JCT College of Engineering Technology Pichanur Coimbatore 641
JCT College of Engineering & Technology , Pichanur, Coimbatore – 641 105. Power Plant Engineering Faculty Name : Dr. Anbalagan Designation : Professor Year/Sem : II/III Dept : EEE 1
Brayton Engines Cycle: Ideal Cycle for Gas-Turbine Gas turbines usually operate on an open cycle. Air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high pressure air proceeds into the combustion chamber, The high-temperature then turbine where the fuel is burned at gases constant pressure. enter the wherethey expand to while atmospheric pressure producing power output. Some of the output power is used to drive the compressor. The exhaust gases leaving the turbine are thrown out (not re-circulated), causing the cycle to be classified as 2 an open cycle.
The basic gas turbine cycle is named for the Boston engineer, George Brayton, who first proposed the Brayton cycle around 1870. The Brayton cycle is used for gas turbines only where both the compression and expansion processes take place in rotating machinery. 3
Gas turbines usually operate on an open cycle. Fresh air at ambient conditions is drawn into the compressor, where its temperature and pressure are raised. The high-pressure air proceeds into the combustion chamber, where the fuel is burned at constant pressure. The resulting high-temperature gases then enter the turbine, where they expand to the atmospheric pressure through a row of nozzle vanes. 4
Open cycle gas turbine engine Closed cycle gas turbine engine 5
The open gas-turbine cycle can be modeled as a closed cycle by using the air-standard assumptions. Here the compression and expansion process remain the same, but a constant-pressure heat-rejection process to the ambient air replaces the combustion process. The ideal cycle that the working fluid undergoes in this closed loop is the Brayton cycle, which is made up of four internally reversible processes 6
Isentropic compression (in a compressor) Constant pressure heat addition Isentropic expansion (in a turbine) Constant pressure heat rejection 7
Efficient compression of large volumes of air is essential for a successful gas turbine engine. This has been achieved in two types of compressors. The axial-flow compressor and the centrifugal – or radial-flow compressor. Most power plant compressors are axial-flow compressors. An axialflow compressio 8
The chemical combination of a substance with certain elements. The function of the combustion chamber is to accept the air from the compressor and to deliver it to the turbine at the required temperature. For the common open-cycle gas turbine, this requires the Internal combustion of fuel. A Combustion Chamber Can 9
Turbine 10
Gas turbines move relatively large quantities of air through the cycle at very high velocities. The gas turbine in its most common form is a heat engine operating through a series of processes. It is similar to the gasoline and Diesel engines in its working medium and internal combustion, but is like the steam turbine in the steady flow of the working medium. 11
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All four processes of the Brayton cycle are executed in steady flow devices so they should be analyzed as steady-flow processes. When the changes in kinetic and potential energies are neglected, the energy balance for a steady -flow process can be express, on a unit-mass basis. The thermal efficiency of an ideal Brayton cycle depends on the pressure ratio of the gas turbine and the specific heat ratio of the working fluid (if different from air). 13
Specific heat ratio: A physical property of a material. The specific heat is defined as the amount of heat required to raise a unit of mass of a substance one degree. In brayton cycle, it is given as; rp=p 2/p 1 14
1 1 k 1 r pk In theory, as the pressure ratio goes up, the efficiency rises. The limiting factor is frequently the turbine inlet temperature. The turbine inlet temp is restricted to about 1, 700 K or 2, 600 F. 15
Actual Gas-Turbine Cycles Some pressure drop occurs during the heat-addition and heat rejection processes. The actual work input to the compressor is more, and the actual work output from the turbine is less, because of irreversibilities. Deviation of actual compressor and turbine behavior from the idealized isentropic behavior can be accounted for by utilizing isentropic efficiencies of the turbine and compressor. 16
Brayton cycle Working fluid – air Ideal gas Specific heats steady or variable High temperature reservoir Open or closed model Steady pressure heat exchange 17
Relation between efficiency and pressure ratio 18
Brayton Cycle With Temperature of the Regeneration exhaust gas leaving the turbine is higher than the temperature of the air leaving the compressor. The air leaving the compressor can be heated by the hot exhaust gases in a counter-flow heat exchanger (a regenerator or recuperator) – a process called regeneration. The thermal efficiency of the Brayton cycle increases due to regeneration since less fuel is used for the same work output. Note: The use of a regenerator is recommended only when the turbine exhaust temperature is higher than the compressor 19 exit
Brayton cycle Ideal • ηth =45. 6% With regenerator • ηth =57% Ideal • ηth =45. 6% With irreversibilities • ηth =24. 9% With regenerator * ηth =56. 8% 20
Brayton Cycle With Intercooling, Reheating, & Regeneration The net work output of a gas-turbine cycle can be increased by either: a) decreasing the compressor work, or b) increasing the turbine work, or c) both. The compressor work input decreased by carrying compression process in cooling the gas in multistage compression intercooling. can be out the stages and between using with The work output of a turbine can be increased by expanding the gas in stages and reheating it in between, utilizing a multistage expansion with reheating. 21
Gas Turbine Advantages o. High power: weight ratio o. Compact o. One-direction motion – vibration o. Fewer moving parts o. Better reliability o. Variety of fuels o. Low emissions Disadvantages o. Higher cost 22
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