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ME 417 Thermodynamics II






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Gas Turbine Engines. Adaptable source of powerWide range of applicationsLighter and more compact than vapour power plantsMore suitable for transportation (aircraft). ME 417 Thermodynamics II - M. Fauchoux. 2. Basic Components. Compressor ? pressurizes incoming airCombustion chamber ? fuel burned
ME 417 Thermodynamics II

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1. ME 417 Thermodynamics II Section 3 Gas Turbine Engines ME 417 Thermodynamics II - M. Fauchoux 1

2. Gas Turbine Engines Adaptable source of power Wide range of applications Lighter and more compact than vapour power plants More suitable for transportation (aircraft) ME 417 Thermodynamics II - M. Fauchoux 2

3. Basic Components Compressor ? pressurizes incoming air Combustion chamber ? fuel burned with air Turbine ? produces work Work used to drive compressor and output to other applications (propel vehicle, etc) ME 417 Thermodynamics II - M. Fauchoux 3

4. Open cycle/mode Most common Air enters compressor Pressure increased Combustion chamber Air mixed with fuel Combustion occurs Temperature increased Combustion products ME 417 Thermodynamics II - M. Fauchoux 4

5. Open cycle/mode cont. Products enter turbine Gases expand Produces work Exhaust products released to surroundings ME 417 Thermodynamics II - M. Fauchoux 5

6. Closed cycle/mode Less common Working fluid remains gas throughout Energy input by heat transfer from external source ME 417 Thermodynamics II - M. Fauchoux 6

7. Closed cycle/mode cont. Gas exiting turbine passes through heat exchanger Gas cooled before re-entering compressor ME 417 Thermodynamics II - M. Fauchoux 7

8. Other components Intercooler Reheat combustors Heat exchangers To increase power output, thermal efficiency, etc ME 417 Thermodynamics II - M. Fauchoux 8

9. Performance of gas turbines Influenced by Component efficiencies Pressure ratio of compressor Turbine working temperature The higher the better ME 417 Thermodynamics II - M. Fauchoux 9

10. Industrial gas turbines Object is production of shaft power Rugged, reliable Size and weight less of a concern Applications Electricity generation Pumping and compressor stations for NG pipelines Ship propulsion Land and rail vehicles Cogeneration plants ? gas turbine drives generator, exhaust gases used as a source of low-grade heat for industrial purposes ME 417 Thermodynamics II - M. Fauchoux 10

11. Aircraft gas turbines Higher power to weight ratio compared to IC engines Small size and lightweight Turboprop, turbojet, turbofan engines ME 417 Thermodynamics II - M. Fauchoux 11

12. Turbojet cycle Diffuser decelerates incoming air Gases expand through nozzle to high velocity Overall change in V of the gases gives rise to the propulsive force (thrust) ME 417 Thermodynamics II - M. Fauchoux 12

13. Review of components Isentropic efficiencies used to compare actual performance with idealized performance ME 417 Thermodynamics II - M. Fauchoux 13

14. Turbines A device in which work is developed as a result of a gas or liquid passing through a set of blades attached to a shaft that is free to rotate ME 417 Thermodynamics II - M. Fauchoux 14

15. Turbines cont. Used for power generation Vapour power systems Gas turbine systems Aircraft engines Superheated gas enters the turbine and expands to a lower pressure as power is generated ME 417 Thermodynamics II - M. Fauchoux 15

16. For a control volume inclosing the turbine Steady state, 1 inlet and 1 outlet Energy rate balance becomes ME 417 Thermodynamics II - M. Fauchoux 16

17. Assumptions For gas flowing through turbine, PE and KE changes are small ?- neglect PE and KE Heat transfer to surroundings occurs, but small compared to work and enthalpy terms ? neglect Q Maximum work ? occurs for smallest value of h2 Isentropic expansion ? h2S Actual work ? less than maximum h2 > h2S ME 417 Thermodynamics II - M. Fauchoux 17

