Gas Turbines and their applications in aircraftspptx

Gas turbines, their components, methods of improving efficiency,

1. Gas Turbines Prepared by Ankur Sachdeva Assistant Professor, ME

2. What are Gas Turbines? Gas turbine engines derive their power from burning fuel in a combustion chamber and using the fast-flowing combustion gases to drive a turbine in much the same way as the high-pressure steam drives a steam turbine. Components of a Gas Turbine Compressor Combustion Chamber Turbine The gas turbine operates on the principle of the Brayton cycle, where compressed air is mixed with fuel and burned under constant pressure conditions. The resulting hot gas is allowed to expand through a turbine to perform work.

3. Applications of Gas Turbine Major applications of gas turbines are found in: As the direct and mechanical drive for various industries Aviation Electrical Power generation Oil and gas industry Marine propulsion Turbo generators Turbo-compressor Automotive sector

4. Difference Between Gas Turbine and Steam Turbine

5. Difference Between Gas Turbine and I.C. Engines

6. Classification of Gas Turbines 1. Based on the cycle of operation/ path of working substance: a) Open Cycle Gas Turbine b) Closed Cycle Gas Turbine 2. Based on the process of heat absorption: a) Constant Pressure Gas Turbine b) Constant Volume Gas Turbine

7. Working of Gas Turbine Intake:  Air is drawn into the engine through an intake. The air is compressed by the compressor section before it enters the combustion chamber. Compression:  The compressor section consists of rotating blades that compress the incoming air. Compressing the air increases its pressure and temperature, preparing it for combustion. Combustion:  In the combustion chamber, fuel is injected and mixed with high-pressure, high-temperature air. The mixture is ignited, leading to rapid combustion. The burning fuel-air mixture produces a high-temperature, high-pressure gas. Expansion:  The hot, high-pressure gas produced during combustion expands rapidly through the turbine section of the engine. The expanding gas flows over turbine blades, causing them to rotate. Energy extraction:  As the turbine blades rotate, they drive the compressor and any other accessories connected to the engine, such as generators or propellers. This extraction of energy from the expanding gas powers the engine and generates thrust in the case of aircraft engines. Exhaust:  The exhaust gases produced during combustion and expansion are expelled from the engine through the exhaust nozzle at high speed, creating thrust in the opposite direction as per Newton's third law of motion.

8. Brayton Cycle Processes Name of the Thermodynamic process 1-2 Isentropic Compression of Air 2-3 Heat Addition at Constant Pressure 3-4 Isentropic Compression of Air 4-1 Heat Rejection at Constant Pressure

9. Expression for Thermal Efficiency

10. Expression for Thermal Efficiency

11. Work Ratio It is the ratio of network output to the total work developed in the turbine or turbines.

12. Back Work Ratio Back work ratio may be defined as the ratio of negative work to the turbine work in a power plant. In gas turbine plants, air is compressed from the turbine exhaust pressure to the combustion chamber pressure. This work is given by –  vdp . As the specific volume of air is very high (even in closed-cycle gas turbine plants), the compressor work required is very high, and also bulky compressor is required . In steam power plants, the turbine exhaust is changed to a liquid phase in the condenser. The pressure of condensate is raised to boiler pressure by the condensate extraction pump and boiler feed pump in series since the specific volume of water is very small as compared to that of air, the pump work (–  vdp ), is also very small. From the above reasons, the back work ratio for gas turbine plants is relatively high compared to that for steam power plants.

13. Methods of Improvement of Thermal Efficiency The following methods are used to improve the efficiency of gas turbine power plants. Gas turbine cycle with regeneration Gas turbine cycle with reheating Gas turbine cycle with intercooling

14. Gas Turbine Cycle with Reheating In order to maximize the work available from the simple gas turbine cycle one of the option is to increase enthalpy of fluid entering gas turbine and extend its expansion upto the lowest possible enthalpy value. Upper limit at inlet to turbine is limited by metallurgical limits while lower pressure is limited to near atmospheric pressure in case of open cycle. For further increasing the net work output the positive work may be increased by using multistage expansion with reheating in between In multistage expansion the expansion is divided into parts and after part expansion working fluid may be reheated for getting larger positive work in left out expansion.

