conception aero turbo machine

8/12/2019 Conception Aero Turbo Machine

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Aircraft Design :

Propulsion Systems

Olivier Leonard

University of Liege

Turbomachinery Group

October 2008

October 2008 ?
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Action - Reaction

Reaction: action which does not have initiative

Newtons law: action opposite reaction of same amplitude

action to be defined (easy for still bodies - less for mobiles)

Stone on a support : action and reaction are equal to the stones weight

Stone falling down : action and reaction depend upon the rate of decrease

of the stones velocity

Stone thrown away : action exerted on the stone reaction by the stone on the support

support may start moving

October 2008 Slide 2/61 ?


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Reaction

Application to aircraft propulsion :

action(initiative) exerted by a mobile (the engine) on the air (the support)

air (the support) moves due to the action

reactionexerted by the air on the engine

Action and reaction areindependentof the ease with which the body moves inthe medium



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Propulsion

Natural forces (wind, water stream, gravity, magnetic field, ...)provide movement

free and ecological but random and risky

Propulsion allows to moveindependently of natural forces

Propulsion may be produced

usingmuscles

usingsources of energy

(wood, coal, hydrocarbons, electricity, atoms, ...)

noisy, polluting, expensive but fast, reliable, isotropic

Propulsion is done by means of anaction on a support

support may be fixed or moving



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Propulsion

On theground: propulsion by friction forces

On thewater: floating is the first concern

propulsion by action on water :

rowing, paddle wheels, propellers, jet pumps...

In theair : lift is the first concern -musclesare not sufficient propulsion by action on air surrounding the engine :

piston engines and propellers

pulse jet engines

ramjet engines

gas turbine engines

rocket engines



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Airbreathing engines : historical perspectives

Lightness is the main quality of an engine steam machines never succeeded

Other qualities of an engine are reliability, easy maintenance, low drag,

efficiency

Reciprocating engines

Wrights engine, 1903 : 4 cylinders in-line, 90 kg, 12 Hp

In-line engineshave a low drag but must be cooled with liquid

drag of the heat exchangers

Rotary cylindersare naturally cooled but noisy, high oil and fuel

consumption, strange inertia effects and high centrifugal forces



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Radial engines do not need extra cooling

light, simple, robust, less vibrations but higher frontal area

The most successful configurations wereV12 cylindersand

radial 7 or 9 cylinders(one or several rows)

Many breakthroughs were issued for aircraft propulsion :

high-quality fuels, supercharging, cooled valves

During World War II power reached 2500 to 5000 Hp

At these speeds the cooling system was bringing too much drag

a technological limit was reached

another type of engine was needed



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Gas turbines engines

Steam cycles worked correctly since end of 19th century

Piston engines worked correctly since end of 19th century

Gas turbines work according to a continuous process

high temperatures are severely limited

the compression work is close to the available work

the success is much dependent on component efficiencies

Principles of gas turbines were known since 18th century

First modern gas turbine by Stolze, 1872 - no output power



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First jet engine byLorinin 1913 - a ramjet

First turbojet engine by Guillaume in 1921

At that time the flight speeds were too low

jet propuslion uneffective and rejected

Stodola and Brown-Boveri improve compressors in the 30ies immediatly

thespecific powerwas 3 times better than for reciprocating engines -

nowadays 20 times

The cycle efficiency was around 15% (30% for piston engines)

nowadays 40%

Patent of FrankWhittlein 1930 for a new turbojet

1st flight in England in 1941 with the Whittle W1 developed by Rolls

Royce and de Havilland



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von Ohainin Germany is supported by Heinkel 1st flight in august 1939 (4900 N, 361 kg)

Messerschmidt and Junkers build theJumo004

(8900 N, 750 kg) with cooled turbine blades - lifespan 25 hours

1st flight in USA in 1942 with a General Electric derived from Whittles

Since then most of the progress are due to advances in

materials, forging, casting, manufacturing

aerodynamics

combustion

dynamics of structures

control of systems



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Airbreathing engines : the pulse jet engine

Developed by the Germans for the V1 rocket

Auto-regulated intermittent cycle

No rotating parts, cheap and simple

Noisy and many vibrations

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Airbreathing engines : the ram jet engine

