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Chapter 4 Gas Turbine

This chapter focused on Gas turbine and thermodynamic cycle associated with it.

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Chapter 4 Gas Turbine

  1. 1. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. GAS TURBINE Prof. Aniket Suryawanshi Asst. Prof. Automobile Engg. Dept. R. I. T. Rajaramnagar
  2. 2. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. OUTLINE OF CHAPTER Working Principle and Applications of gas turbine. Types of Gas turbine. Modified Bryton cycle: Regeneration Modified Bryton cycle: Reheat Modified Bryton cycle: Intercooling. Numerical on gas turbine work ratio and efficiency. Gas turbine irreversibility's and losses.
  3. 3. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. The simple gas turbine power plant mainly consist of a gas turbine coupled to a axial air compressor and combustion chamber which is placed between the compressor and turbine in the fuel circuit. The gas turbine use gas as working medium by which heat energy is converted into mechanical work or thrust. Gas is produced in the engine by the combustion of the fuel in the combustion chamber. Gas turbine is a rotary internal combustion engine. Hot gases coming out from Combustion Chamber discharges over the blades of turbine wheel. It causes the spinning action of the turbine blade about the shaft. GAS TURBINE POWER PLANT - INTRODUCTION
  4. 4. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Atmospheric air is compressed to high pressure and temperature in the axial compressor. This high pressure and temperature air is passed through nozzle in the CC, where fuel is injected in the form of spray and combustion takes place. Resulting combustion products enters in the expansion zone where it expands through a turbine to produce shaft work. The exhaust gases leaving the turbine are finally discharged into the atmosphere. Most of the power plant uses it to the run the auxiliaries devices of the plant such as cooling fan, water pumps and the generator itself also. GAS TURBINE POWER PLANTS – WORKING PRINCIPLE
  5. 5. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. GAS TURBINE POWER PLANTS – ADVANTAGES Compared to Steam-Turbine and Diesel Propulsion Systems, Gas Turbine offers : • Greater Power for a given size and weight, • High Reliability, • Long Life, • More Convenient Operation. • Engine Start-up Time reduced from 4 hrs to less than 2 min…!! • Less area requires for storage of fuel. • Less maintenance cost, • Simple construction, no need of boiler, condenser as in the other cases. • Kerosene, Paraffin, benzene and powdered coal like cheaper fuels used. • Less requirement of water, so can be installed at water scarcity area. • Less pollution. • Easy handling.
  6. 6. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. GAS TURBINE POWER PLANTS – DISADVANTAGES • Manufacturing a turbine blades is much difficult and costly. • For same power o/p gas turbine produces 5 times more exhaust gases than I. C. engine. • Gas Turbine blades require special cooling system. • Lower thermal efficiency as 15% to 20% in gas turbine as 25% to 30% in I. C. engine. (66% of the power developed is used to drive the compressor. Therefore the gas turbine unit has a low thermal efficiency.) • High frequency noise from the compressor is objectionable. • Requires Special metals and alloys to cast parts of the gas turbine. ( The running speed of gas turbine is in the range of (40,000 to 100,000 rpm) and the operating temperature is as high as 1100 – 12600C.)
  7. 7. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Two Major Application Areas : 1. Aircraft Propulsion 2. Electric Power Generation. Electric Power GenerationAircraft Propulsion GAS TURBINE POWER PLANTS – APPLICATIONS
  8. 8. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. GAS TURBINE POWER PLANTS – APPLICATIONS
  9. 9. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. GAS TURBINE POWER PLANTS – AEROSPACE APPLICATIONS
  10. 10. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. • 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 high-temperature gases then enter the turbine where they expand to atmospheric pressure while 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 an open cycle. BRAYTON CYCLE: OPEN CYCLE FOR GAS-TURBINE ENGINES Gas turbines usually operate on an open cycle.
  11. 11. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. • The compression and expansion processes remain the same, but the combustion process is replaced by a constant-pressure heat addition process from an external source. • The exhaust process is replaced by a constant-pressure heat rejection process to the ambient air. BRAYTON CYCLE: CLOSED CYCLE FOR GAS-TURBINE ENGINES The open gas-turbine cycle can be modelled as a closed cycle, using the air-standard assumptions.
