Department of Mechanical Engineering ME 322 Mechanical Engineering Thermodynamics. Lect 27b. Jet Aircraft Propulsion

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1 Department of Mehanial Engineering ME 3 Mehanial Engineering Thermodynamis Let 7b Jet Airraft Propulsion

2 Airraft Propulsion Thrust produed by inreasing the kineti energy of the air in the opposite diretion of flight Slight aeleration of a large mass of air Engine driving a propeller Large aeleration of a small mass of air Turbojet or turbofan engine Combination of both Turboprop engine

3 Airraft Gas Turbine Engines Turboprop Small ommuter planes Turbofan Larger passenger airliners Turbojet high speeds 3

4 The Turbojet Ideal Turbojet Pressure drop with aeleration Ram effet - pressure rise with deeleration a-1 Isentropi inrease in pressure (diffuser) 1- Isentropi ompression (ompressor) -3 Isobari heat addition (ombustion hamber) 3-4 Isentropi expansion (turbine) 4-5 Isentropi derease in pressure with an inrease in fluid veloity (nozzle) 4

5 The Turbojet with an Afterburner The turbine exhaust is already hot. The afterburner reheats this exhaust to a higher temperature whih provides a higher nozzle exit veloity 5

6 Turbojet Irreversibilities Isentropi effiienies - Diffuser - Compressor - Turbine - Nozzle Fluid Frition effets - Combustion hamber 6

7 The Turbojet Model First Law analysis of the omponents in the yle Wt m 1 h3h4 W m h h in Q m h h 3 Air is the working fluid throughout the omplete yle Combustion is replaed with a heat transfer W W W net t V V ha h V V g a g Va h1 ha g a V V h h V V g g V g h h

8 Turbojet Performane There is no net power output of the turbojet engine. Therefore, the idea of net power and thermal effiieny are not meaningful. In turbojet engines, performane is measured by, Propulsive Fore (Thrust) The fore resulting from the veloity at the nozzle exit Propulsive Power The equivalent power developed by the thrust of the engine Propulsive Effiieny Relationship between propulsive power and the rate of kineti energy prodution 8

9 Turbojet Performane Propulsive Fore (Thrust) in (a) exit (5) F mv mv g g exit in m F V V g 5 a In this equation, the veloities are relative to the airraft (engine). For an airraft traveling in still air, Propulsive Power The power developed from the thrust of the engine W p FV airraft m W V V V p 5 a a g V V V airraft in a 9

10 Turbojet Performane Effiienies Overall Effiieny Thermal Effiieny Propulsive Effiieny propulsive Kineti energy prodution rate Wp m ke ke air 5 a overall thermal propulsive thermal mair ke5 ke m HV fuel Propulsive power fuel / m g V V V mair V 5 V a g air 5 a a V a propulsive V V V / V 1 5 a 5 a a Kineti energy prodution rate Thermal power available from the fuel V5 VaVa V V V V 5 a 5 a 1

11 Turbojet Example Given: A turbojet engine operating as shown below Find: P 6 kpa T 3 K V m/s m 5 kg/s PR d.85.9 t T3 14 K 1. n P5 6 kpa (a) (b) () (d) (e) (f) The properties at all the state points in the yle The heat transfer rate in the ombustion hamber (kw) The veloity at the nozzle exit (m/s) The propulsive fore (lbf) The propulsive power developed (kw) The propulsive effiieny of the engine 11

12 Turbojet Example P 6 kpa T 3 K V m/s m 5 kg/s PR t T3 14 K P5 6 kpa 1. d 1. n Note: An array position of [] is allowed in EES! 1

13 Turbojet Example Strategy: Build the property table first. This will require some thermodynami analysis. Consider eah omponent in the yle. P 6 kpa T 3 K V m/s m 5 kg/s PR d.85.9 t T3 14 K 1. n P5 6 kpa Diffuser V V 1 m h m h1 g g h V h1 g 13

14 Turbojet Example Compressor P 6 kpa T 3 K V m/s m 5 kg/s PR t T3 14 K P5 6 kpa h h 1 s h h 1 1. d 1. n Turbine h h 3 4 t wt w h3 h4s Combustion Chamber 14

15 Turbojet Example Nozzle P 6 kpa T 3 K V m/s m 5 kg/s PR t T3 14 K P5 6 kpa 1. d 1. n At this point, the property table is omplete! 3 s 4s 4 15

16 Turbojet Example Now, we an ontinue with the rest of the thermodynami analysis. P 6 kpa T 3 K V m/s m 5 kg/s PR d.85.9 t T3 14 K 1. n P5 6 kpa Combustion Heat Transfer Rate Q m h h in 3 Nozzle Exit Veloity V 4 V 5 m h4 m h5 g g h h 4 5 V 5 g 16

17 Turbojet Example Now the propulsive parameters an be alulated, P 6 kpa T 3 K V m/s m 5 kg/s PR d.85.9 t T3 14 K 1. n P5 6 kpa m F V V g 5 a W FV FV p airraft propulsive air W p m ke ke 5 17

18 Turbojet Example Solution (Key Variables): P 6 kpa T 3 K V m/s m 5 kg/s PR t T3 14 K P5 6 kpa 1. d 1. n 18

19 Turbojet Example Analysis How is the energy input to this engine distributed? Qin 4,164 kw P5 T5 V5 m 6 kpa K 986 m/s 5 kg/s out exess thermal energy transfer 5 1,617 kw 5.% Q m h h P T V m 6 kpa 3 K m/s 5 kg/s kineti energy prodution rate m m kenet V5 V 11,548 kw 47.8% Wp 4, 13 kw 36.5% 7,335 kw 63.5% m ke exess 19