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UNIT - IV JET ENGINE PROPULSION
The document provides information about jet engine propulsion, including the major components and processes involved. It discusses the global momentum analysis and equations for jet engines. It also covers types of propulsion systems, classifications of jet engines, and the basic operation and components of jet engines such as the compressor, combustor, turbine, and nozzle. Key components and their functions are described, including how compressed air is mixed with fuel and ignited to produce thrust through exhaust exiting the nozzle.
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UNIT - IV JET ENGINE PROPULSION
- 1. ME 6604 –GAS DYNAMICS & JET PROPULSION UNIT – IV JET ENGINE PROPULSION Mrs.N.PREMALATHA ASSOCIATE PROFESSOR DEPARTMENT OF MECHANICAL ENGG
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- 3. Momentum Equation pinlet pexit Vac Vjet ∑= dt dM F cm surface Reynolds Transport Theorem: inletexit cvcm MM dt dM dt dM −+= Newton’s Second Law of Motion
- 4. inletexit cv surface MM dt dM F −+=∑ For a frictionless flight, pressure forces are only the surface forces… inletexit cv ductwallexitexitinletinlet MM dt dM FApAp −+=−− ∑∑ Steady state steady flow inletexitductwallexitexitinletinlet MMFApAp −=−− ∑∑ airairjetjetductwallexitexitinletinlet VmVmFApAp −=−− ∑∑ airairjetjetexitexitinletinletductwall VmVmApApF +−−= ∑∑
- 5. airairjetjetexitexitinletinletductwall VmVmApApF +−−= ∑∑ Pressure Thrust Momentum Thrust At design cruising conditions : Pressure thrust is zero. airairjetjetthrust VmVmF −= atmexitinlet ppp ==
- 6. Generation of Thrust: The Capacity ( ) acairjetfuelairT VmVmmF −+= acairjetjetT VmVmF −= Thrust ( ){ }acjetairT VVfmF −+= 1 f : Fuel-air ratio
- 7. Dynamic Equilibrium :Cruising Vehicle For a cruising vehicle: ( ){ } Vehicleon1 dragVVfmF acjetairT =−+= ( ){ } 2 1 2 air ac acdragacjetair V ACVVfm ρ=−+
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- 10. Drag Coefficient ofan Air Craft
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- 12. Drag Coefficient ofan Air Craft
- 13. Propulsive Power orThrust Power: ( ){ }acjetairacacTp VVfmVVFP −+== 1 Specific Thrust S ( ) acjet air T VVf m F S −+== 1 Measure of compactness of a jet engine:
- 14. Thrust Specific FuelConsumption TSFC ( ){ } ( ){ }acjetacjetair fuel T fuel VVf f VVfm m F m TSFC −+ = −+ == 11 Measure of fuel economy:
- 15. Aviation Appreciation Propulsion Efficiency JettheofPowerKineticAvailable PowerThrust propulsion=η ( ){ }22 1 2 acjet air acT propulsion VVf m VF −+ = η ( )( ) { }22 )1( 2 1 acjet air acacjetair propulsion VVf m VVVfm −+ −+ = η
- 16. Jet Characteristics • Quantitiesdefining a jet are: – cross-sectional area; – composition; – velocity. jetjetjetjet VAm ρ= acairjetjetjetT VmVAF −= 2 ρ acairjetjetT VmVmF −= Of these, only the velocity is a truly characteristic feature and is of considerable quantitative significance.
- 17. Jet Characteristics ofPractical Propulsion Systems System Jet Velocity (m/s) Turbofan 200 - 600 Turbojet (sea-level, static) 350 - 600 Turbojet (Mach 2 at 36000 ft) 900 - 1200 Ramjet (Mach 2 at 36000 ft) 900 - 1200 Ramjet (Mach 4 at 36000 ft) 1800 - 2400 Solid Rocket 1500 – 2600 Liquid Rocket 2000 – 3500
- 18. Nozzle : SteadyState Steady Flow First Law : No heat transfer and no work transfer & No Change in potential energy. in jet cv jetin cv Wgz V hmgz V hmQ + ++= +++ 22 22 jetin V h V h += + 22 22
- 19. Combined analysis ofconservation of mass and first law 22 += + jetjet jet inin in A m h A m h ρρ A SSSF of gas through variable area duct can interchange the enthalpy and kinetic energy as per above equation. Consider gas as an ideal and calorically perfect. 0 22 22 Tc c V Tc c V Tc p p jet jetp p in inp = += +
- 20. γ γ 1− = jet in jet in p p T T Isentropic expansionof an ideal and calorically perfect gas.
