Whittle harnesses the immense serge of power in a jet engine and this over time has been improved upon to make it more durable, efficient and lighter. (Bellis, 2001)

When Sir Frank Whittle first started running tests on his new experimental propulsion system, the engines hot sections melted due to the immense heat generated by the combustion unit. At the time there was no material strong enough to withstand such temperatures. Metal manufactures in the early 1940’s came up

with a brilliant blend of Nickel, chrome, steal and molybdenum to create the first super metal strong enough to survive inside the engine. (Bellis, 2001)

Figure 2 First jet engine

From this era onwards, manufactures have been making stronger and more durable alloys to improve the life of the turbine blades.

1.2 Turbine Blade science

Newton’s third low of motion dictates that every action has an equal but opposite reaction. This is one of the basic principles governing the working of a jet engine and can be applicable to the turbine blades themselves.

Turbine blades are used everywhere, ever since the notion that a fluid in motion can give mechanical power. The water wheel used to power windmills use free flowing water as a driving force that is harnessed by the paddles on the water wheel and this principle is the same as the one used to convert kinetic power to mechanical driving power in hydroelectric dams. More efficient hydrodynamic designs allow hydro turbines Figure 3 Removed turbine sectionto convert more power with slower moving water. Blades inside the jet engine work in a similar manner. The high velocity gas expanding at an immense rate pushes against the turbine blades in its path to continue its flow and this acts on all the blades around the axel of the turbine blade arrangement which provides the push to move the whole assembly whether clockwise or anti clockwise. This completes the conversion of kinetic force into mechanical shaft rotational force.

1.3 The Jet engine

The concept of the jet engine propelled the aviation industry to a new era of exploration and air travel. Today’s modern jet liners carry approximately 4.5 million passengers daily to and from locations all over the globe, and it’s all thanks to the innovations and advancements to make engines run smoother and cleaner than ever before. (M, 2005)

In the jet engine the turbine blade is what gives the rotational power to turn the main fan and compressor sections. Some engines have a multistage arrangement that connects with a shaft to the other parts that need a driving force to keep developing thrust.

Figure 4 Jet engine cut through

2. Critical analysis

An aircraft is always kept in perfect condition to make sure everything is in working order and all the vital components are checked repeatedly to maintain quality and airworthiness.

Focusing on the engine, I carried out interviews and surveys to find out what part of the engine needed the most attention and maintenance. This led to the conclusion that the turbine blades were the most heavily affected part of the engine.

The national carrier has a very large fleet of Boeing 777’s and being one of the most used aircrafts, it is subjected to multiple inspections and problems.

2.1 Data collection

To collect enough data to accurately understand what part of the engine I need to focus on lead me to produce a survey that if filled out will give me a better understanding on what approach to take. The feedback from this survey directed me to have meetings with a number of engineers working in the industry to start with the second task of data collection. After narrowing down, I used an interview process to question the engineer to make sure what part of the engine I needed to focus on.

2.2 Findings

- Aircraft: Boeing 777 - 300ER
- Engine: GE90 - 115BL1
- Frequency: Inspected after every 1500 to 2000 turbine cycle
- Preferred inspection method: Bore scope inspection
- Most common default: Fatigue cracks

Figure 5 GE90 - 115BLI

Abbildung in dieser Leseprobe nicht enthalten

2.3 Aircraft information Survey

This survey is to help me gather valuable information about the maintenance problems and service of the aircraft.

Abbildung in dieser Leseprobe nicht enthalten

3. Maintenance

The maintenance procedure is a vital part in the aviation industry to keep the aircrafts airworthy. The manufacturer and aviation governing bodies lay out ground rules and limitations on how to go about maintaining every part of any given aircraft. The turbine blades on the GE90 - 1151B1 are routinely checked for any defects and damage, every 1500 cycles to keep them in the acceptable range of operation recommended by the manufacturer. (Bellis, 2001)

On wing inspections, these inspections are done while the engine is still attached to the wing and then there are off wing inspections where the engine has to be separated and taken to the engine workshop. During this phase the engine is thoroughly checked for any damage and standard maintenance is carried out. Parts of the engine like the cowling, fan blades, compressor, turbine blades and combustion chamber and all the electronics are separated and individually cleaned and inspected and then put back in place. All activities are strictly done in accordance with the engine maintenance manual. (Bellis, 2001)

3.1 Turbine blades

There are several methods the turbine blade is inspected, and each of these methods can be used if not all to a certain degree of inspection.

3.1.1 Visual inspection

This type of inspection, as the name suggests is a preliminarily inspection carried out visually either using an inspection device or by sight. (Siemens, 2014)

1. Bore scope: These devices are used to peer into the required part of the engine, in this case the turbine blades. Flexible bore scopes have an optical tube that can be adjusted and moved in certain directions to locate the part that needs to be inspected. The images are relayed on to a screen or an eye piece is available for the observer to inspect the blade.Figure 6 Bore Scope

2. Magnifying glass: After disassembly the blade can be inspected with a magnifying glass to see if there is any obvious cracks or deformation on the blades surface.

Advantages and limitations

This type of inspection is very cost effective and doesn’t require the aircraft to be grounded and can be done while the component is still in service, but it requires the skills of a very seasoned inspector.

