Gas Turbine Cycles refers to the path of the working of the whole process. A gas Turbine is a prime mover in which gradual changes in the momentum of fluid are utilized to produce rotation of the mobile member. In this article, I will discuss the gas turbine cycles and the workings of these gas turbine cycles.
Gas Turbine Cycles
The gas turbine cycles are basically of two types:
- Open cycle.
- Closed cycle.
The classification of the gas turbine cycle refers to the path of the working substance. In an open cycle, the working substance or fluid, usually air, flows in and out. Heat is added by burning fuel in the air and the combustion products are released to the atmosphere. Such open-cycle gas turbines, by definition, are internal combustion engines because the combustion process is internal to the cycle.
In a closed cycle, the working fluid is confined within the plant. The same fluid having specified mass flows in the system in a closed cycle, and there is the transfer of heat and work taking place between the system and the surroundings. The working fluid does not come in contact with the combustion product.
The heat is added through a heat exchanger. the combustion taking place externally. The working fluid is usually hydrogen or helium due to its good thermodynamic properties, although air can also be used.
For automotive applications, both open-cycle and closed-cycle gas turbine engines can be used. But the open cycle gas turbine engines give better performance.
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1. Open Cycle Gas Turbine
It consists of a compressor, combustion chamber, and turbine. The atmospheric air is compressed in the compressor. The compressed air goes to the combustion chamber, where the burning of the fuel takes place. The combustion product goes to the turbine, which passes through the blades and rotates the turbine shaft. The combustion product is then exhausted to the atmosphere. This leads to an open-cycle gas turbine.
The open cycle is simpler than the closed cycle because the cooler is not required. Also, the heater is replaced by the combustion chamber, and the fuel is burnt directly in the gas stream. The weight of the gas flowing through the turbine will be more than that flowing through the compressor by the weight of the fuel added. There is an advantage b using a gas cycle if the gas is air since fuel can be burnt directly into the working fluid. The combustion of the gas will differ from that of pure air.
In sophisticated calculations, the weight of the fuel is taken into account, but since the air-fuel ratios are very large, the weight of the fuel is neglected in simpler analysis. Also, the gas stream after combustion may be treated as having properties of air. Thus, open-cycle analysis can be taken up in the same way as closed cycle analysis.
Fig represents the open cycle on the T-ø and H-ø diagram. Air is sucked at 1. The process 4- 1 taking place in the cooler in the closed cycle is eliminated since the exhaust from the turbine at 4 is rejected to the atmosphere. Line 4-1 has no meaning in open cycle gas turbine. It does not represent any process in any component.
Open Cycle Gas Turbine With Heat Exchanger or Regenarator
It is seen in the open cycle gas turbine that the exhaust gases from the turbine are rejected to the atmosphere at a sufficiently high temperature (about 430°C), much higher than that of the air discharged from the compressor. Hence a large quantity of heat is wasted by the exhaust gases, which might be utilized in heating the compressed air by the use of a heat exchanger regenerator.
The exhaust gases from the turbine first go in the heat exchanger, where the heat of the compressed air and exhausted into the atmosphere. The compressed air is heated in the heat exchanger and then goes into the combustion chamber. It gives fuel economy. The fuel required per unit mass of air will be less giving a higher overall efficiency of the plant. Different sizes of heat exchangers are suitable for different purposes.
The simplest form of the heat exchanger for this purpose would be the double pipe type, with one gas steam flowing through the inner pipe and the other stream through the annular between the inner and the outer pipe.
By the use of such a heat exchanger in the open cycle gas turbine, it is theoretically possible to raise the temperature of the compressed air from T2 to Tx = T4 and lower the temperature of the gas leaving the turbine from T4 to TY = T2, as shown in Fig. Thus, the heat transfer takes place at each interval of the heat exchanger with an infinitesimal temperature difference. Thus, the process may be regarded as reversible and the cycle as ideal.
In actual practice, such an arrangement of an ideal heat exchanger is not possible. The temperature difference causing heat transfer i s a fi nite value and i n thi s case, equal to (T4 – Tx ) = (Ty) = T2).
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Open Cycle Gas Turbine With Intercooling and Reheating
It is obvious that the regenerator improves the thermal efficiency of the cycle but does not improve the work ratio. The work ratio may be improved either by decreasing the compressor work or by increasing the turbine work.
An inter-cooler is used when the high-pressure ratios are involved to cool the air from the low-pressure compressor before it goes into the high-pressure compressor. It reduces the overall power required for compression. Although the inter-cooler reduces the compression work, there is some pressure loss. But even then it gives overall economy.
The air-fuel ratio in a gas turbine is quite high. Hence, the combustion products exhausted after expansion in the high-pressure turbine are still rich in oxygen and are subjected to combustion once again in the second combustion chamber before entering the low-pressure turbine.
The temperature attained by the air after re-heating is the same as that before it enters the high-pressure turbine. The compressor may be driven by power from the high-pressure turbine. The use of heat exchangers, intercoolers, and re-heaters increases the overall efficiency at the expense of increased cost.
