Abstract: This paper introduces the technical characteristics of gas turbines and their applications in various fields, elaborates on the system composition of gas turbines, focuses on the study of their structural characteristics and technical and economic performance, and looks forward to their future technical development trends. As a common type of thermal engine, gas turbines play an important role in aviation propulsion, ship power, offshore platforms, thermal power generation, military vehicle power, oil and gas transportation, and have played an indelible role in promoting the maintenance and development of national economic construction and national defense industry.

Keywords: gas turbine; steam turbine; diesel engine; internal combustion engine; economy

1 Introduction and classification of gas turbines

Gas turbines are a type of thermal engine that can convert the chemical energy of fuel into the thermal energy of gas, and then convert part of the thermal energy into mechanical energy through a turbine. The overall classification of gas turbines is shown in Table 1.

Gas turbines have the advantages of light weight and small size, fast start-up, easy maintenance, reliable operation, high degree of automation, and low cost. Compared with steam turbines, the main disadvantages of gas turbines include low power per unit, short service life, and high requirements for fuel types. Gas turbines for locomotives have long been successfully trial-produced. The Long March 2 gas turbine locomotive successfully manufactured in my country uses heavy oil as fuel, which fully saves fuel costs. As the disadvantages of gas turbines are continuously overcome, gas turbines have been widely used in many fields in recent years.

2 Application and development of gas turbines

Gas turbines can achieve rapid start-up and are often used in thermal power plants to bear peak loads or as standby units [5]. Gas turbines are suitable for bearing basic loads in small-power thermal power plants. Due to the advantages of small size, light weight, simple maintenance, and good maneuverability, gas turbines are also often used in mobile power plants or train power plants. In the field of ships, gas turbines are often used as acceleration units of surface ships and the main power source of high-performance ships such as hydrofoils and hovercraft. In the aviation field, gas turbines are widely used. At present, gas turbines have played a relatively important role in national economic construction and the national defense industry system. The application classification of gas turbines is shown in Table 2.

As early as the 1970s, my country had the initial ability to design and manufacture gas turbines on its own, but due to limited oil resources at that time, the application of gas turbines in thermal power plants was not promoted. my country has relatively abundant coal resources, so it vigorously developed coal-fired steam turbines at that time. There are many advantages to using gas turbines for power generation. The technical advantages and disadvantages of gas turbines for power generation are shown in Table 3.

At present, countries around the world are focusing on the study of coal-fired gas-steam combined cycle. The gas turbine uses the gas generated by coal combustion as a high-temperature heat source, and makes full use of the exhaust waste heat to supply the steam turbine with subsequent power output, supply hot water and steam to the outside, and build a thermal power station with combined heat and power. This gas-steam combined cycle has a high thermal efficiency. In terms of coal-fired technology of gas turbines, the current focus of research is on the integrated coal gasification gas-steam combined cycle (IGCC) and the pressurized fluidized bed gas-steam combined cycle (PFBC-CC). By adopting these two types of combined cycles, the power generation efficiency can be significantly improved and the pollution of flue gas emissions can be alleviated.

3 System composition of gas turbines

The gas turbine is composed of components such as a compressor, a combustion chamber, a turbine and a regenerator. The compression, heat absorption and expansion work processes in the thermal cycle of the gas turbine are respectively carried out in the compressor, the combustion chamber (sometimes equipped with a regenerator) and the turbine, and are all in a continuous operation state. Therefore, the gas turbine, like the steam turbine, is a continuous flow thermal engine, which is different from the interrupted flow thermal engines such as steam engines and internal combustion engines. Gas turbines can use open cycles or closed cycles. Gas turbines mainly use open cycles, and a small number of gas turbines use closed cycles.

In a gas turbine using a closed cycle, the working fluid is heated by external combustion (air boiler, nuclear reactor or other heat exchanger) after pressurization, and the heat is discharged through the heat exchanger after expansion and work. Gas turbines using a closed cycle can use gases other than air as working fluids, such as helium. Steam turbines can also be regarded as a generalized form of closed-cycle gas turbines. Steam turbines use water and its steam as working fluids, use water pumps instead of compressors to increase pressure, are heated by boilers, and then exhaust heat through condensers to achieve the corresponding thermodynamic cycle.