3.1.2 Liquid penetrating testing

This method involves applying a fluorescent dye on the surface that needs to be inspected, this will then highlight the points that have any damage or imperfections. (Siemens, 2014)

Before the dye or penetrant can be applied on the turbine blade, it has to be cleaned to get rid of any contaminants that might be clogging or filling in the cracks on the blades surface. After the surface has been cleaned it is ready to be applied with the fluorescent dye. The time taken for the penetrant to seep through into the cracks and damaged areas depend on the size of the object being tested and the size of the defects. The next step is to remove the excess dye on the surface of the blade. This makes sure that the flaws can be detected easily and there is free surface for the other layer. The developer layer is set next, to give a better contrast to see the defects more

clearly and also forces the dye bleed out to make the

Figure 7 LP testing

imperfections more visible.

For the final inspection the blade is taken to a dark room where an ultra violet light is set on the blade to show the blotched areas on the surface. This indicates the cracks and defects on the blade and are marked so the location of the defects are easier to find when the part is taken elsewhere.

Advantages and limitations

This method is low cost and can be carried out on any non-porous material, but the surface has to be meticulously cleaned to make sure the dye can penetrate properly.

3.1.3 Magnetic particle testing

This method uses an induced magnetic force to help indicated the faulty areas on the surface of the blade. Magnetizing the part can be done in an indirect way and a direct way. Applying a current through the blade will generate a magnetic field around the part then applying a magnetic field from the outside on to the blade is the indirect method. After being magnetized any defect on the blade will allow the magnetic flux to leak out, iron particles are add to the surface which will then be attracted to these leakages. Any buildup of iron particles in particular areas show that this location has a defect and then appropriate measures can be taken to fix the defect. (Siemens, 2014)

Advantages and limitations

This procedure is low cost and gives more accurate depictions of possible imperfections on the surface of the blade, but this method can only be carried out on ferromagnetic material and turbine blades with no protective coating.

3.1.4 Eddy current inspection

Eddy current testing is one of the electromagnetic testing methods of NDT testing and uses electromagnetic induction to find faults in conductive materials. The fields are created when an alternating current is passed through the conductor, this field size increases and decreases as the current is at maximum and at minimum.

Figure 8 Eddy current schematic

Abbildung in dieser Leseprobe nicht enthalten

If another conductor is brought close to the first field a current will be induced at the second unite. Eddy currents can also be described as electrical current the moves in a circular path around the conductor. (Siemens, 2014)

Uses of eddy current inspection

1. Detecting small cracks
2. Measuring thickness of certain material
3. Measuring thickness of coatings
4. Measuring the conductivity for identification of material, heat damage detection and monitoring heat treatment.

Advantages of eddy current inspection

1. Can detect very small cracks and other surface deformations
2. Can detect imperfections near the surface as well as near surface defects
3. No time taken for processing results, immediate response
4. Wide variety of anomalies can be detected
5. Easy to use
6. No contact necessary with the test piece and the unite Drawbacks of eddy current inspection

1. Limited to be used only on conductive materials

2. The probe should be near the test piece

3. Cannot be undertaken unless if the inspector is certified to use this procedure

4. Coatings, pint and surface finishes could interfere with the readings (Siemens, 2014)

This technique of inspection has a lot of potential and will continue to grow in popularity as then technology moves forward

4. Improvements

4.1 Dynamic spin testing

Inside the jet engine there are millions of moving parts all working in harmony to keep the unite running smoothly, any small defect can produce problems for the other components so it is vital that everything is checked and is in working order. Since the major components in the engine are working around an axis there is no shuddering or vibrations felt anywhere on the aircraft due to the engines, but under cetin conditions there is a flow vibration occurring when in the flow of air in the turbine section. As the frequency of this vibration matches the natural frequency of the blade, this could cause the blade to get destroyed. The natural frequency is the frequency level that that certain blade will vibrate under, when this frequency is met the turbine blade will tend to vibrate as well. This can create fatigue cracks and compromise the integrity of the blade. The most common solution is to change the tune of the component. High cycle fatigue testing allows to change and check the results of the tuning. (Microsc., 2014)

Due to this fact the blades have a tendency to fail in the air and it could have very bad consequences. The most alarming factor is that the type of failure can occur at any given time. The most blades that suffer high cycle fatigue get rejected when inspections are done

4.1.1 Current methods for blade damping

Dampening or modifying the geometry of the integrally bladed rotors and blades can have positive effects on the parts, these methods are used to authenticate the results. (Li, 2013)

4.1.1.1 Finite element analysis (FEA)

This method is not a preferred method to carry out testing to generate viable results because this computer aided testing can’t simulate all the possible environments of damage in an engine. Matching the rpm and simulating how the new improved

version holds at certain stages and at certain levels of vibration is the current potential of this technology.

Figure 9 FEA

Abbildung in dieser Leseprobe nicht enthalten

4.1.1.2 Bench testing

This method of testing involves realistically simulating vibrations that can occur inside the engine. Vibration measuring instruments and machines specially designed to rapidly vibrate a blade can depicts the conditions of an engine. But this method is limited to vibration simulations and not centrifugal loading, the rotation also plays a vital role in the analysis of stress and vibration since the centrifugal loading can accelerate the damage being done by the vibrations.

4.1.1.3 Engine test

The most straight forward approach is to simulate the environment itself. This method involves in producing the improved test turbine blade and installing it on a controlled engine run test. The real engine that uses the turbine is set up and run with the new modified blade. Various date collection methods will gain information at every stage of the blades cycle. But costing of the test facility and engine as well as staff

Figure 10 Blade off test

Abbildung in dieser Leseprobe nicht enthalten

makes this method very expensive, not to mention if the blade still fails it will destroy a good test engine.

