unit - iv jet engine propulsion

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.