  12. 12. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. The ideal cycle that the working fluid undergoes in the closed loop is the Brayton cycle. It is made up of four internally reversible processes: 1-2 Isentropic compression; 2-3 Constant-pressure heat addition; 3-4 Isentropic expansion; 4-1 Constant-pressure heat rejection. The T-s and P-v diagrams of an ideal Brayton cycle. Note: All four processes of the Brayton cycle are executed in steady-flow devices thus, they should be analyzed as steady-flow processes. BRAYTON CYCLE: IDEAL CYCLE FOR GAS-TURBINE ENGINES
  13. 13. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. The energy balance for a steady-flow process can be expressed, on a unit–mass basis, as The heat transfers to and from the working fluid are: The thermal efficiency of the ideal Brayton cycle, is the pressure ratio.where BRAYTON CYCLE: IDEAL CYCLE THERMAL EFFICIENCY Processes 1-2 and 3-4 are Isentropic, P2 = P3 and P4 = P1. 1 1 2 2 3 3 1 1 4 4 T P P T T P P T                        , 1 1 1th Brayton pr              
  14. 14. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. 14 The combustion process is replaced by a heat-addition process in ideal cycles. Air-standard assumptions: 1.The working fluid is air, which continuously circulates in a closed loop and always behaves as an ideal gas. 2.All the processes that make up the cycle are internally reversible. 3.The combustion process is replaced by a heat-addition process from an external source. 4.The exhaust process is replaced by a heat- rejection process that restores the working fluid to its initial state. Cold-air-standard assumptions: When the working fluid is considered to be air with constant specific heats at room temperature (25°C). Air-standard cycle: A cycle for which the air-standard assumptions are applicable. AIR-STANDARD ASSUMPTIONS
  15. 15. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Inlet conditions to a Brayton cycle are 1 bar and 300 K. The cycle pressure ratio is 6.5. The temperature at the inlet to the turbine is 1500 K. Calculate the performance parameters of the cycle. Process 1→2 :   1 1.4 1 2 2 1.4 2 1 1 (300 ) 6.5 512.132 T P T K T P K             rp =6.5 300 K 1500 K Process 3→4 :   1 1.4 1 3 3 1.4 4 4 4 (1500 ) 6.5 878.679 T P T K T P K             EXAMPLE ON IDEAL BRAYTON CYCLE
  16. 16. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Compressor Work :      2 1 2 1 1.005 (512.132 300) 213.19 . C P kJ kJ W h h C T T K kg K kg                    Steady – Flow – Energy – Equation : Heat Input :      3 2 3 2 1.005 (1500 512.132) 992.81 . in P kJ kJ Q h h C T T K kg K kg                    Turbine Work :      3 4 3 4 1.005 (1500 878.679) 624.43 . T P kJ kJ W h h C T T K kg K kg                    Heat Out :      4 1 4 1 1.005 (878.679 300) 581.57 . out P kJ kJ Q h h C T T K kg K kg                    EXAMPLE ON IDEAL BRAYTON CYCLE
  17. 17. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Net Power In :  624.43 213.19 411.24T C kJ W W W kg               Plant Efficiency :     411.24 / 41.42 % 992.81 / th in kJ kgW kJ kgQ       ….ANS Alternatively;   1.4 11 1.4 1 1 1 1 41.42 % 6.5 th pr                      ….ANS EXAMPLE ON IDEAL BRAYTON CYCLE…CNTD
  18. 18. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. 18 • 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 or frictional effects . 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. Turbine: Compressor: ACTUAL GAS-TURBINE CYCLES…CNTD
  19. 19. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. 300 K 1500 K rp =6.5 In the plant of Example 1, let the compressor and the turbine have the isentropic efficiencies of 0.8 each. Calculate the performance parameters of the cycle. From the earlier results, kg kJ W kg kJ Q kg kJ W KT KT T in C 43.624 81.992 19.213 679.878 132.512 4 2         EXAMPLE ON ACTUAL BRAYTON CYCLE
  20. 20. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech.     12 12 12 12 12 12 , , TT TT TTC TTC hh hh W W A S AP SP A S ActC TheorC C             300 K 1500 K rp =6.5 300 300132.512 8.0 2    AT KT A 165.5652  Similarly;     S A SP AP S A TheorT ActT T TT TT TTC TTC hh hh W W 43 43 43 43 43 43 , ,             679.8781500 1500 8.0 4    AT KT A 94.10024  EXAMPLE ON ACTUAL BRAYTON CYCLE…CNTD
  21. 21. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Compressor Work :                     kg kJ K Kkg kJ TTChhW APAC 49.266300165.565 . 005.11212 Turbine Work :                     kg kJ K Kkg kJ TTChhW APAT 54.49994.10021500 . 005.14343 Heat Input :                      kg kJ K Kkg kJ TTChhQ APAin 51.939165.5651500 . 005.12323 Net Power In :           kg kJ WWW CT 05.23349.26654.499 Plant Efficiency : ….ANS%8.24 51.939 05.233    in th Q W  EXAMPLE ON ACTUAL BRAYTON CYCLE…CNTD
  22. 22. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Heat Input :                      kg kJ K Kkg kJ TTChhQ APAin 51.939165.5651500 . 005.12323 Alternatively;          kg kJW W C SC AC 49.266 8.0 19.213, ,  Actual Compressor Work :         kg kJ WW STTAT 54.49943.6248.0,, Actual Turbine Work : Net Power In :           kg kJ WWW CT 05.23349.26654.499 Plant Efficiency : ….ANS%8.24 51.939 05.233    in th Q W  EXAMPLE ON ACTUAL BRAYTON CYCLE…CNTD
  23. 23. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. The fraction of the turbine work used to drive the compressor is called the back work ratio. BWR is defined as the ratio of compressor work to the turbine work 𝑟𝑏𝑤 = 𝑤𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟 𝑤𝑡𝑢𝑟𝑏𝑖𝑛𝑒 The BWR in gas turbine power plant is very high, normally one-half of turbine work output is used to drive the compressor BACK WORK RATIO
  24. 24. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. The fraction of the turbine work that becomes the net work is called the work ratio. Work Ratio is defined as the ratio of net work to the turbine work 𝑟𝑤 = 𝑤 𝑛𝑒𝑡 𝑤𝑡𝑢𝑟𝑏𝑖𝑛𝑒 WORK RATIO
  25. 25. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. The early gas turbines (1940s to 1959s) found only limited use despite their versatility and their ability to burn a variety of fuels, because its thermal efficiency was only about 17%. Efforts to improve the cycle efficiency are concentrated in three areas: 1. Regeneration by preheating the air leaving the compressor with turbine exhaust gases (regeneration or recuperation). 2. Reheating of gases after each stage of expansion to obtain more work from the turbine (reheating). 3. Intercooling during compression stages to reduce the work input to the compressor (intercooling). IMPROVEMENTS OF GAS TURBINE’S PERFORMANCE
  26. 26. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. BRAYTON CYCLE WITH REGENERATION • Temperature of the 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 (Fig. 9-38 & Fig. 9-39). • 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 exit temperature.