The new method that had been engineered is the Dynamic spin test

This method is the closest artificial simulations of the real working conditions inside a jet engines turbine section. Blades are spun at close to 10,000 rpm with the real temperatures give as well as the vibrations. This engine ready state depicts the environment the blade will be in inside a turbine section and comes very close to replicating it entirely.

(EPFL, 2013)

Major points of the new test method:

1. Very fine speed controls allows the test piece to be held on a certain RPM to simulate the maximum amount of vibrations at realistic blade cycles.

2. Simulating working condition temperatures allows the test piece to be studied very carefully under real operational conditions, this pushes the mechanical as well as thermal stress factors so all possible degrees of stress can be simulated

3. The system is designed to automatically shut down when the blade grows a fracture or any kind of deformation while the test is begin carried out. The test piece can then be taken off the jig and sent for analysis.

4. Very accurate vibrations allows the test piece to experience the full array of possible scenarios as well as test the effective ness of the new damping improvements. (Li, 2013)

Advantages

1. This method allows engineers to run the improved test pieces under very close engine working conditions. The accuracy level of the changeable variations has never been fully replicated in this manner before for this purpose. The contained testing also adds a level of risk avoidance and cost effectiveness as compared to real live engine runs.

2. Effectively producing the vibrations to match that of the engine is a key point in this test method, and the speed control allows the vibrations to be set and studied at a wide scope of possible RPM’s and the ability to continually maintain the set RPM like a turbine section rotor allows the test piece to be supplemented with a variety of frequency’s to see how they would propagate through the blade and grow in frequency.

3. The constant held speed also helps in looking into the stress loads on the blade, this spin test can hold the blade at a continues set speed to simulate real working cycle of an engine. This gives a much more realistic time based reading.

(Li, 2013)

4. With all the possible scenarios that can be simulated the test gains a level of practicality allowing more than one reading to be taken for number of variable characteristics of a blade in motion.

4.2 New visual inspection method

4.2.1 The IPLEX video scope

Current challenges in visual inspections

The range of possible parts for inspectors to cover is tremendous, carrying out a visual inspection is the easiest method to identify a problem without actually getting to the part or look at it through sophisticated methods like ultrasonic inspections. Getting accurate imagery of the blades is important to save time and money. (Olympus, 2014)

Trained inspectors need to look into all the turbine blades on the assembly to successfully inspect a certain section. Since the inspection opening is located in a certain areas the blade assembly has to be moved to the right angle to be inspected, normally this is done by an operator manually turning the blades to fit in the viewing field, this takes more than required man power and cost. Trying to tackle this problem an engine turning tool has been implemented to be used when inspecting any rotary assembly with improve efficiency and reduces the work load.

This new inspection tool comes with a built in data storage library that effectively stores the images taken from each sample or stage.

Advantages of the IPLEX video scope (Olympus, 2014)

This new type of video scoping allows engineers to inspect parts of the engine in much more detail than conventional bore scope instruments will allow. The display portrays very high resolution live imagery with multiple end tips to focus into a verity of different parts of the turbine assembly.

5. Blade destruction analysis and damage to aircraft

5.1 Examples of turbine blade failure

Spinning at around 10,000 revolutions per minute the turbine blades are frequently subjected to monumental amounts of force, revolutionary new materials help keep the blades in working condition. There is no room for error, because any moving part especially at this speed will have devastating effects if there is a failure. The centrifugal force acting on the blade can easily destroy the whole engine if it ruptures and hits the wall of the turbine section.

Most modern engine designs come with a containment ring that goes around the turbine section of the engine, this stops the turbine blade from moving out of the engine. After separation the blade travels down the length of the engine and exits from the exhaust nozzle. Manufactures aim to prevent any of the debris from reaching the pressurized parts of the aircraft. (Carter, 2004)

Many situations can lead to a turbine blade to fail and engine manufactures and turbine blade manufactures always research on methods to decrease the risk of this happening. These are leading causes for turbine blade failure and how manufactures aim to improve on:

Thermal fatigue failure - When subjected to extreme temperatures in the turbine section any material would start to deform over time. There is no exception for turbine blades, temperatures go up to 1500˚C this puts a great stress on the moving parts of the section. Blades move from a range of 8000 to 15,000 reevaluations per minute, adding the process of thermal degradation of the metal will cause an accelerated form of deformation if the metal used for turbines aren’t stiff enough. Turbine manufactures come up with revolutionary new materials called super alloys that have an engineered molecular structure to combat high thermal creep and deformations e.g. Nickel based super alloys like INCONEL® and HASTELLOY®. These materials can work under very hot conditions of up to 1000˚C without any forms or deformation. (Matthey, 1995)

Corrosion - Exhaust gases exiting the combustion chamber can be very corrosive if protective measure are not taken, impurities in the fuel cause sulphidation and oxidation reactions with the alloys which can cause excessively damage to blades. Most modern alloys have a percentages of certain alloys to combat the effects of corrosion in the turbine blades.

5.2 Damage to aircraft

There are two categories of damage in a turbine section failure, contained and uncontained.

