Material Selection And Design For Gas Turbine Blades

Material Selection and Design for Gas Turbine Blades

Sample Answer

Introduction

Gas turbine engines have been widely used in aircraft, marine and land-based power generation. Gas turbine blades are uses in gas turbine engines, where hundreds of them rotate at very high speeds on a series of coaxial disks.  Basically, the engine arrangement is comprised of an inlet duct, which is sequentially followed by compressor section, then combustion chamber, high and low pressure sections of turbine, and exhaust duct in that order as shown in Fig.1.  Principally, gas turbines operate by simply converting heat energy into mechanical energy.  This conversion is achieved by taking a large volume of air, compressing it to high pressure, mixing the compressed air with fuel and then igniting it in the combustion chamber. As a result of rapid expansion of this ignited air-fuel mixture through the turbine blades, power is generated or propulsion is obtained.  Since thermodynamic efficiency is an important parameter in gas turbine design, especially for those used in aerospace due to the need applications where fuel efficiency and power to weight ratio are of concern, there is a need to achieve the greatest temperature difference across the engine. As such, designers usually try to maximize high temperature especially in the high pressure turbine section. However, there are constraints to achieving this. Most of these constraints have something to do with gas turbine blades (Power, 1995, p.117-126).

Fig.1 Cross section showing the various parts of a gas turbine engine (Golinval, p.4)

GAS TURBINE BLADES

Functions and Uses of Gas Turbine Blades

In a gas turbine engine, the blades are held onto the engine shaft by a hub. The purpose of the blades is to facilitate compression of the inlet gas. The compression of inlet gas in a gas turbine raises both its temperature and pressure. Further rise in temperature and pressure is achieved by combustion of fuel inside the combustor. The high temperature and high pressure gases escape through another set of turbine blades where the energy from this exhaust gas is extracted thus lowering the temperature and pressure. The extraction of energy from the kinetic motion of the exhaust gas is also the work of the blades. When the kinetic energy of the exhaust gas is converted into mechanical energy in the turbine blades, the entire stage of blades rotate. The rotating blades in turn rotate the rotor shaft connected to the generator where the mechanical energy is converted into electrical energy. Basically, the purpose of the blades is to extract kinetic energy of exhaust air into mechanical energy which is then used to turn the gas turbine shaft.

Gas turbine blades are arranged in stages with each stage performing a particular function. In most gas turbines, the first stage blades, also called impulse blades have zero reaction; while the second-stage and third-stage blades are called reaction blades where they usually produce 50% reaction. On the other hand, the impulse stage produces almost twice the output of a comparable 50% reaction stage. However, the efficiency of an impulse stage has been found to be less than that of a 50% reaction stage (Boyce, 2012, p.79)

Functions of the Gas Turbine Blades in Terms of the Materials’ Constraints on the Design. 

There are fundamental challenges towards the turbojet technology. One of them is the challenge of increasing the thermodynamic cycle efficiency by increasing the compressor pressure ratio while the other is the problem of increasing the ratio of power-output to engine weight by increasing the turbine inlet temperature. However, these come with mechanical challenges such as design of discs and analysis of the dynamic behavior of multiple-rotor systems and prediction of critical speeds. Then there is the challenge of high temperatures given the environmental conditions under which blades are operating. Given that the temperatures are so high to be tolerated by metals such as steel, the idea is to cool the blades, use a different design such honeycomb instead of solid blades or use an alloy, or a combination of at least two of these. Other methods used involve various heat-treatment procedures whose end results is improvement in the grain structure of the turbine blades. During operation, the turbine blades are subjected to intense vibrations. These vibrations introduce another challenge which requires that the design of the blades should withstand these vibrations. In addition, there are a number of stresses subjected to the turbine blades during operation. Examples of these stresses are bending stresses produced by aerodynamic loads on the blades, thermo-mechanical stresses produced by temperature gradients between bore and rim and dynamical stresses of vibratory origin (Power, 1995, p.117-126).  

Appropriate materials used for the manufacture of gas turbine blades and mechanical and thermal properties of those materials.