Invented by aFrench, developed by the Germans, improved by the

Americans : first successful flight in 1945

Needs a gas generator at low speeds and a variable geometry

Efficient at Mach 3+



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Airbreathing engines : the gas turbine engines

A continuous process allows very high mass flows

high power output

Gas turbine engines are based on agas generator:

core compressor + combustion chamber(s) + high pressure turbine

powering the compressor

Iso-p lines in the (T,s) plane show that there is some enthalpy left behind

the gas generator - existing engines differ by the propulsion system using

this residual enthalpy

The simplest configuration is a nozzle but cycle optimization induces

ejection velocities up to 2500 km/h

poor propulsive efficiency for most civil and military planes

A subsonic propeller is efficient up to 600 km/h



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There is a need for intermediate solutions !This is possible since thrust is proportional to mass flow and to gas V

Turbojet : simple nozzle

rather low mass flow rate but high output velocity

used for missiles, drones, Concorde or Lockheed SR71

often combined with reheat



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By-pass engine : part of the mass flow is by-passed after a LPcompressor or a wide singlestage fan

the ejection velocity and the noise are lowered

the propulsive efficiency is increased from high subsonic to medium

supersonic

compatible with higher mass flows

may be combined withreheat

this cold flux may generate up to 85% of the total thrust

the by-pass air may be released to atmosphere through a nozzle or

mixed with the core hot stream before ejection



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Turbopropeller : a subsonic propeller is powered through a gearbox by anadditional turbine section

almost no thrust is generated by the hot stream exhaust

the drag is lower than the drag of a piston engine

it is the most efficient solution for the low subsonic range



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Airbreathing engines : synthesis

Today aircraft propulsion is performed bygas turbinesRamjet is used only for the highest speeds or for missiles

Piston engines are now used only for the smallest planes

Gas turbine engines induced a real disruption

Action on air may be measured as an increase of its momentum theuseful poweris measured by the variation of the gas kinetic energy

Agas turbine engineis made up of

agas generatordelivering a high enthalpy mass flow

anozzleturning this enthalpy into kinetic energy

aturbinethat converts enthalpy into mechanical energy and drives a

fan or a propeller



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Thrust production process

Thrust= reaction force exerted by the fluid on the engine

Compressor blades and diffusers are the main sources of thrust

Combustion induces air expansion and a positive thrust

Turbine blades and nozzle induce a negative thrust

Uninstalled thrustis T = qm,9V9 qm,0V0+ (p9 p0)A9



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Airbreathing engines : performance

Gas turbineperfomanceare described in terms of

Installed thrust

A given engine may be installed on different aircrafts

influences of the pod or the fuselage must be identified

Specific thrust= thrust divided by the air mass flow

Specific fuel consumption= fuel mass flow divided by thrust

Cycle efficiency= quality of transformation of fuel heating value into

useful power = rate of production of kinetic energy

Propulsive efficiency= quality of the transformation of the cycles useful

power into power used to propel the vehicle

Component efficiencies= intake, compressor, combustion

chamber, turbine, nozzle...



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Propulsive efficiency

Thepropulsive efficiencymeasures the quality of the transformation of theuseful power delivered by the cycle into power utilized for propelling the vehicle

T =Propulsive power

Useful power =

Thrust speed

Useful power

Thrust reads

T = qm(V9 V0) + (p9 p0)A9| {z }0

thenT =

2(V9 V0)V0

(V9 V0)(V9+ V0) =

2V0

V9+V0=

2

1 +V9

V0



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The difference between the useful power and the propulsive power is trapped

in the air

Pair = 1

2qm(V

2

9 V

2

0) qm(V9 V0)V0 =

1

2qm(V9 V0)