5.2.1 Contained

A contained failure of the turbine blade is when the destruction is limited to the internal structure of the jet engine. Separation of the blades at full RPM can destroy the entire aft section of the engine. As one blade detaches from the root it touches the inner wall of the turbine section, from then it spins and destroys the other turbine blades and keeps moving backwards until the engine expels them from the back. This is a design feature in all engines to make sure it is expelled in a straight line to prevent it from effecting the aircraft. Rings around the turbine section in the walls of the casing creates a strong barrier for when the blades detach they do not travel outside the vicinity of the engine unite. The engine losses power and shuts off after a contained failure, vibrations from the failure dames the shafts and compromises the whole engine. (EPFL, 2013)

5.2.2 Uncontained failure

Looking back at history there have been numerous events where uncontained engine failure has caused so much destruction outside the engine that the plain is left inoperable. An uncontained failure occurs when the engine casing and turbine section walls cannot contain the initial blast of the blades leaving the root and coming into contact with the wall of the turbine section. When this happens the sides of the exploding engine shoot out shrapnel that destroys everything in the vicinity of the engine, this destruction is enough to paralyses the whole aircraft. (EPFL, 2013)

Initial impact

As the pieces of metal leave the engine it can rupture through the fuselage, wings, fuel tanks, hydraulic pressure line and electrical lines.

1. This shrapnel could rupture through the passenger compartment which would cause depressurization and serious injuries to passengers if hit.
2. Damage to hydraulic systems could compromise the primary and secondary flight controls due to bleeding of hydraulic oil and loss of pressure.
3. If electrical connections get caught in the failure there could be loss of electrical power to the aircraft and its components.

Figure 11 Uncontained engine failure

Abbildung in dieser Leseprobe nicht enthalten

6. Cause of blade failure

6.1 Thermo - mechanical fatigue

6.1.1 Creep

Creep deformation is a phenomena that can happen to any material when stress is applied to it. When a certain piece of material is subjected to a certain amount of stress over a long period of time, the stress will create strain points on the metal that will eventually cause the piece to permanently deform. Metals can experience creep deformation while being applied with forces below the yield strength of that certain metal, and the addition of heat accelerates this process.

The rate at which the deformation occurs is based on the time of exposure, the properties of the material, the amount of stress that is loaded and the temperature the material is exposed to. For example in the turbine section the turbine blade that weighs 300 grams is rotating around at 10,000 RPM, will experience a centrifugal force that is equivalent to 3 tons of force acting on it, adding the fact that it is operating at 1400˚C, is always exposed to the risk of having creep failure. For turbine blades the smallest change in its dimensions will cause the fast moving blade to hit the sides of the turbine section wall. Creep is the most common form of deformation over material under pressure for a long time, any material is susceptible to creep as it gets closer to the melting point. As a general statement, the creep of any material can be detected at 30% of the materials melting point on the Kelvin or Rankine scale. (Cambridge, 2004)

This form of deformation is the most important for turbine blade manufactures, todays super alloys are specifically engineered to combat the effects of creep and to eliminate it completely at the operating temperatures.

6.1.1.1 Microstructure analysis of creep

The initial stages of creep deformation starts with the formation of cavities and voids in the microstructure. In the presence of a small stress the cavities will form on the grain boundaries that are oriented to the direction of which the stress is coming from, and the cavities that have formed will start to multiply and enlarge. The spread and expansion varies depending on the type of precipitate in between the grains and the allowance it has. With the load continuously acting on the material the cavities eventually expand and propagate into cracks in the boundaries of the grain which leads to creep failure. At extreme stress levels both grains and grain boundaries are at risk of getting cavities, and these can be formed by a process called plastic growth within the grain and by grain boundary sliding.

The size of the cavities range from 0.1 microns to 10 microns, due to the size of the initial cavities and no NDT tests available to it very hard to predict an impending failure due to creep formation. Surface fatigue can be detected by a process called metallographic replication (Cambridge, 2004)

6.1.1.2 Nickel based super alloys

This is the most famous and widely used material in turbine blade manufacturing. Nickel is the parent metal used and other alloys are added to create a material that can survive in temperatures above a 1000˚C without showing any signs of deformation. The high creep resistant nature of the metal is thanks to its microstructure that is designed to overcome the process of creep deformation at any given homogenous temperature. Single crystal structured microstructures have the highest creep resistance in the super alloy family. (Cambridge, 2004)

6.1.2 Corrosion

As the combustion phase is underway the exhaust gas coming out mixes with sodium chloride at high temperatures to make a by-product, sodium sulphate. This material then gets deposited on the nozzle guide vanes and rotary turbines which means that the parts are under oxidation attack. Corrosive material like these sediments will reduce the working life of the turbine blades due to the impurities reacting with the alloy to make it week.

6.1.2.1 Hot corrosion

This type of corrosion can be named an accelerated state corrosion because of the active amount of salts like sodium chloride and sodium sulphate and these salts combine to make deposits on the blades damaging oxide layer on the surface.

Characteristics of hot corrosion

There two main types of hot corrosion, type 1 and type 2. A number of effecting parameters may change the process and time for these types of corrosion to take effect.