Given extreme conditions under which the blades are operating, there are few materials that can withstand such environment. Selection of materials is therefore narrow and there are ongoing efforts to come up with a variety of materials with improved characteristics.  Materials chosen for blades should be in a position to operate for tens of thousands of hours or millions of miles of flight. That is why they should be of good tensile strength; and resistance to low and high cycle fatigue, rupture, production/processing techniques, corrosion/oxidation and thermal mechanical fatigue. The materials chosen also should result in a product with good physical properties such as smoothness. Gas turbine blades in the first stage have to withstand the harshest conditions which combine environment, stress, and elevated temperature; a combination which is usually the limiting factor of the turbine. Some of the high temperature alloys are those of chromium, nickel, cobalt, iron, tungsten, molybdenum, aluminum, cobalt, vanadium, carbon, titanium and barium. However, since temperature is not the only issue, the blades are usually made from vacuum cast, nickel-base super alloys that are strengthened through solution and precipitation-hardening heat treatments (Schilke, 2004, p.4).

Nickel-base super alloys have been used for some time and the only modifications introduced are coatings, manufacturing technique, design and addition of more elements to increase the structural strength, creep resistance/fatigue and hot corrosion resistance. Nickel-based alloys such as Inconel alloy 716 is a precipitation-hardenable high-strength alloy with improved machinability, with good resistance to corrosion and oxidation over a wide range of environments.   Modern nickel-base super alloys are suitable because they offer a good combination of desired characteristics such as high-strength, castable, good fatigue resistance, good manufacturability and excellent creep strength. Above all, well manufacture nickel-based alloys such as those with single grain can retain these characteristics for a long period of time in its service life. Although nickel-based alloys are not excellent in resistance to oxidation/hot corrosion, advanced high temperature hot-corrosion and oxidation resistant ceramic coatings have been developed (Kraus et al, 2007, pp.169-172; Kutz, 2002, p.244; Mutassim, 2008, p.38-42; Schilke, 2004, p.4-9).

Three processes used in the manufacture of gas turbine blades.

The manufacture of gas turbine blades involves a number of processes. For example, casting is currently the most preferred method in the manufacture of blades. Casting has gained wide acceptance because it results in a directionally solidified or a single-crystal blading material. The blade is usually cast in a Bridgman Furnace. In a Bridgman furnace, the process is controlled by time-dependent parameters such as withdrawal speed and heater temperatures (Emmerich, 2012, p.2).  Earlier, gas turbine blades were forged. However, forging had its limitations. For example, producing blades with forging methods is hard due to high quotient of surface forging volume. Forging also is not preferable due to large heat losses encountered during the process. In addition, the large heat losses results in the formation of large temperature gradient formed on the cross-section of the forged blade. Forging is also not a precise process and the resultant product is multicrystalline unlike the case of casting where a single-crystal microstructure can be achieved (Sinczak, 2010, p.83-90).

Main limitations of the manufacturing process of gas turbine blades. Three of the limitations from the list that influence function of gas turbine blades and ways to reduce the effects of such limitations

Manufacturing of gas turbine blades comes with a lot of challenges. A gas turbine blade is designed to withstand the harshest of environmental conditions especially high temperatures exiting the combustion chamber. The idea has been to use a number of processes such as nickel-based alloys and more recently coatings and air channels. It appears the metallurgical limits has been pushed to the extreme and no more materials could be developed that can operate at greater temperatures. Even if those materials are to be achieved, certain parts of the gas turbine manufactured from other materials will have to be redesigned to withstand increased temperatures. Eventually, complications will lead to high costs of turbine. That is why manufacturing methods such as us of coatings, single crystal and machining air channels comes in. The level of precision and accuracy required is high. In addition, the processes are usually very expensive since they require specialized manufacturing methods that can result in parts utilized in harsh environment. Reduction of limitations is achieved by incorporating high technology manufacturing processes.