Gas turbines used in chemical, metallurgical and other fields are also called industrial gas turbines. In a broad sense, systems or equipment such as gas-steam combined cycle and turbochargers of internal combustion engines can also be classified into this category. Turbochargers that are widely used in internal combustion engines can be regarded as a combined power unit of gas turbines and internal combustion engines [19]. In this type of power unit, the high-pressure compression process, combustion process and high-pressure expansion process are carried out intermittently in the internal combustion engine; the low-pressure compression process is carried out continuously in the turbocharger, and the low-pressure expansion process is carried out continuously in the exhaust gas turbine.

4 Research on the structural characteristics of gas turbines

4.1 General overview

The two major industrial systems that the development of the gas turbine industry depends on are the steam turbine industry and the aircraft engine industry. Influenced by these two types of industrial systems, gas turbines have gradually formed two structural forms with obvious differences in the process of development: light structure and heavy structure.

4.2 Research on design concepts based on gas turbine structural characteristics

4.2.1 Research on design concepts based on heavy gas turbine structural characteristics

The design process of heavy gas turbines is mostly based on the traditional design habits of steam turbines, that is, thick and thick parts are used. Thick parts have good rigidity under normal temperature conditions, but when the temperature changes drastically, the thermal stress gradually increases with the thickness of the parts, and thick parts are prone to deformation and fracture. Therefore, when designing heavy gas turbines, it is difficult to use strong cooling methods with large temperature gradients, nor is it suitable to use excessively high gas temperatures. It is also necessary to prevent parts from generating excessive thermal stress through long-term gradual warm-up and turning measures. It is also difficult to ensure that parts will not deform violently through relevant measures. Since the pressure of the working fluid of the open cycle gas turbine is not very high, there is a significant difference from the steam turbine. Therefore, large parts such as the cylinder and rotor of the gas turbine do not need to be designed as heavy as the steam turbine. On the premise of ensuring a certain strength and rigidity, the thinner the part size, the safer it is. Once it is too thick, it will increase the thermal stress and easily cause thermal fatigue.

Since the pressure ratio of the gas turbine is not high, the gas flow pressure loss in its pipeline, combustion chamber, heat exchanger, air filter and muffler has a relatively large impact on the performance of the gas turbine. Therefore, when designing the structure of the gas turbine, the flow loss should be minimized. To this end, a larger flow cross-section can be used to reduce the flow velocity of the working fluid, and short and straight pipelines and better streamlined parts can be used. In heavy-duty gas turbines, the working fluid flow velocity is low, but the body is large, the equipment layout is scattered, and the pipeline is long and curved, which is not conducive to reducing the flow resistance. In the compressor of the gas turbine, the working fluid flows against the pressure gradient, so the corresponding aerodynamic problem is more complicated than the expansion process. The flow efficiency of the working fluid in the gas turbine blades has a significantly greater impact on the overall efficiency of the device than that of the steam turbine, requiring the gas turbine to adopt a blade shape with higher aerodynamic performance. The application of traditional steam turbine design ideas in the field of gas turbines is limited.