18. Isentropic turbine efficiency Typical values ? 70-90% Can be represented on an h-S diagram (Mollier) ME 417 Thermodynamics II - M. Fauchoux 18

19. ME 417 Thermodynamics II - M. Fauchoux 19

20. Nozzles and diffusers Flow passages of changing cross-sectional area Nozzles accelerate flow Diffusers decelerate flow ME 417 Thermodynamics II - M. Fauchoux 20

21. For a control volume on whole nozzle/diffuser Assumptions Only work is flow work ? no work input ? Wcv = 0 Heat transfer to surroundings small ? neglect Q For gases, PE change is small ? neglect PE ME 417 Thermodynamics II - M. Fauchoux 21

22. Isentropic Nozzle efficiency Ratio of actual kinetic energy to ideal kinetic energy exiting nozzle Typical values - 95% or greater ME 417 Thermodynamics II - M. Fauchoux 22

23. Compressors and pumps Purpose is to raise the pressure or elevation of a substance flowing through it Work input is required Compressors - gases Pumps - liquids ME 417 Thermodynamics II - M. Fauchoux 23

24. For a control volume around compressor Assumptions Neglect changes in KE and PE Heat transfer to surroundings is small - neglect Q ME 417 Thermodynamics II - M. Fauchoux 24

25. Isentropic compressor efficiency Ratio of ideal compression to actual compression Depends on h2 - smaller h2, less work input required Smallest possible value is going to be h2S Represent on an h-s diagram ME 417 Thermodynamics II - M. Fauchoux 25

26. ME 417 Thermodynamics II - M. Fauchoux 26

27. Heat exchangers Purpose is to add or remove heat Numerous applications Home heating and cooling Automotive systems Electrical power generation Examples ME 417 Thermodynamics II - M. Fauchoux 27

28. For a control volume around heat exchanger Often multiple inlets and outlets Assumptions No work in or out of control volume - W = 0 No major changes in KE or PE For our purposes - 1 inlet and 1 outlet ME 417 Thermodynamics II - M. Fauchoux 28

29. Other components Intercooler Used to cool a fluid between stages of a multi-stage process Example - between two compressors to lower T while increasing P Reheat Used to heat a fluid between stages Example - between two turbines to increase T to as high as possible ME 417 Thermodynamics II - M. Fauchoux 29

30. Open cycle Closed cycle ME 417 Thermodynamics II - M. Fauchoux 30

31. Air standard Brayton cycle As with IC engines, actual analysis of gas turbine cycles is complicated Named for George Brayton - developed continuous combustion process Assumptions Working fluid is air (ideal gas) Temperature rise from combustion replaced with heat transfer from external source Heat exchanger after turbine brings gas back to state point 1 to complete thermodynamic cycle No pressure drop due to friction in piping 4 internally reversible processes ME 417 Thermodynamics II - M. Fauchoux 31

32. Provides qualitative insight into gas turbine performance Notes on chalkboard Net Work ME 417 Thermodynamics II - M. Fauchoux 32

33. Some of the work produced by the turbine is used to operate the compressor This is expressed as the back work ratio Typically 40 - 80% A large portion of the work developed by the turbine is needed to run the compressor ME 417 Thermodynamics II - M. Fauchoux 33

34. Thermal efficiency ME 417 Thermodynamics II - M. Fauchoux 34

35. Cold-Air Brayton Cycle Constant specific heats - cp, cv and k ME 417 Thermodynamics II - M. Fauchoux 35

36. Isobaric processes 2-3 and 4-1 P4 = P1 P3 = P2 Isentropic processes 1-2 and 3-4 ME 417 Thermodynamics II - M. Fauchoux 36

37. The greater the pressure rise across the compressor, the greater the thermal efficiency (for cold-air standard analysis) ME 417 Thermodynamics II - M. Fauchoux 37

