unit - iv jet engine propulsion
TRANSCRIPT
ME 6604 – GAS DYNAMICS & JET PROPULSION
UNIT – IV JET ENGINE PROPULSION
Mrs.N.PREMALATHAASSOCIATE PROFESSOR
DEPARTMENT OF MECHANICAL ENGG
Global Momentum Analysis
Momentum Equation
pinletpexit
Vac Vjet
dt
dMF cmsurface
Reynolds Transport Theorem:
inletexitcvcm MM
dtdM
dtdM
Newton’s Second Law of Motion
inletexitcv
surface MMdt
dMF
For a frictionless flight, pressure forces are only the surface forces…
inletexitcv
ductwallexitexitinletinlet MMdt
dMFApAp
Steady state steady flow
inletexitductwallexitexitinletinlet MMFApAp
airairjetjetductwallexitexitinletinlet VmVmFApAp
airairjetjetexitexitinletinletductwall VmVmApApF
airairjetjetexitexitinletinletductwall VmVmApApF
Pressure Thrust Momentum Thrust
At design cruising conditions : Pressure thrust is zero.
airairjetjetthrust VmVmF
atmexitinlet ppp
Generation of Thrust : The Capacity
acairjetfuelairT VmVmmF
acairjetjetT VmVmF Thrust
acjetairT VVfmF 1
f : Fuel-air ratio
Dynamic Equilibrium : Cruising Vehicle
For a cruising vehicle:
Vehicleon 1 dragVVfmF acjetairT
2
12
airac
acdragacjetairVACVVfm
Drag on Aircraft
Generation of Lift
Drag Coefficient of an Air Craft
Generation of Lift
Drag Coefficient of an Air Craft
Propulsive Power or Thrust Power:
acjetairacacTp VVfmVVFP 1
Specific Thrust S
acjetair
T VVfmFS 1
Measure of compactness of a jet engine:
Thrust Specific Fuel Consumption TSFC
acjetacjetair
fuel
T
fuel
VVff
VVfmm
Fm
TSFC
11
Measure of fuel economy:
Aviation Appreciation
Propulsion Efficiency
Jet theofPower Kinetic Available PowerThrust
propulsion
2212 acjetair
acTpropulsion
VVfmVF
22)1(
2
1
acjetair
acacjetairpropulsion
VVfmVVVfm
Jet Characteristics
• Quantities defining a jet are:– cross-sectional area;– composition;– velocity.
jetjetjetjet VAm
acairjetjetjetT VmVAF 2
acairjetjetT VmVmF
Of these, only the velocity is a truly characteristic feature and is of considerable quantitative significance.
Jet Characteristics of Practical 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
Nozzle : Steady State Steady Flow
First Law :
No heat transfer and no work transfer & No Change in potential energy.
in jet
cvjetin
cv WgzVhmgzVhmQ
22
22
jetin
VhVh
22
22
Combined analysis of conservation of mass and first law
22
jetjet
jetinin
in Amh
Amh
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
22Tc
cV
Tcc
VTc pp
jetjetp
p
ininp
1
jet
in
jet
in
pp
TT
Isentropic expansion of an ideal and calorically perfect gas.
Types of Propulsion Systems
Air-BreathingUse atmospheric air (+ some fuel) as main propellant
RocketsCarry 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
Basic Operation of Jet 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.
Classifications of Jet Engine
• Turbojets:
• Turbine used 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
Jet Engine Combuster
Film cooling via convection
FUEL IS MIXED WITH 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 RUNNING
THE COMBUSTION PROCESS
Jet engines are also 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
Major components
• Air intake• Compressors• Combustors • Turbines • Nozzles • Fuel system• Cooling system
Intermediate components
• Turbo pumps• Afterburners( reheat)• Thrust reversers
Fan-The fan is a 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
The second part “bypasses” the core of the jet engine. It goes through a duct which surrounds the core to the back of jet enginewhere it produces much of force that propels the airplane forward. This cooler air helps toquiet the jet engine as well as adding thrustto the jet engine.
Compressor - The compressor 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.
Combustor - In the 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°.
Turbine - The high-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.
The turbines of the jet spin around thousands of times. They are fixed on shafts which have several sets of ball-bearing in between them.
Nozzle - The nozzle 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.
The nozzle may be 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.