4.2.2 Research on design concepts based on the structural characteristics of light gas turbines

The design of light gas turbines has absorbed a lot of scientific research results and technical experience in the field of aviation engines. On this basis, in terms of investment cost or operating performance, material consumption and plant investment are greatly saved. Due to the thin parts, small size, floating structure and large thermal expansion gap, thermal stress is greatly reduced. Due to the small number of stages and large inter-stage enthalpy drop, the gas temperature at the nozzle outlet can be further reduced. At the same time, with the help of a strong air cooling scheme, certain advantages have been achieved in the strength, reliability and life of high-temperature materials. It can make up for the adverse effects caused by high gas temperature and high speed, and can adapt to rapid startup and drastic changes in operating conditions. When using the same materials, due to the stronger cooling effect, light gas turbines can use higher gas temperatures than heavy gas turbines. Some high-temperature parts can be made of ferrite or pearlite with small thermal expansion coefficient, good thermal conductivity and high strength, which can significantly reduce the cost of high-temperature metals for light gas turbines. In terms of efficiency, although the flow rate of the working fluid inside the light gas turbine is high and the circulation is simple, the efficiency of light gas turbines has gradually exceeded that of heavy gas turbines due to the use of higher gas temperature and pressure ratio, short straight pipes with less flow loss, and more efficient blade design. The reduction of mass inertia, volume inertia and thermal inertia is more conducive to the rapid start-up of the unit, and the operating adjustment characteristics of the unit have also been significantly improved. The simple and compact equipment is more suitable for full automatic control. Manufacturers who focus on the research and development of heavy gas turbines can absorb the design ideas of light gas turbines, thereby further promoting the development of the gas turbine industry. 5 Research on the technical and economic performance of gas turbines in the application process

5.1 The overall situation of the development of the technical and economic performance of gas turbines

In recent years, high-power thermal engines are mainly steam turbines, but gas turbines are obviously lighter and smaller than steam turbines, and are not restricted by water sources when selecting plant sites.

In terms of the analysis of the thermodynamic properties of the working fluid, there are also large differences between gas turbines and steam turbines. The temperature of the working fluid of the steam turbine is lower than that of the gas turbine, but its pressure is significantly higher. Ultra-high pressure will cause a series of problems, such as requiring the unit structure to be thicker, thereby increasing the manufacturing cost. The technical problems caused by excessive pressure will further restrict the improvement of the economic performance of the steam turbine. The gas turbine has fully improved the design and manufacturing costs by adopting a lightweight structure.

At present, medium and small power thermal engines are still mainly internal combustion engines, among which diesel engines have the highest economic performance. Most modern diesel engines use turbochargers, which can be regarded as a special combination of internal combustion engines and gas turbines. The gas turbine has a compact structure, light size, and its maneuverability is significantly better than that of a diesel engine. Even though the thermal efficiency of gas turbines is relatively low, the overall operating economy is gradually approaching that of diesel engines because they can burn low-quality fuels and effectively save lubricating oil and maintenance costs.

5.2 Advantages of gas turbines in technical and economic efficiency

5.2.1 Light and small device

Under the premise of the same power, the weight and volume of the gas turbine unit are only a few of those of steam turbines or internal combustion engines. Therefore, gas turbines are more suitable as power units for mobile equipment, especially mobile equipment with high requirements for structural compactness. Since gas turbines occupy a small area, they can effectively save plant infrastructure costs, which is significantly better than steam turbine power plants. In addition, the construction period of gas turbine power plants is shorter, and sometimes it only takes a few months from design to commissioning.

5.2.2 Strong fuel adaptability and less pollution

Gas turbines can burn cheaper fuels, such as heavy oil, coal gas, etc., and can even be combined with nuclear reactors to operate as closed-cycle gas turbines. The same gas turbine can burn different liquid or gas fuels, and usually there is no need to make large-scale adjustments to the fuel supply system. The emission performance of gas turbines is generally good. Except for the need to take additional countermeasures for emissions such as NOx, the unit has little impact on atmospheric pollution, and the noise is also controlled within an acceptable range.

5.2.3 Effectively save water and lubricating oil

Gas turbines do not use steam as a working fluid, nor do they use water cooling. They require less water and can operate continuously in water-scarce areas. Unlike internal combustion engines, the working fluid of gas turbines flows continuously in the unit, and strict piston ring sealing equipment is not required. The wear of the parts is less, and the amount of lubricating oil is reduced.

5.2.4 Fast start-up and high degree of automation

The process of starting a gas turbine from a cold state and accelerating it to a full load state usually takes only tens of seconds to a few minutes. It takes several minutes to several hours for an internal combustion engine or steam turbine to start from a cold state and accelerate it to a full load state. Gas turbines start quickly in severe cold, have a high degree of automation, are easy to remotely control, and do not require special monitoring personnel on site.