  27. 27. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Effectiveness of the regenerator, Effectiveness under cold-air standard assumptions, Thermal efficiency under cold-air standard assumptions, EFFECTIVENESS OF THE REGENERATOR Assuming the regenerator is well insulated and changes in kinetic and potential energies are negligible, the actual and maximum heat transfers from the exhaust gases to the air can be expressed as If written in terms of temperatures only, it is also called the thermal ratio
  28. 28. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Thermal efficiency of Brayton cycle with regeneration depends on: a) ratio of the minimum to maximum temperatures, and b) the pressure ratio. Regeneration is most effective at lower pressure ratios and small minimum-to-maximum temperature ratios. FACTORS AFFECTING THERMAL EFFICIENCY Can regeneration be used at high pressure ratios?
  29. 29. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. BRAYTON CYCLE WITH REHEATER •A high pressure and a low pressure turbine are used in a Reheat Brayton Cycle •A reheater is a heat exchanger that increases the power output without increasing the maximum operating temperature but it does not increase the efficiency of the cycle •The capital cost to build a reheater alone cannot be justified because the thermal efficiency • does not increase.
  30. 30. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. •Multistage compression with intercooling improves the efficiency of a compression process. •A Brayton Cycle with Intercooling uses two or more compression stages with one or more intercoolers, as shown below. The power requirement for compression is reduced, but QH also increases. •Again, the capital cost to build an intercooled compressor alone cannot be justified because the thermal efficiency does not increase. BRAYTON CYCLE WITH INTERCOOLING
  31. 31. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. A gas-turbine engine with two-stage compression with intercooling, two-stage expansion with reheating, and regeneration and its T-s diagram. For minimizing work input to compressor and maximizing work output from turbine: Tmax limited by materials, Tmin limited by environment BRAYTON CYCLE WITH INTERCOOLING, REHEATING & REGENERATION
  32. 32. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. 32 The work input to a two-stage compressor is minimized when a) equal pressure ratios are maintained across each stage. b) Complete intercooling is performed This procedure also maximizes the turbine work output. Thus, for best performance we have, CONDITIONS FOR BEST PERFORMANCE 𝑇3 = 𝑇1
  33. 33. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. 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 can be decreased by carrying out the compression process in stages and cooling the gas in between (Fig. 9-42), using multistage compression with intercooling. 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. BRAYTON CYCLE WITH INTERCOOLING, REHEATING & REGENERATION
  34. 34. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. As the number of compression and expansion stages increases, the gas-turbine cycle with intercooling, reheating, and regeneration approaches the Ericsson cycle. Intercooling and reheating always decreases thermal efficiency unless accompanied by regeneration. Why? Therefore, in gas turbine power plants, intercooling and reheating are always used in conjunction with regeneration. BRAYTON CYCLE WITH INTERCOOLING, REHEATING & REGENERATION
  35. 35. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. • The thermal efficiency of an ideal Brayton cycle depends on the pressure ratio, rp of the gas turbine and the specific heat ratio, k of the working fluid. • The thermal efficiency increases with both of these parameters, which is also the case for actual gas turbines. PARAMETERS AFFECTING THERMAL EFFICIENCY
  36. 36. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. • The highest temperature in the cycle is limited by the maximum temperature that the turbine blades can withstand. This also limits the pressure ratios that can be used in the cycle. • The air in gas turbines supplies the necessary oxidant for the combustion of the fuel, and it serves as a coolant to keep the temperature of various components within safe limits. An air– fuel ratio of 50 or above is not uncommon. PARAMETERS AFFECTING THERMAL EFFICIENCY
  37. 37. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. PARAMETERS AFFECTING THERMAL EFFICIENCY
  38. 38. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. PARAMETERS AFFECTING THERMAL EFFICIENCY
  39. 39. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. PARAMETERS AFFECTING THERMAL EFFICIENCY
  40. 40. AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech. Thank You !

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