15. Gas Turbine Cycle with Reheating A mbient air enters compressor at 1 and compressed air at high pressure leaves at 2. Compressed air is injected into combustion chamber for increasing its temperature upto desired turbine inlet temperature at state 3. High pressure and high temperature fluid enters high pressure turbine (HPT) for first phase of expansion and expanded gases leaving at 4 are sent to reheat combustion chamber (reheater) for being further heated. Assuming perfect reheating (in which temperature after reheat is same as temperature attained in first combustion chamber), the fluid leaves at state 5 and enters low pressure turbine (LPT) for remaining expansion upto desired pressure value.

16. Gas Turbine Cycle with Regeneration The turbine exhaust temperature is normally much above the ambient temperature. There exists potential for tapping the heat energy getting lost to surroundings with exhaust gases. Using a heat exchanger called a regenerator, which shall preheat the air leaving the compressor before entering the combustion chamber, thereby reducing the amount of fuel to be burnt inside the combustion chamber (combustor). Regenerative air standard gas turbine cycle has a regenerator (counter flow heat exchanger) through which the hot turbine exhaust gas and comparatively cooler air coming from the compressor flow in opposite directions.

17. Gas Turbine Cycle with Regeneration

18. Gas Turbine Cycle with Intercooling Net work output from gas turbines can also be increased by reducing negative work, i.e. compressor work. Multistaging of compression process with intercooling in between is one of the approach for reducing compression work. First-stage compression occurs in low-pressure compressor (LPC) and compressed air leaving LPC at ‘2’ is sent to an intercooler where the temperature of compressed air is lowered down to state 3 at constant pressure. In the case of perfect intercooling the temperatures at 3 and 1 are the same. An intercooler is a kind of heat exchanger where heat is picked up from high-temperature compressed air. .

19. Gas Turbine Cycle with Intercooling Thermal Efficiency may not increase because of the increase in the amount of heat addition

20. Gas Turbine Cycle with Reheating and Intercooling

21. Gas Turbine Cycle with Reheating and Regeneration

22. Gas Turbine Cycle with Intercooling, Reheating and Regeneration

23. Isentropic Efficiency

24. Stage Efficiency Let us consider an axial flow compressor comprising of number of stages having equal stage efficiency for all constituent stages as η s.

25. Stage Efficiency due to diverging nature of constant pressure lines on T-S diagram,

26. Polytropic Efficiency Polytropic efficiency is defined as the isentropic efficiency of an elemental stage in the compression or expansion process such that it remains constant throughout the whole process. Polytropic Efficiency of compressor, η poly,c Polytropic Efficiency of turbine, η poly,t

27. Deviation of Actual Cycles from Ideal Cycles Frictional effects within the compressor and turbine which causes an increase in specific entropy of working fluid across these components. Friction which shall cause drop in pressure of working fluid across the constant pressure processes. Irreversibilities in the combustion chamber. Fluid velocities in turbomachines are very high and there exists substantial change in kinetic energy between inlet and outlet of each component. In the analysis carried out earlier the changes in kinetic energy have been neglected whereas for exact analysis it cannot be. In case of regenerator the compressed air cannot be heated to the temperature of gas leaving turbine as the terminal temperature difference shall always exist. Compression process shall involve work more than theoretically estimated value in order to overcome bearing and windage friction losses.

28. Numerical Problem (AKTU 2022-23) A gas turbine unit receives air at 1 bar, 300 K, and compresses it adiabatically to 6.2 bar. The compressor efficiency is 88%. The fuel has a heating value of 44186 kJ/kg and the fuel-air ratio is 0.017 kg fuel/kg of air. The turbine’s internal efficiency is 90%. Calculate the work of the turbine and compressor per kg of air compressed and thermal efficiency. For products of combustion cp = 1.147 kJ/kg K, γ = 1.33.

29. Numerical Problem (AKTU 2022-23)

30. Numerical Problem (AKTU 2022-23)

31. Numerical Problem (AKTU 2021-22) A Brayton cycle has a pressure ratio of 4. Inlet conditions are 1 bar, 27˚C. Find the air flow required for 100 KW power output if the maximum temperature in the cycle is 1000˚C. Take 𝛾 =1.4 and Cp= 1 KJ/Kg.K

32. Numerical Problem (AKTU 2018-19) Consider an ideal gas-turbine cycle with two stages of compression and two stages of expansion. The pressure ratio across each stage of the compressor and turbine is 3. The air enters each stage of the compressor at 300 K and each stage of the turbine at 1200 K. Determine the back work ratio and the thermal efficiency of the cycle, assuming: (a) no regenerator is used and (b) a regenerator with 75% effectiveness is used.