(Cambridge, 2004)

1. Type 1 - High temperature hot corrosion

This type of corrosion takes effect at the temperature range of 850 - 950˚C and initially starts with the breakdown of alkaline metal salt on the surface of the turbine blade, and progresses to eat through the blade step by step. Initially the corrosive material eats the oxide film on the blade, then drains the chromium out of the alloy, this results in the formation of porous scaling on the blade. The active salt in the type of attack is Sodium sulphate, this type of salt can be found in the air streams above open oceans, airlines flying over large water bodies could develop corrosion in the blades if proper inspection isn’t carried out. In some grades of fuel used in aircrafts there are traces of vanadium, and if this deposit gets on to the turbine blades, it would destroy the blades. This contaminant has the capability to form in temperatures lower than 535˚C.

The following is the progression of the reaction from start till failure

Phase 1 - localized breakdown of the oxide layer produces some growth and faint roughening of the area, but the blade still retains its integrity and chromium.

Phase 2 - Structure is stable but the surface is starting to show more prominently since the oxide layer is being eaten away as well as the chromium

Phase 3 - The oxide layer around the affected area is completely gone as well as the chromium from the area, the corrosion has reached deep into the blade and when checked the blade will be removed and deemed useless.

Phase 4 - in this phase the reaction has gone all over the blade and the loss of structural parts is obvious.

1. Type 2 - low temperature hot corrosion

Active between 650 to 800˚C this type of corrosion mainly has pitting and mild reactions with compounds with low melting points like cobalt sulphate

(Cambridge, 2004)

6.1.3 Foreign object damage

As the name suggests this kind of damage is from ingestion of foreign debris or even parts from the jet engine itself. These debris can greatly affect the performance and integrity of the engine, while the engine components like the turbine blade move at very high speeds any small collision with an object could compromise the structural performance of the blade. Damaged blades have small scratches on the tips and edges of the structure showing where the debris collided. If this isn’t inspected and replaced it will create cracks that will propagate through blade causing it to fail catastrophically. Technical references to Foreign object damage is mainly associated with the stages before the combustion chamber, debris cannot reach the turbine blades because of the size, and can only be effected by particulates tough and small enough to survive through the previous three sections. Metal fragments, dust, sand and volcanic ash could all cause extreme scoring and damage on the turbine blades.

7. Preventing blade failure

7.1 Material

Nickel based alloys and super alloys

With sections in the gas turbine engine reaching temperatures above 1400˚C, engineers had to come up with a new type of metal that could with stand such conditions and so super alloys were created. Today’s modern alloys have a high tolerance against thermally induced creep, corrosion and as well as overall mechanical strength. Over the years, ever since the first strengthened metals were mad for the first generation Figure 12 Grain bounderies engines, a lot has changed in the composition or concentrations of metal used. The alloys have a base, and alloying elements are added to it to give it its desired characteristics. (Estrada, 2007)

The microstructure of the alloy can be easily explained by considering them like blocks and elements like tantalum to hold it all together, this gives it strength at high temperatures and improved oxidation resistance. Most common alloys used in the manufacturing of turbine blades for jet engines is nickel based, as in the parent metal. Nickel based super alloys have a high tolerance for thermal creep fatigue, oxidation and corrosion via sulphidation. (Estrada, 2007)

Nickel based alloys have two specific subcategories of strength one of which is solid solution strengthen and the other, precipitation strengthened. Hastelloy X is a good example of the first kind of super alloy, this type of alloy is generally used for low strength purposes and low heat applications. The other subcategory is the precipitation strengthened alloy, the presence of a stiffening alloy (precipitate) in between the lattice structure. These alloys are used in the harshest environments such as turbine blades in the turbine section of a jet engine, these precipitates help create the most resilient forms of super alloys. (Estrada, 2007)

These are some of the common phases in modern super alloys:

Gamma (γ) - In this phases the compounds in the structure of the super alloy is in a FCC ( face centered cube) orientation where the molecules are arranged in such a way that they are all face to face and contains a large portion of solid - solution elements like Molybdenum, Tungsten, Cobalt and Chromium.

Gamma prime (γ) - This is the key phase that gives super alloys their strength and resilience. The use of precipitating phase (precipitates in between the gamma grid). This combination of structures and formations allows the super alloy to perform under the most demanding conditions.

(Matthey, 1995)

Every day new materials get tested and implemented to meet the ever changing demands in the engineering world. These are two examples:

Ceramic matrix composites

Ceramic by itself has exceptional heat shielding properties, but the material is very brittle and cannot function under high stress forces. CMC or ceramic matric composites aims to brake the barrier between advanced heat shielding functionality and heavy

fatigue load baring capabilities. This new revolutionary composite-

Figure 13 Cooling holes on a turbine blade

Abbildung in dieser Leseprobe nicht enthalten

-will soon pass the role super alloys play in the gas turbine engine. (Estrada, 2007)

The key point of this material is the fact that it’s stronger than most super alloys, it can withstand far higher temperatures and weighs less than half of what the competition has to offer. CMC does not need extra internal cooling to increase the life span of the component, this composite material’s thermal capacity allows it to operate in these conditions with little to no cooling, which increases the amount of air available for the bypass making the engine more efficient.

(Estrada, 2007) (Milne, 2014)

The fracture toughness, compressive behavior and the tensile strength are all properties that a monolithic ceramic would not be categorized under, the ceramic matrix of the compound allows it to perform well in these areas as well as surpass most alloys. Fiber reinforced composites tend to deform slowly over time unlike conventional ceramics, where their brittle nature cannot hold any stress that can compromise the material.