38. Temperature considerations Highest temperature occurs at inlet to turbine Same as Otto and Diesel cycles Limit on temp is about 1700 K for a turbine Due to metallurgical considerations Shows importance of advanced materials research, blade cooling, convective heat transfer, etc. ME 417 Thermodynamics II - M. Fauchoux 38

39. Compare 2 cycles: A and B ME 417 Thermodynamics II - M. Fauchoux 39

40. In order for B to develop the same net power as A, it will need to operate at a higher mass flow rate This means larger, heavier gas turbine May need to trade off thermal efficiency for reduced weight in the face of high temperature limits ME 417 Thermodynamics II - M. Fauchoux 40

41. Example 7 Air enters the compressor of an idea air-standard Brayton cycle at 100 kPa, 300 K, with a volumetric flow rate of 5 m3/s. The compressor pressure ratio is 10. The turbine inlet temperature is 1400 K. Determine (a) the thermal efficiency of the cycle, (b) the back work ratio and (c) the net power developed in [kW]. ME 417 Thermodynamics II - M. Fauchoux 41

42. Effect of irreversibilities and losses Friction -inside the compressor and turbine Means they are not truly isentropic Entropy will increase across the component Pressure drop in heat exchangers Means they are not truly isobaric Not as significant as friction losses Typically ignored Heat transfer to surroundings Means compressor and turbine not adiabatic Typically small and therefore ignored ME 417 Thermodynamics II - M. Fauchoux 42

43. Changes that friction makes to the T-s diagram ME 417 Thermodynamics II - M. Fauchoux 43

44. To develop a large net work, need very efficient compressors and turbines Typically 80-90% Recall: Isentropic turbine efficiency Isentropic compressor efficiency ME 417 Thermodynamics II - M. Fauchoux 44

45. Example 8 Consider example 7 but include in the analysis that the turbine and compressor each have an isentropic efficiency of 80%. Determine for the modified cycle (a) the thermal efficiency of the cycle, (b) the back work ratio and (c) the net power developed in [kW]. ME 417 Thermodynamics II - M. Fauchoux 45

46. Regeneration ME 417 Thermodynamics II - M. Fauchoux 46

47. Regeneration Exhaust gases leaving the turbine are at much higher temperatures than the surroundings Can we make use of this heat? Use a heat exchanger - called a regenerator - to preheat air from the compressor before it enters the combustor Counterflow exchanger - hot turbine exhaust gases flow in one direction, cooler air from compressor flows in the other direction Assume no pressure drop across regenerator ME 417 Thermodynamics II - M. Fauchoux 47

48. ME 417 Thermodynamics II - M. Fauchoux 48

49. Regenerator Reduces the amount of fuel that must be burned in combustion chamber to achieve same output temp Therefore Reduces heat added (Qin) Net work remains the same Increases thermal efficiency ME 417 Thermodynamics II - M. Fauchoux 49

50. Air Standard Brayton Cycle + Regeneration ME 417 Thermodynamics II - M. Fauchoux 50

51. Turbine exhaust gas (T4) is cooled in regenerator to Ty Air exiting compressor (T2) heated in regenerator to Tx Combustor raises temp from Tx to T3 instead of T2 to T3 Heat added Work produced by turbine Work required by compressor ME 417 Thermodynamics II - M. Fauchoux 51

52. Heat rejected to surroundings Thermal efficiency ME 417 Thermodynamics II - M. Fauchoux 52

53. Regenerator Effectiveness If ideal (ie: reversible) - Tx = T4 Realistically - Tx < T4 Higher effectiveness means higher Tx Typically 60-80% ME 417 Thermodynamics II - M. Fauchoux 53

54. Increase effectiveness - increase surface area Causes an increase in the pressure drop due to friction ME 417 Thermodynamics II - M. Fauchoux 54

