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A gas turbine is a rotary machine in which the chemical energy of the fuel is converted into mechanical energy or kinetic energy in terms of shaft power. In other words, it is a mechanical power or thrust-delivering machine. It uses a gaseous working fluid for this purpose. The generated mechanical power can be used by industrial devices. There is a continuous flow of the working fluid in a gas turbine. Power generation gas turbines are the ones that produce shaft power. To propel an aircraft, gas turbines are used that convert fuel energy into kinetic energy for the generation of thrust. Fig. 1 below shows a typical representation of a Gas turbine.
Let''s understand the basic operating principle of a gas turbine with the following example:
Imagine there is a rocket in which fuel is going to burn thereby creating high-pressure exhaust gas. According to energy conservation law, in high-pressure exhaust gas, the chemical energy of the fuel is converted into mechanical energy. The thrust of the exhaust gas tries to move the rocket forward when the rocket is fired. Now the question is if one fixes the rocket body with a mechanical structure in order to prevent its movement. What will happen?
In such a case, the high-pressure exhaust gas releases but in a backward direction. Now another case is that what if we add a set of turbine blades to this back-fired exhaust gas?
The released mechanical energy which is in the linear backward direction will transform into rotational movement of the turbine shaft which is a big success. This means the chemical energy of the fuel gas is transformed into rotational mechanical energy of the turbine shaft as shown in Fig. 3.
In simple words, in a gas turbine, hot gases move through a multistage gas turbine. It has both stationary and moving blades just like a steam turbine. The stationary blades adjust their velocity and guide the moving gases to the rotor blades. The turbine''s shaft is coupled to a generator.
In a gas turbine power plant, there is a generator known as an electrical machine and this generator needs a prime mover which is a gas turbine in order to generate electricity as shown in Fig. 4.
It transforms the fuel''s chemical energy into mechanical energy or in other words converting natural gas into mechanical energy. The generated mechanical energy is then transferred to the generator''s shaft through a gearbox. Now the turbine can create electrical energy as shown in Fig. 5.
This prime form of electrical energy usually has a low or medium level of voltage. In order to manage power loss in transmission lines, step-up transformers are used to increase this voltage and the increased voltage is provided to the electrical energy which in turn is transmitted through the transmission lines and delivered to the grid as shown in the below Fig. 6.
With the combustion chamber between the air compressor and turbine, both the air compressor and turbine are mounted on either end on a common shaft. Gas turbines require a starting motor as they are not self-starting. The use of an air compressor is to suck the air and compress it thereby increasing its pressure. Axial design type compressors (multi-stage) are preferred for the most advanced and large gas turbines.
There is a multistage gas turbine from where hot gases move and the kinetic energy is transformed into shaft horsepower. A gas turbine has both stationary and moving blades just like a steam turbine. The purpose of stationary blades is to guide the moving gases to the rotor blades and then adjust their velocity. The turbine''s shaft is coupled to a generator.Exhaust Module:
The hot gases from a gas turbine exit through the exhaust section. The exhaust case consists of an inner and outer housing.
The below listed are types of gas turbines:Open-cycle gas turbineClosed-cycle gas turbineAero derivative gas turbineScale jet enginesAuxiliary power unitJet enginesOpen-cycle gas turbine
Open cycle gas turbine consists of three parts mainly a combustion chamber, turbine, and compressor. The compressor raises the pressure by taking in the ambient air. Fuel is burnt to add heat to the air in the combustion chamber thereby raising its temperature. The heated gases from the combustion chamber are then passed to the turbine where it does its mechanical work while expanding. The below figure (Fig. 10) shows an image of an open-cycle gas turbine.
The working fluid used in a closed-cycle gas turbine is air or any other suitable medium that comes out from a compressor and is heated in a heater by some external source at a relatively constant pressure. The heated high-pressure and high-temperature air is then passed to the turbine. The fluid from the turbine is then cooled to its original temperature by some external cooling agent and then passed to the compressor. This way the working fluid is constantly used in the system and the required heat is given to the fluid by the heat exchanger without significant change in its phase. The below figure (Fig. 11) shows an image of a closed-cycle gas turbine.
These are smaller types of gas turbines used to supply auxiliary power to aircraft. Auxiliary gas turbines are used to supply air conditioning and ventilation. They supply compressed air power to jet engines. They also supply mechanical power to the gearbox to start larger jet engines or to drive shafted accessories.
The factors that limit the size and efficiency of a gas turbine are the firing temperature, compression ratio, mass flow, and centrifugal stresses.The most critical areas in the gas turbine design that determines the engineefficiency and life are the hot gas path, i.e., the combustion chambers and the turbine first stage stationary nozzles, and rotating buckets. The components in these areas represent roughly 2% of the total cost of the gas turbine, but they control the gas turbine output and efficiency.
Nickel superalloys are normally used for gas turbine nozzles and buckets. These are coated under a vacuum with special metals (platinum-chromium-aluminide) to protect against the hot corrosion occurring at high temperatures in presence of contaminants like sodium, vanadium, and potassium.
The operating performance of gas turbines can be increased by the continuing improvements in firing temperatures and compression ratios. Air-cooled nozzles and buckets using bleed air from the compressor are major advancements to increase the firing temperature. This limits the metal temperatures of the nozzles and buckets to withstand hot corrosion and creep.
To provide additional turbine power output by increasing the Final compressor pressure, additional compressor stages can be added to the compressor rotor assembly to give a higher compression ratio.
Natural gas, diesel oil, residual or crude oil can be used as gas turbine fuel.With an increase in ambient temperature and altitude, the air density reduces. This causes a significant reduction in the power output and efficiency of the gas turbine. Ambient air temperature and elevation changes do not affect steam plants and diesel.
Frequently used codes and standards that govern the design, construction, testing, etc of a gas turbine are
The factors that impact the performance of a gas turbine are
The major share of gas turbine manufacturing is controlled by the following organizations:
The main differences between a gas turbine and a steam turbine are listed in the following table:
To learn more about gas turbines click on the below-mentioned subjects, review the course and enroll:
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