The vertical distance between only two constant pressure lines goes on decreasing to the left and goes on increasing to the right. The compression takes place in two stages, 1-3 and 4-5, and the air is cooled at constant intermediate pressure between the two stages. If there is single-stage compression, the vertical distance 4-5 is less than the vertical distance 3-2. Therefore the work done on the compressor with inter-cooling at intermediate pressure has decreased.
It is possible to cool the air from T3 to atmospheric temperature T1, i.e., T1 = T3• This is called perfect inter cooling. For minimum compressor work and perfect inter-cooling, the work input for two stages is equal, or work is equally shared between two stages.
Similarly, with re-heating, the vertical intercepts move towards the right, and hence work done by the turbine increases. With ideal re-heating, i.e., re-heating to the metallurgical temperature limit T = T6, the turbine work is increased.
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2. Closed Cycle Gas Turbine
The air alone circulates through the compressor and turbine. Cold air enters the compressor, the compressed air passes to the heater where heat is supplied and the temperature of the air is raised. The hot air now enters the turbine where it expands to a lower pressure. The air exhausted from the turbine flows to the cooler where heat is rejected and the air is restored to its initial conditions. Thus, the same air remains in continuous circulation through the cycle.
The heat is not wasted from the turbine. For ideal performance, the expansion will be assumed reversible adiabatic. The cooler should cool the air without any pressure loss. Therefore, the ideal limit will be constant pressure cooling. While flowing through the heater, the effect of friction or turbulence may lead to pressure loss. For deal performance, the heating is assumed at constant pressure.
The compressor should be of reasonable size, and capable of handling large quantities of air. The condition for best performance of the compressor is reversible adiabatic or isentropic compression. Thus, with all the ideal processes, the closed cycle is called the Brayton cycle.
Working of Closed Gas Turbine Cycles
Air has been mostly used as a working fluid. Other gases are also used. The gas used in the close cycle should have a high adiabatic coefficient, i.e., the ratio of the specific heat at constant pressure to the specific heat at constant volume. The higher the value of this ratio, the lower will be the pressure ratio required for a compressor and turbine designed to give maximum efficiency at chosen values of the maximum and minimum temperature in the system.
The values of v for mono, di- and triatomic gases are 1.66, 1.4, and 1.3 respectively. Thus, a mono-atomic gas should preferably act as the working fluid in the closed cycle. Another consideration for selecting a gas is its density. The greater the density of the gas, the smaller the dimension of the flow passage and hence smaller the size of the unit.
The mono-atomic gases suggested are argon, krypton, and xenon having densities of 0.38, 2.87, and 4.53 relative to air. Carbon dioxide is also used though it is diatomic having a density of 1.52. In recent years, helium has been extensively used for closed cycle gas turbines. The specific heat of helium at constant pressure is about five times that of air, therefore for each kg mass flow the heat drop and hence energy dealt with by the helium machines is nearly five times that in the case of air.
For the same temperature ratio and the plants of the same output, the cross-sectional area required for helium is much less than that for air. The surface area of the heat exchanger for helium can be kept as low as that required for the gas turbine plant using air. Thus the size of the helium unit is considerably small.
Advantages of Closed Cycle Gas Turbines
The advantages of closed cycle gas turbines are-
- The combustion products do not mix with the working fluid, because the heating is external. Hence any fuel of high calorific value may be used.
- There is no corrosion or accumulation of deposits of carbon or tar on the blades and nozzle of the turbine. There is no need for internal cleaning.
- For a given output the size of the compressor and turbine are very Because the air is pre cooled in the pre-cooler before entering the compressor, its specific volume is decreased. Moreover, much higher pressure than the atmospheric can be maintained around the whole cycle.
- The cycle approximates to reversible, which gives higher thermal
- If helium is used as the working medium, molybdenum alloys can be used in designing the turbine, which has high-stress properties at elevated temperatures above I000°C. By doing so plant efficiency of over 50% can be achieved.
- The waste heat from the combustion gases from the heater and pre-heater can be further used for heating water. The hot water from the pre-cooler and intercooler can be used for hot water supply for industrial or domestic purposes.
- A smaller heating surface is required in the intercooler, pre-cooler, and heater because the air at a higher heat transfer is co-efficient.
A gas turbine is used in various propulsion engines. It is worked on the Brayton or Joule Cycle. The two different types of gas turbine cycle are- open cycle and closed cycle. In simple terms, it is a heat engine utilizing the expansion from the combustion of fuel and air in a combustion chamber.
Frequently Asked Questions(FAQ’S)
What is the Brayton or Joule Cycle?
The gas turbine engines work on the Brayton or Joule cycle. It consists of two reversible adiabatic and two constant pressure processes.
How does a gas turbine cycle work?
In a gas turbine cycle, air is compressed in the compressor, mixed with fuel and ignited in the combustion chamber, and the resulting high-pressure, high-temperature gases are expanded through a turbine to produce mechanical work, which can be used to drive various applications like aircraft engines, power plants, and more.
How are gas turbine cycles used in power generation?
Gas turbine cycles are commonly used in power generation, either as standalone units or as part of combined cycle power plants. In combined cycles, the waste heat from the gas turbine is used to generate additional power through a steam turbine.