55. Reheat Temperature of the gas entering the turbine is limited because of metallurgical reasons Solution - multi-stage turbines with a reheat between stages ME 417 Thermodynamics II - M. Fauchoux 55

56. ME 417 Thermodynamics II - M. Fauchoux 56

57. Reheat In order to do this, must provide excess air so that not all of the air is burned in the first combustor Work from the turbines increases Work required by compressor unchanged Therefore net work increases Does thermal efficiency increase? ME 417 Thermodynamics II - M. Fauchoux 57

58. ME 417 Thermodynamics II - M. Fauchoux 58

59. Gas reheated at constant pressure in reheat combustor Total work of two-stage turbine greater than single turbine Thermal efficiency not necessarily higher - qin increases Turbine exhaust temp greater with reheat - greater potential to benefit from regeneration ME 417 Thermodynamics II - M. Fauchoux 59

60. Example 9 (9.62) Air enters the turbine of a gas turbine at 1200 kPa, 1200 K, and expands to 100 kPa in two stages. Between the stages, the air is reheated at a constant pressure of 350 kPa to 1200 K. The expansion through each turbine stage is isentropic. Determine, in kJ per kg of air (a) the work developed by each stage, (b) the heat transfer for the reheat process and (c) the increase in net work as compared to a single stage of expansion with no reheat. ME 417 Thermodynamics II - M. Fauchoux 60

61. Intercooling The net work output of a gas turbine can be increased by reducing the compressor work input Accomplished with by cooling the air while it is compressed ME 417 Thermodynamics II - M. Fauchoux 61

62. ME 417 Thermodynamics II - M. Fauchoux 62

63. Intercooling The smaller area to the left of process 1-2 indicates less work is required for compression with cooling However - to cool a gas as it is being compressed requires large amounts of heat removal Solution - intercooling Multi-stage compression with cooling between ME 417 Thermodynamics II - M. Fauchoux 63

64. ME 417 Thermodynamics II - M. Fauchoux 64

65. Increase pressure from P1 to P2 in two stages Intermediate pressure Pi ME 417 Thermodynamics II - M. Fauchoux 65

66. Air Standard Brayton Cycle + Intercooling Process 1-c Isentropic compression Temperature and pressure increase Process c-d Constant pressure heat rejection through intercooler Temperature decreases Process d-2 Isentropic compression Temperature and pressure increase ME 417 Thermodynamics II - M. Fauchoux 66

67. Air Standard Brayton Cycle + Intercooling Intercooling reduces the amount of work input to compression process Does not necessarily increase thermal efficiency Temperature of the air exiting multi-stage compressor is cooler Air entering the combustor is cooler More heat is required to achieve desired temperature entering turbine Therefore lower thermal efficiency Could combine with regeneration/reheat to increase efficiency ME 417 Thermodynamics II - M. Fauchoux 67

68. Example 10 A regenerative gas turbine with intercooling and reheat operates at steady state. Air enters the compressor at 100 kPa, 300 K with a mass flow rate of 5.807 kg/s. The pressure ratio across the two-stage compressor is 10. The pressure ratio across the two-stage turbine is also 10. The intercooler and reheater each operate at 300 kPa. At the inlets to the turbine stages, the temperature is 1400 K. The temperature at the inlet to the second compressor is 300 K. The isentropic efficiency of each compressor and turbine is 80%. The regenerator effectiveness is 80%. Determine (a) the thermal efficiency, (b) the back work ratio and (c) the net power developed. ME 417 Thermodynamics II - M. Fauchoux 68

69. Gas turbine engines used for aircraft propulsion Light and compact High power to weight ratio Aircraft gas turbine Open cycle Jet Propulsion Cycle Jet Propulsion Cycles

70. Jet Propulsion Cycle compared to Brayton Cycle Gases not expanded to atmospheric pressure in turbine Work from turbine only used to drive compressor/auxillary equipment Gas only expands to pressure that produces necessary amount of work Therefore - net work is zero