(Estrada, 2007) (Milne, 2014)

Other nickel based super alloys like TSM - 152 and TSM - 196 have been shedding new light in the capability of these alloys. Changing the gamma alpha phase structure and adding different percentages of alloying metals like rhenium, tungsten and molybdenum have yielded in alloys that can keep its stiffness under very high stresses at 1100˚C for 1000h. Continues research will provide more possibilities and more verities of super alloys to be used in the next generation of jet engines.

(Estrada, 2007) (Milne, 2014)

7.2 Coating the blade

Coatings

A coating is a certain layer of substance that gets applied on a surface to prevent the surface from getting damaged, similarly the coating on turbine blades is used to provide an extra layer of protection against the heat and corrosive elements in the turbine section.

Thermal barrier coating (TBC)

The average operating conditions of a turbine blade has gone up to 1000˚C and this has been pushed to 1400˚C with the help of ceramic thermal barrier coatings and the cooling mechanism. This thin layer of coating allows the blade to operate freely at these temperatures and prevent it from being subjected to corrosion. Ceramic is a natural insulator and is widely used in areas where protection against heat is required and the necessity for thermal barriers is present. The coating is applied to last a set number of cycles or service life time period, with in this time the coating should hold up to attacks from the stress of loading and unloading forces as well as exhaust gases and fine particle debris from combustion. It should also stand neutral and not react or weaken the metal the coating is being applied too. (Milne, 2014)

Prior to applying the protective coating there are three steps to be taken to apply properly and are as follows:

1. The initial step is to treat the surface of the blade with a chemical thermal treatment and this contains aluminum or a mixture of elements like silicon and yttrium plus the base aluminum. After this treatment a coating is formed on the blade due to the reaction with the aluminum and the alloys surface.
2. Second a top coat of evaporated alloys is set on to the blade using a vacuum to evaporate and then condense all the elements evenly on to the blade.
3. The final step is to form the coating using powder by the process of arc plasma spraying.

7.3 Existing cooling methods

An ideal way to increase the engines power output is to produce a hotter turbine entry temperature that will result in a faster expanding gas, but 1500˚C can melt the blades in the turbine section so a cooling mechanism will be implemented. The simplest way to control the temperature of the turbine blades is to cool the blade with air at a lower temperature moving through the blade, where heat exchange happens and the blade remains cool. Air is taken from the compressor section through pathways and brought in to the turbines at 650˚C, this is enough to cool the blades down to the desired temperature. Even though the super alloys are made to work in these conditions, that can’t be exposed for prolonged periods of time. The operating temperature inside the blade is brought down to around a 1000˚C, from the 1200˚C to 1600˚C range to extend the life of the blade. Over the past 60 years the inside working temperature has doubled due to the advancements in blade and part cooling. Since the 1950’s the process of cooling the turbine blades further has been an issue, at the time due to improper burning and slower exhaust gas speeds the temperature did not go above the super alloys working condition range. Advancements combustion chamber burning processes lead to hotter exhaust gas temperatures and faster moving blades, inevitably moving past the safe operating zones of the super alloys used to make the turbine blades. Current working temperatures will not exist in 30 years’ time, there will be new cooling methods and more resilient materials available for the hot section to perform at much higher operating temperatures than today’s jet engines. (M, 2011) (Je-Chin Han, 2008)

The air cooling process can be delivered in two ways: Internally, this is when the air is forced into sectioned passage ways through the inside of the turbine blade and absorbs the hot air and carries it out the exhaust, while the external cooling method involves pushing air out of external cooling holes on the surface of the blade to create a thin film of cold air to help deflect some heat.

7.3.1 Internal cooling

- Convection cooling

This method is the most straight forward method the blades can be cooled internally, the heat from the hot gas outside heats the blade and conduction through the blade heats up the inside of the blade as well. Convection transmittance absorbs the heat and carries it from the hub and then out through the out let holes in the blade tips. The inside of the blade is like a radiator, adding more surface area for the cool air to pass through makes it an efficient design.

- Impingement cooling

Like the convection cooling method the impingement cooling relies on convection to transfer the heat from the blade and out with the exiting air. The cavernous winding architecture of the convection cooling is not as prominent in the blades that use impingement cooling method.

The leading edge of the blade is the where

Figure 14 Cooling

most of the heat loading takes place, and

the impingement cooling throws the air at a much higher velocity at these area and pushes the air along the side of the internal structure of the blade. This method is more suitable for the curved section of the leading edge because the high velocity air gets trapped and swirls around extracting the maximum amount of heat before it is passed on, and the walls on the side of the blade, the mid chord area and the vane can also implement this process but isn’t as effective.

(Je-Chin Han, 2008) (M, 2011)

- Multiple jet impingement

Multiple jet acting around the same area to increase the effectiveness of the heat exchange process, but this method is not a very viable process due the factors given bellow:

Jet impingement makes use of high velocity jets to strike the inner surface of the blade and extract the heat, but adding multiple jets will create disruption in the flow. Mass flow rate is constant, air bleed ducts in the compressor stages divert cold air through the air pathways to the mouth of the turbine blade, and a cone shaped inlet section squeezes the air at much higher velocity to keep a constant mass flow rate. When these high velocity air paths meet inside the blade it causes a disturbance in each other’s performance by slowing the flow of air. Each jet Is located in specific areas of the blade to hit the desired part of the inner surface, and as two jets meet the purpose of the high velocity impact is lost. After the rate of flow is compromised the air absorbs heat using normal convection methods.