5.2.5 Fast maintenance and reliable operation

Gas turbine equipment is simple and easy to achieve serialization, standardization and generalization. At the same time, it can be designed into a box-type structure, which is convenient for detection and maintenance, effectively saving maintenance costs.

6 Outlook on the main technical development trends of gas turbines at this stage

6.1 Overall situation of gas turbine technology development

In recent years, energy and pollution issues have put forward higher requirements for power machinery, but these requirements may conflict with each other. In order to reduce operating costs, gas turbines can burn low-cost heavy oil, but the exhaust gas pollution generated when burning heavy oil is more serious and it is easy to corrode the blades. Therefore, the maximum temperature of the working fluid needs to be controlled accordingly during operation, but it will reduce the thermal efficiency of the unit. Large equipment such as regenerators and waste heat boilers are needed to compensate for the reduction in efficiency, which increases the manufacturing cost.

The new generation of light gas turbines modified from aviation has higher parameters such as temperature and pressure ratio used in the working fluid, and the thermal efficiency has been improved, but this type of unit can usually only burn light fuel, which increases the operating cost. At present, the use of heavy fuel oil in light gas turbines is still under further research.

At present, in addition to being widely used in the field of peak loads, gas turbines have gradually taken on basic loads. If gas turbines are to be used as a power source for basic loads, there is still a need to further improve efficiency and use cheaper fuels to reduce pollution, increase unit power, and reduce maintenance costs. If gas turbines are used as a power source for land-based vehicles, it is also necessary to focus on optimizing the variable operating performance of gas turbines and reducing the cost of materials and manufacturing processes. In the automotive industry, the overall output of automobiles is high and the impact is wide. Automobiles powered by gas turbines have not yet been widely promoted. At present, it is necessary to continue to optimize indicators such as the economy of the unit before it is possible to put gas turbine vehicles into large-scale production.

6.2 Improve unit efficiency

6.2.1 Increase gas temperature

(1) Develop high-temperature materials. Gas turbine blades operate at high speeds at high temperatures and need to adapt to rapid temperature changes. If the blade material encounters thermal stress, thermal fatigue, thermal corrosion and creep, it will have a serious impact on the life of the unit. In recent years, the development of high-temperature alloys can effectively increase the maximum temperature of the gas. However, the temperature resistance limit of high-temperature alloys cannot be infinitely increased. Therefore, there are currently two main measures: one is to use a blade surface protective layer, using aluminum, chromium and composite materials to improve the high-temperature corrosion resistance when burning heavy fuels; the other is to vigorously develop special ceramic blades. This field is still under continuous research.

(2) Improve cooling technology.
In order to ensure the normal operation of the unit, the combustion chamber and turbine blades of the gas turbine need to be cooled. In recent years, more advanced cooling methods have been adopted, such as film cooling and divergent air cooling.

6.2.2 Increase the unit pressure ratio

In order to improve efficiency, simple cycle gas turbines need to increase the pressure ratio while increasing the gas temperature. On the one hand, the single-stage pressure ratio can be increased and a transonic stage can be used; on the other hand, the whole machine pressure ratio can be increased, and adjustable guide vanes and dual rotors can be used, thereby widening the operating range of the compressor.

6.2.3 Make full use of waste heat

Making full use of the exhaust heat of the gas turbine can effectively improve the overall efficiency of the unit. Use a regenerator. The use of a regenerator in a gas turbine can significantly improve the efficiency of the unit. Currently, a high-efficiency, lightweight regenerator is being developed. Use a gas-steam combined cycle. This type of combined power unit can use the waste heat of the exhaust gas discharged by the gas turbine to heat the steam, thereby improving the overall efficiency of the unit. It is a common solution for newly built thermal power plants.