71. Gases exit turbine at high P, T Accelerated through nozzle to provide thrust to power aircraft Difference in momentum at inlet and outlet of engine Inlet - low velocity air Outlet - high velocity gases Pressures at inlet and outlet the same (atmospheric) Thrust

72. Velocities are relative to aircraft velocity Vinlet = Vaircraft although fuel is injected, the AF is high making the difference in small Propulsive Power [kW]

73. Propulsive efficiency Measure of how efficiently the thermal energy released during combustion is converted to propulsive energy

74. Simplest jet engine Diffuser, combustor, nozzle High speed applications Engine must move relative to ambient air to have flow through engine and to develop pressure rise Pressure rise associated with deceleration known as ?ram effect? Ramjet

75. Ramjet

76. Next level of engine complexity Diffuser, compressor, combustor, turbine, nozzle Turbine extracts some energy from fluid to drive compressor Turbojet

77. Turbojet

78. Also called single spool One turbine, one compressor Older models, simpler Single-shaft design

79. Twin spool Two compressors, two turbines One high pressure shaft (rotates faster) One low pressure shaft Two-shaft design

80. Additional combustion chamber after turbine Similar to a reheat Adds nearly instantaneous thrust and acceleration Used on military aircraft Thermodynamically inefficient Afterburner

81. Afterburner

82. Heavier but more efficient than turbojet Always multi-spool Fan section upstream of LP compressor Both driven by LP turbine Turbofan

83. Portion of air from fan enters LP compressor Continues through engine, exits through nozzle Some air bypasses Exits through fan nozzle Produces additional thrust Turbofan with fan exhausted

84. Turbofan

85. Bypass air not exhausted through fan nozzle Bypasses engine but mixed with turbine exhaust All air exits from one nozzle Turbofan with fan mixed

86. Airflow accelerated through exhaust nozzle Largest component of thrust comes from propeller Turbine drives propeller Turboprop

87. Turboprop

88. Comparison of Engine Types

89. Assumptions Internally reversible processes Working fluid is air (ideal gas) KE neglected at exit of all components except nozzle PE neglected throughout Air Standard Turbojet

90. Air at 26 kPa, 230 K and 220 m/s enters a turbojet engine in flight. The air mass flow rate is 25 kg/s. The compressor pressure ratio is 11, the turbine inlet temperature is 1400 K, and air exits the nozzle at 26 kPa. The diffuser and nozzle processes are isentropic, the compressor and turbine have isentropic efficiencies of 85% and 90%, respectively, and there is no pressure drop for flow through the combustor. KE is negligible everywhere except at the diffuser inlet and nozzle exit. Determine a) P, T at each state, b) rate of heat addition in combustor [kW] and c) velocity at nozzle exit [m/s]. Example 11 (prob 9.77)

91. Couples two power cycles such that the energy discharged by heat transfer from one cycle is used as the heat input for the second cycle Gas turbine - high temp gases at exhaust of turbine can be put through regenerator Alternative - use high temp gas as an input to a vapour power cycle Combined gas turbine - vapour power cycle Topping Cycle Combined gas turbine - vapour power cycle

93. Heat exchanger balance No heat losses to surroundings Thermal efficiency

94. Consider the addition of an afterburner to the turbojet in Example 11 that raises the temperature at the inlet of the nozzle to 1300K. Determine the velocity at the nozzle exit in m/s. Example 12 (prob 9.80)

95. Air enters the diffuser of a ramjet engine at 40 kPa, 240 K, with a velocity of 2500 km/h and decelerates to negligible velocity. On the basis of an air-standard analysis, the heat addition is 1080 kJ/kg. Air exits the nozzle at 40 kPa. Determine a) the pressure at the diffuser exit, in kPa and b) the velocity at the nozzle exit, in m/s. Neglect KE except at the diffuser inlet and nozzle exit. Example 13 (prob 9.83)


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