(Je-Chin Han, 2008)

7.3.2 External cooling

- Film cooling

As the name implies this proses makes use of a thin layer of air or film of air to produce an effective barrier over the immediate surface of the blade. The leading edge of the blade is where the air flow comes out of, small holes running along the front of the blade from the root to the top diverts air evenly to create a flowing film of cool air. The leading edge is initial part of the blade that starts with curve, the air flow moves beyond this point and sticks to the surface to effectively act as a heat shield. The thin layer of air has two roles, it sticks to the surface of the blade because of the cold air density which acts as a

Figure 15 Film cooling

convection flow that absorbs some of the heat from the surface, and the other is to act as a barrier from the hot gases to limit the heat transfer to the blades surface. A key point that has to be noted in this method of cooling is that, the addition of cool air into the turbine section reduces the efficiency of the turbine section, but this is overlooked due to the fact that the cooling of the blade increases the performance of the turbine section by increasing the operating temperature.

(EPFL, 2013)

- Pin Fin cooling

As the air is fed into the

turbine blade numerous

cooling techniques makes use of the initial cool air, but after that has been spend the air flow is sent out from the trailing end of the blade. The

pin fin arrangement makes use of this exiting air flow to cool

Figure 16 PinFin cooling

the blade further. The two end walls create a gap running down the end of the blade from the root to the top, this area is connected by small cylindrical shafts that fix between the end walls from one side to the other side.

(EPFL, 2013)

When the exiting cool air passes around the small cylindrical pins it disrupts the flow creating a wake as well as vortexes, these horseshoe vortexes spin around the pins as the flow moves on. The agitated air plus the vortexes mix the air which helps in the process of heat transferring. The control factor in this arrangement of cooling is the pins and fins themselves. The gap between each pin and the shape play a major role in the efficiency of using this technique, as well as the size of the pins. All of these factors changes the amount of heat that can be exchanged.

7.3.3 Newer designs

- Effusion cooling

This method is similar to the film cooling effect where there are cooling ports allowing air to form a film of cool air, similarly in the effusion cooling system the turbine blade is made with small orifices along the sides of the turbine blade. This creates a cooling layer over

the majority of the blades surface and the act of heat exchange cools the blade. (EPFL, 2013)

8. ANSYS Thermo-mechanical analysis

ANSYS, Inc. is an engineering simulation software (computer-aided engineering, or CAE). By using this program I have analyzed how the turbine blade behaves under a certain conditions. Hot expanding gases from the combustion chamber hits the stator nozzle guide vanes and moves on to hit the turbines at this stage most engines experience a temperature of around 1200˚C to 1400˚C. Using a centrifugal load parameter of 12,000N and the force of the oncoming gas as 80623N, as well as blade cooling at 650˚C, I have recreated the forces acting on the blade as it would be inside an engine. The analysis will show how the blade reacts to the temperature as it increases

8.1 First reading

For the first reading the temperature of the blade is at normal operating conditions, which is fixed at 1000˚C, the forces acting on the blade remain constant.

Figure 17

Abbildung in dieser Leseprobe nicht enthalten

The normal stress acting on the blade at 1000˚C

Figure 18

Figure 19

Abbildung in dieser Leseprobe nicht enthalten

At normal operating conditions the cooling the blade goes under tension which is the elongation of the frontal face and the compression of the back and the root section. As the expanding gasses hit the blade if pushes the blade back slightly, added with the temperature and centrifugal force, stress and compression takes place. In figure 19 the top root section has some degree of compression from the slightly elongating leading edge.

Total deformation of the blade at 1000˚C

Figure 20

Abbildung in dieser Leseprobe nicht enthalten

Figure 21

As figure 20 shows the maximum deformation happens at the tip of the blade, red indicates the most movement that has been detected as the simulation is run. The tip of the blade has displaced the most from the original point and the root has minimum because it is at the base. The intense heat as well as the centrifugal force and the force of the expanding gas combines to attain these results.

8.2 Second reading

For the second reading the temperature of the blade is at 1200˚C, the forces acting on the blade remain constant.

Figure 22

Abbildung in dieser Leseprobe nicht enthalten

Figure 23

Abbildung in dieser Leseprobe nicht enthalten

Figure 24

In figure 23 and 24 the blade is undergoing stress around most of the blade except at the root section where compression is taking place. Around the perforations for the cooling the stress is concentrate where the air is being forced out. In figure 23 the root shows that there is a considerable amount of compression occurring.

Total deformation of the blade at 1200˚C

Figure 25

Abbildung in dieser Leseprobe nicht enthalten

Figure 26

Compared to the readings taken at 1000˚C, the total deformation occurring at the tip of the blade has increased from 0.0154m to 0.0155m. Again the change is towards the top, which in fact is the weakest part of the blade since it is the furthest away from the root.

Abbildung in dieser Leseprobe nicht enthalten

8.3 Third reading

For the third reading the temperature of the blade is at 1400˚C, the forces acting on the blade remain constant.

Figure 27

Abbildung in dieser Leseprobe nicht enthalten

Figure 28

Abbildung in dieser Leseprobe nicht enthalten

Figure 29

At 1400˚C the stress on the front of the blade can clearly be seen as well as the immediate cooling air outlet area where it shows considerable amounts of stress. The increase in temperature reduces the strength of the material considerably and hence leads to creep deformation, these patches are signs of such stress. The root is experiencing compression forces like the readings taken at 1000˚C and 1200˚C Total deformation of the blade at 1400˚C

Figure 30

Abbildung in dieser Leseprobe nicht enthalten

Figure 31

As the previous readings the deformation is located only at the top of the blade but there is an increase of the maximum displacement of 0.015567.