6.3 Use low-cost fuels, use new energy, and reduce pollution emissions

Treatment before burning heavy fuels. If a gas turbine burns heavy oil for a long time, its turbine blades are prone to corrosion. Therefore, if heavy fuel is required, the fuel needs to be pre-treated to reduce its negative impact on the unit. Heavy oil needs to be precipitated, filtered, washed and fed first.

Coal gasification and liquefaction. Gas turbines usually cannot burn coal directly, but if this goal is to be achieved, the coal needs to be gasified or liquefied before burning. When it is necessary to burn gas with a lower calorific value, the combustion equipment should also be adjusted. At the same time, the combustion equipment that burns coal gas is large in size, which will affect the compactness of the gas turbine structure, and the coal ash needs to be cleaned in time.

Use nuclear fuel. Currently, closed-cycle gas turbines combined with nuclear reactors are also under development, and the combination of high-temperature gas-cooled reactors and helium turbines has good prospects.

Develop low-pollution high-temperature combustion chambers. By designing new combustion chambers, optimizing cooling schemes and improving combustion processes accordingly, pollution emissions can be reduced.

6.4 Increase unit power

In addition to being limited by parameters such as working fluid temperature and pressure, the increase in unit power of a gas turbine is also affected by the flow rate of the working fluid, which is determined by the flow rate, density and flow area of ​​the unit.

Increase the flow area. On the one hand, the blade length of the gas turbine can be increased, but the vibration generated during high-speed operation needs to be improved through design, and the impact of the three-dimensional flow of the working fluid needs to be considered. On the other hand, the unit can be assembled in parallel, and the flow rate can be increased by using several parallel gas generators and a turbine.

Increase the flow rate of the working fluid. When using a supersonic compressor and turbine, the impact of the three-dimensional flow of the working fluid also needs to be considered.

Increase the density of the working fluid. At present, the basic pressure of the working fluid can be increased by adopting a closed cycle solution, so that the density of the working fluid increases proportionally, thereby increasing the unit power.

6.5 Achieve efficient monitoring and maintenance

Strengthen real-time monitoring of the unit. Automatic monitoring is achieved through computers, and a certain number of measuring points are arranged at key parts of the unit. Problems can be discovered early without opening the cylinder, thereby achieving timely maintenance, improving the unit reliability factor, and reducing the accident rate accordingly.

Adopt box-type structure. The use of such a structure can effectively simplify the maintenance process and improve the unit utilization rate.

6.6 Achieve efficient operation under variable operating conditions

When the gas turbine is in variable operating conditions, the compressor operation performance is easy to deteriorate, resulting in a decrease in unit performance. The following optimization measures can be adopted:
First, use adjustable stators. The compressor and turbine of the gas turbine can improve the flow of the working fluid by using adjustable stators, thereby alleviating the deterioration of the unit performance.
Second, use a multi-axis system. By adopting multi-axis systems such as split shafts, dual shafts, and triple shafts, the economy of the unit under variable operating conditions can be improved.
Third, use a regenerator. By using a regenerator, the efficiency of the unit at low load can be improved accordingly.
Fourth, use a closed cycle. Closed-cycle gas turbines can adapt to changes in operating conditions through flow regulation, and the efficiency reduction at low load is relatively small.

6.7 Adopting advanced process technology

Since gas turbine blades have high requirements for profile design, and high-temperature alloys have high hardness and are difficult to process, special processes and equipment are required to manufacture gas turbine blades. In order to reduce the cost of design and manufacturing, new processes that are being vigorously promoted and improved include vacuum smelting, directional crystallization, powder metallurgy, ceramic metallurgy, high-speed forging, shot peening, laser processing, CNC processing, explosive forming, high-power spinning, plasma spraying, electron beam welding, argon arc welding, laser welding and brazing.

7 Conclusion

Gas turbines have developed rapidly in recent years, and their technical level has been steadily improved. At present, higher requirements are put forward for the design, manufacture, testing, application and maintenance of gas turbines. Through in-depth research and development of this type of thermal engine, the development of my country’s economic construction and national defense industry can be further promoted.