- 21. Types of PropulsionSystems Air-Breathing Use atmospheric air (+ some fuel) as main propellant Rockets Carry entire propellant (liquid/solid fuel + oxygen) Piston, Gas Turbine and Ramjet Engines Gas Turbine Engines (most aircraft jet engines): • Use high-temperature gases to power a propeller or produce direct thrust by expanding and accelerating the exhaust gases through a nozzle. • Three main types: Turbojet, Turbofan and Turboprop
- 22. Basic Operation ofJet Engines – • Air enters the trough the intake duct (cowl). • Air compressed by passage through the compressor. • Mixed with fuel in the combustion chamber. • Fuel is ignited, Pressure and Temperature raised • Some of the pressure used to turn a turbine; • Turbine shaft drives the compressor. • Hot, high pressure air forced through a nozzle. • The reaction force is the engine thrust.
- 23. Classifications of JetEngine
- 24. • Turbojets: • Turbineused to drive the compressor. • All intake air passes through the combustion chamber and exits through the nozzle. • All thrust produced by hot, high-speed exhaust gases. •Turbofans (Fan-Jet): • A large propeller in the intake cowl, in front of compressor. • Dramatically increases the amount of air pulled in the intake. • Only a small percentage passed through the engine, the rest of cold air is Bypassed. • Part of the thrust through the hot exhaust gases and part by the cold bypassed air. Produces cooler exhausts and quieter engines. • High by-pass ratio are most commonly used in larger commercial aircraft. •Turboprops: • Jet engine used to turn a large propeller, which produces most (90% or more) of the thrust. Used in smaller aircraft. Classifications of Jet Engine
- 25. Jet Engine Combuster Filmcooling via convection
- 26. FUEL IS MIXEDWITH AIR BEFORE ENTERING THE CYLINDERS THE FUEL/AIR MIXTURE IS THEN COMPRESSED THEN IT IS IGNITED BY A SPARK IN A PISTON ENGINE: - ONCE FOR TWO REVS OF THE ENGINE (IN THE 4 STROKE CYCLE) AIR IS COMPRESSED AND FORCED INTO THE COMBUSTOR FIRST THEN THE FUEL IS SPRAYED IN UNDER PRESSURE IT IS THEN IGNITED BY A SPARK (BUT ONLY ONCE FOR STARTING) IN A JET ENGINE: - COMBUSTION IS THEN CONTINUOUS WHILST THE ENGINE IS RUNNIN THE COMBUSTION PROCESS
- 27. Jet engines arealso called as gas turbines. The engine sucks air in at the front with a fan. A compressor raises the pressure of the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine. As the jets of gas shoot backward, the engine and the aircraft are thrust forward. TURBO JET ENGINES
- 28. Major components • Airintake • Compressors • Combustors • Turbines • Nozzles • Fuel system • Cooling system
- 29. Intermediate components • Turbopumps • Afterburners( reheat) • Thrust reversers
- 31. Fan-The fan isa first component in a turbo fan. The large spinning fan sucks in large quantity of air. Most of the fan blades are made up of titanium.It then speeds this air up and splits it into two parts. One part continues through the “core” or center of the jet engine, where it is acted upon by other jet engine components. AIR INTAKE
- 32. The second part“bypasses” the core of the jet engine. It goes through a duct which surrounds the core to the back of jet engine where it produces much of force that propels the airplane forward. This cooler air helps to quiet the jet engine as well as adding thrust to the jet engine.
- 34. Compressor - Thecompressor is the first component in the jet engine core. The compressor is made up of fans with many blades and attached to a shaft. The compressor squeezes the air that enters it into progressively smaller areas, resulting in an increase in the air pressure. This results in an increase in the energy potential of the air. The squashed air is forced into the combustion chamber.
- 36. Combustor - Inthe combustor the air is mixed with fuel and then ignited. There are as many as 20 nozzles to spray fuel into the air stream. The mixture of air and fuel catches fire. This provides a high temperature, high-energy airflow. The fuel burns with the oxygen in the compressed air, producing hot expanding gases. The inside of the combustor is often made of ceramic materials to provide a heat-resistant chamber.The heat can reach 2700°.
- 37. Turbine - Thehigh-energy airflow coming out of the combustor goes into the turbine, causing the turbine blades to rotate. The turbines are linked by a shaft to turn the blades in the compressor and to spin the intake fan at the front. This rotation takes some energy from the high-energy flow that is used to drive the fan and the compressor. The gases produced in the combustion chamber move through the turbine and spin its blades.
- 38. The turbines ofthe jet spin around thousands of times. They are fixed on shafts which have several sets of ball-bearing in between them.
- 39. Nozzle - Thenozzle is the exhaust duct of the jet engine. This is the jet engine part which actually produces the thrust for the plane. The energy depleted airflow that passed the turbine, in addition to the colder air that bypassed the engine core, produces a force when exiting the nozzle that acts to propel the engine, and therefore the airplane, forward. The combination of the hot air and cold air are expelled and produce an exhaust, which causes a forward thrust.
- 40. The nozzle maybe preceded by a mixer, which combines the high temperature air coming from the jet engine core with the lower temperature air that was bypassed in the fan. The mixer helps to make the jet engine quieter.