Turbine blade machining is an activity than involves several process requiring very high precision, accuracy and tolerances (Sandvic,2013) Well machined blades have better performance and longer life expectancy. However, the machining processes and complexity of the machines themselves means that the blades will never be identical. As such, some will tend to wear out more than the others. Forging of gas turbine blades is also one of the methods of manufacturing them. Although it used to present its related challenges, it is in longer the case. Currently, the technology applied in forging has been developed to an extend where wrought nickel alloy turbine blades can be forged to very accurate dimensional tolerances with good consistency of metallurgical properties. A good understanding of practical in combination with theoretical knowledge of nickel super alloys as well as the processes of forging has been utilized to meet the ever-rising temperature and stress conditions experienced in gas turbines. Generally, a combination of new forging techniques, materials coupled with computer-aided designs and manufacturing techniques should be developed to compete with advanced casting processes (Wright and Smith, 1986, pp.742-747).

Fig.2. Photographs depicting conventional casting and directional solidification (Gell et al,1980, p.206)

Casting has been used in the manufacture of gas turbines. It has been observed that single materials and directionally solidified have greater performance than multi-crystalline materials. As such, casting has been employed in the manufacture of gas turbine blades. However, in an attempt to produce cost and energy efficient gas turbines, single crystal technology has been limited by the of grain defects in the gas turbine blades. It is on these grounds that a new casting technology should be developed to overcome the problem of grain defect in order to improve the quality of single crystal blades (Technologie Allianz, 2013).

In summary, more research should go into developing new and better methods of machining, casting, forging, manufacturing of air channels, ceramic coatings and also alloying elements.

Three main causes of service failure for gas turbine blades.

Gas turbines have been evolving with time and this evolution has tended towards higher entry temperatures and higher pressure ratios. As such, the problems of blades in the hot section have been rising with this evolution. Whether it is the aircraft or industrial application of gas turbines, blade failures are the greatest concerns for both the designers and users.  Blading problems in gas turbines have been found to account up to 42% of gas turbine failures with severe effect on plant availability. On industrial gas turbines, available statistics indicate that other rotor components and turbine blades account up to 28% of all the primary failures of gas turbine failures while turbine nozzles and other gas turbine parts account for approximately 18%. Failure of gas turbine blades can be traced to various sources with various failure modes which can be troubleshooted in a number of ways. For example, there are various blade failure modes such as environmental attack, fatigue, embrittlement, creep, and erosion. The causes of blade failure have been found to be manifold. These include foreign object damage such as devices or particles entering the gas turbine chamber, erosion, sulphidation, vibration, creep and fatigue. The predominant causes of failure are fatigue, creep and corrosion. Although there is a wide array of sophisticated design tools available, still, blade failures are prevalent in turbines and compressors. Studies and observations have shown that these failures are attributed to a number of causes. One study found out that the gas turbine blade failure is caused by the presence of unavoidable aerodynamic and mechanical excitations and combines failure modes. Another factor responsible for failure is exceedingly complex vibration characteristics of blades under actual operating conditions which have been found to differ completely with analytical predictions. In certain circumstances, non-uniformity of blades which are otherwise supposed to be identical combined with quality problems during manufacture of components or assembly; or both. When gas turbine airfoils operate in hostile environment, which is usually the case, damage mechanisms of oxidation, creep, thermal fatigue and hot corrosion often work in combination resulting in compound failure modes. Available information form research and industry also indicate the rapid rate of progression of high frequency fatigue failures is responsible for gas turbine blade failure (Meher-Homji and Gabriles, 1998).

Estimation of the service life of gas turbine blades when subjected to creep and fatigue loading.

According to Meher-Homji and Gabriles (1998, p.136), estimation of a gas turbine blade life is complicated by factors such as presence of stress enhancers and corrosive environment. As such, estimation of blade life is a complex work requiring substantial analytical input. Nevertheless, there are two philosophies in use on practical level. One of this philosophies is the ‘operation hour concept’ which expresses hot section life as a function of actual operation hours of a gas turbine blade, number of starts and drips, as well as other operational profile data, with each start adding 20 hours. This concept assumes that mechanisms leading to damage operate in a strong way. Where this concept is to be used, the manufacturer provides an empirical formula in form of an equation that helps in estimating life consumption of hot section parts.  However, other quarters assumes that while the interactions exist, they represent second order effects. As such, base maintenance intervals on operating hours and the independent counts of the number of starts; for example, 1200 starts for a hot gas inspection or 24000 hours of operation. Modification of calculations will be done for varying types of fuels, and other factors including peaking service, emergency loading conditions and water injection. Basically, the equation for estimation of life incorporating thermal fatigue and creep is expresses as:

[T/T_f] + [N/N_p] < D

Where:

T_f = time to failure due to creep

N_p = number of cycles to crack

D = critical damage parameter. For super alloys, it is taken as 1.0 while for steels it is taken as 0.75.