8.4 Fourth reading

The fourth set of readings aim to show how the blade behaves in excess amounts of heat, where it exceeds the normal operating temperature. These conditions and parameters will show what happens at these temperatures.

Figure 32

Abbildung in dieser Leseprobe nicht enthalten

The normal stress acting on the blade at 2000˚C

Figure 33

Figure 34

Abbildung in dieser Leseprobe nicht enthalten

Total deformation of the blade at 1400˚C

Figure 35

Figure 36

At this temperature the blade undergoes massive bending forces that cause large amounts of compression strain to form all over the blade. The displacement from the origin of the tip of the blade is also very large (0.036m compared to the 0.0156m from 1400˚C). The extreme temperature causes the meatal to lose its structural rigidity as well as its potential to hold stress

Abbildung in dieser Leseprobe nicht enthalten

With constant cooling and constant forces acting on the same body, the change of temperature causes thermos-mechanical creep deformation that alter the shape of the turbine blade. The acceptable limit as to which the blade can still function is 1400˚C, after this point the blade losses its characteristics and cannot function efficiently and has the tendency to fracture and fail, which then would cause engine failure.

Conclusion

Research and implementation of new ideas in the advancements of overall turbine performance is on the rise due to the fact that technology is always improving. More efficient engines using the cutting edge in material and thermal science entering the market always pushes the envelope and creates new opportunities for companies to make more advancements in their products.

The research and findings in this report explore all the possible advancements made in making turbine blades more resilient and able to perform in ever increasing workloads. Super alloys such as INCONEL® and HASTELLOY® have the highest number of in-service turbine blades which will eventually be over shadowed by new companies coming up with more innovative materials that brings forth more advanced materials. Such developments will deliver jet engines like General Electric’s CMC LEAP and beyond, helping today’s research and performance analysis the future for tomorrow’s technology.

Recommendations

1. Invest more on new technology to scan and inspect turbine blades more efficiently, this will reduce the time taken to manually inspect every blade. Focus on making x ray or exterior scanning more realistic to improve the accuracy of the results as well as the duration from test to result.
2. Composite materials are light and durable, using these in the turbine section to replace conventional super alloys would make improve power to wait ratio as well as maintenance costs. Ceramic composites that are lighter and more thermally stable in the conditions of the turbine section will perform better than super alloys due its natural capacity to perform
3. Research more into designing safer containment features when an engine component fails such as the turbine section.

Bibliography

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List of Figures

Figure 1

http://www.aviation-history.com/airmen/frank_whittle.htm

Figure 2

http://www.coventryobserver.co.uk/2012/09/27/news-Film-celebrates-Sir-Frank-Whittle- invention-51147.html

Figure 3

http://www.enginehistory.org/G&jJBrossett/JetEngines/301Low%20pressure%20turbine.JPG

Figure 4

http://www.rchelisite.com/images/turbine/turbine_diagram.png

Figure 5

http://www.geaviation.com/engines/img/thumb-ge90.jpg

Figure 6

http://www.omega.com/Temperature/images/HHB400_l.jpg

Figure 7

http://www.hodgsonndt.com/images/slider/slider_3.jpg

Figure 8

https://www.nde-ed.org/GeneralResources/MethodSummary/ET1.jpg

Figure 9

http://www.mece.ualberta.ca/~wmoussa/lab/new-15.gif

Figure 10

http://www.rolls-royce.com/Images/bladeOff_tcm92-44803.jpg

Figure 11

http://upload.wikimedia.org/wikipedia/commons/b/b1/Delta_Airlines_Flight_1288_Engine_Failu re.jpg

Figure 12

http://www.tms.org/Meetings/Specialty/Superalloys2000/gpimage.gif

Figure 13

http://www.yxlon.com/Resources/Applications-

en/Castings/TurbineBlades_0681x0454.jpg?width=

56 | P a g e

Figure 14

http://www.tpg.unige.it/research/blade_cooling.jpg

Figure 15 http://d2n4wb9orp1vta.cloudfront.net/resources/images/cdn/cms/MMS_0313_RT_turbine- blade.jpg

Figure 16 http://turbomachinery.asmedigitalcollection.asme.org/data/Journals/JOTUEI/28686/011101j.1.jpe g

Figure 17

Figure 18

Figure 19

Figure 20

Figure 21

Figure 22

Figure 23

Figure 24

Figure 25

Figure 26

Figure 27

Figure 28

Figure 29

Figure 30

Figure 31

Figure 32

Figure 33

Figure 34

Figure 35

Figure 36

58 of 58 pages

Details

Title
Thermomechanical Failure of Turbine Blades. Analysis and Recommendations
College
Coventry University
Course
Aerospace Engineering
Grade
First class
Author
Year
2015
Pages
58
Catalog Number
V305308
ISBN (Book)
9783668054127
File size
1933 KB
Language
English
Tags
turbine, blade, analysis, thermomechanical, fatigue
Quote paper
Imal Liyanage (Author), 2015, Thermomechanical Failure of Turbine Blades. Analysis and Recommendations, Munich, GRIN Verlag, https://www.grin.com/document/305308

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