When additional failures are considered, estimating life of a gas turbine blade becomes too complex.

Three ways of improving service life of gas turbine blades and measures that need to be taken in order to overcome the causes of service failure. (Maximum length: 450 words)

The idea behind the design of an ideal gas turbine engine is to try to maximize the operating temperature of the turbine section of the engine, which also happens to be the high pressure turbine section. However, there are constraints regarding this since the operating temperatures and stresses become higher with increase in temperature and even a small further increase in temperature can adversely reduce the service life of the blades. Nevertheless, it has been possible to achieve higher operating temperatures while improving the service life of the turbine blades at the same time. This has been made possible by improvement in manufacturing techniques, choice of materials and engineering designs. In engineering design, one of the greatest developments has been the incorporation of special cooling systems into the high pressure turbine blades. The cooling system is a network of channels in each blade which permits a relatively cold air to be passed at pressure through the middle of the blade. The pumped air then flows out of large number of small holes on the blade surfaces thus enveloping gas turbine blade in neither a film of cooler air causing a reduction in the operating temperature of gas turbine blades without necessarily causing any substantial reduction in neither the operating efficiency nor the operating temperature of the gas turbine engine (Power, 1995, p.117-126).

Fig.3. Network of air cooling channels as seen in a section of a 2-inch long high pressure turbine blade (Power, 1995, p.117-126)

Apart from cooling, there have been advances in the manufacturing of gas turbine engine blades. One method involves the surface treatment. Since gas turbine blades in gas turbine engines operate highly oxidizing atmospheres due to elevated temperatures they can be easily contaminated with sea water salts or even fuel residues.  Gas turbine blades are expensive to produce but despite being subjected to high stresses during service life, they must be totally reliable during their design life. Research has indicated that coating the base metal super alloy with a protective layer which is capable of resisting not only high temperatures but also hot corrosion is one of the most economical ways to maintain blade properties. This coating is achieved by applying conventional aluminide coatings although platinum aluminineds have also been found to offer improved corrosion resistance. Research has also resulted in the development of a platinum aluminide diffusion coating that is advantageous over commercial systems (Hill et al in Lin and Zhu, 2009, p.123; Wing and McGill, 1981, p.15-21).

Metallurgical constraints imposed by high temperature working environment of gas turbine blades have been overcome by other manufacturing processes such as use of single crystal nickel based super alloys although there are other elements that are added to contribute to the optimum mechanical properties of turbine blade under high temperature conditions. Cobalt alloys with large quantity of alloying elements has been used with the aim of producing gas turbine blades with high resistance to creep at very high temperatures, high strength at elevated temperatures and resistance to hot corrosion. The single crystal gas turbine blade can operate at relatively higher temperatures as compared with multi-crystalline turbine blades; thus they can be used where there is a need to increase the thermal efficiency of gas turbine cycle. When compared with crystalline structures, single crystal structures have the ability to withstand creep at greater temperatures since they do not have grain boundaries. Single crystal turbine blades do not have does not have grain boundaries along directions of axial stress unlike the case in crystalline blades (Askeland and Fulay, 2010, p.452; Carter, 2005, p.237-247; Gell et al, 1980, p. 205-214; Rao, 2011, p.299).

To overcome the causes of service failure, appropriate materials should be used to manufacture the blades. It is also important that the operation of the turbine be within the blade material design temperature. Fuel used in the engine must be of high quality and free of impurities that can enhance hot corrosion. The engine chamber should be free of any debris that can otherwise enhance wear and tear of the plate parts. And because the blades are going to wear out in the end, there should be regular inspection and servicing until the service life is attained.

References

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