2

Improving the propulsive efficiency implies lowerV9 V0

Optimizing the cycle efficiency means high TET values

By-pass air is used to obtain a globally lower ejection speed

For ...600... km/h the turboprop is the best solution

Propfans use advanced propellers to obtain by-pass ratios 25...50

Turbofans are optimized for ...900... km/h with by-pass ratios 5...9

By-pass engines can go up to Mach 2.5 with a by-pass ratio 0.5...1



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Gl b l ffi i


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Global efficiency


P i th C t


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Progress since the Comet

A car doubles its consumption from 100 km/h to 150 km/h

An aircraft can go higher to go faster

a car at 200 km/h burns about 10 liters of fuel per 100 km and per seat

same for last piston engine aircrafts (DC.6) flying at 500 km/h same for the early jets (Comet, Caravelle, 707, DC.8) flying at 800 km/h

same for modern turbofans at 900 km/h and for Concorde at Mach 2

At commercial speeds the consumption per seat and per km has been

reduced by 70 % in 50 years

The noise has been reduced by 75 %, the weight by 30 %




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Integration issues


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Integration issues

Lowering the specific fuel consumption may be done by increasing the

cycle and the propulsive efficiencies

Increasing the propulsive efficiency is done by decreasing the

specific thrust the mass flow must be augmented to keep the thrust level

the size of the engine must be increased

the weight, the drag, the cost increase

The integration of the engine is more difficult or impossible the optimal engine configuration depends on the mission of the vehicle


Integration issues
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Integration issues

For a long rangecivil airplaneSFC is THE criterion

at high subsonic speeds, the best configuration is a huge flow of air

fairly accelerated through awide fan(2 to 3 m)

For a high speedmilitary planethe engine must be narrow and specificthrust is THE criterion

the usual configuration is a moderate flow of air strongly accelerated

through anarrow fanwith multiple stage

Military planes must have a wide range of performance variable cycles (reheat)


Integration issues


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Integration issues


Integration issues


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Integration issues

The jet engine is theoretically rather simple but is actually very complexbecause ofmany conflicting design goals:

high speed air flow through the engine

steady high temperature levels active cooling

engine must be light

consumption must be low even for military engines

lifespan up to 20000 hours !

easy inspection and maintenance (on plane)

total security and reliability

Thehighest technological capabilitiesare required


Integration issues


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Integration issues

How many engines ?

Criteria : performance, reliability, enonomy

For a given performance at take-off, 2 large engines are less expensive

than 3 or 4 smaller ones

More engines increase the redundancy and the reliability of propulsionforce but also of electrical and auxiliary systems

More engines increase the probability of failure

Failed engines 1 2 3 4

Total engines

1 P

2 2 P P2

3 3 P 3 P2 P3

4 4 P 6 P2 4 P3 P4

P = 0.02 ... 0.1

per 1000 hours


Integration issues


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Integration issues

ETOPS rules have been modified with time

from 60 to 180 minutes (with failure) for a twin engine a/c

twin engine a/c are becoming most popular

Twin engine a/c must climb with 50 % thrust, 4 engine a/c with 75 %

2 engine a/c are oversized for cruise : weight, cost, drag (T/W 0.30) 4 engine a/c need more maintenance (T/W 0.20)

3 engine a/c are losing favor because of installation issues (T/W 0.25)

Final choice :

estimation of T/W selection of an existing engine scale the available engine


Why jet propulsion did not succeed on the ground


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A vehicle must be powered at a velocity of 40 m/s

At that speed its drag is 2000 N and the required power is 80 kW

Why not a propeller to power it ?




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y jet p opu s o d d ot succeed o t e g ou d

Thrust is proportional to the air mass flow and toV

propeller will be large for a smallV propeller will be small for a substantial V

Propeller 1expels air at 200 m/s, provides a Vof 160 m/s and has an

average velocity of 120 m/s

Propeller 2expels air at 50 m/s, provides a Vof 10 m/s and has an

average velocity of 45 m/s

Mass flow ofpropeller 2= 16 mass flow ofpropeller 1

Propeller 2must be more than 6 times biggerfor the same thrust

Propeller 1leaves in the air an energy propotional to (160 m/s)2

which is dissipated behind the vehicle




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y j p p g

Propeller 1consumes 2.667 more fuel than propeller 2

Propeller 2consumes 1.125 more than a direct wheel drive

Propeller 2is too big

If ajet engineis used : Ve 600 m/s the necessary mass flow and engine size would be small

the fuel consumption would be 8 times the standard one

If arocket engineis used withVe = 40 m/s the propulsive efficiency is

unity but a mass flow of 50 kg/s is required

If arocket engineis used withVe = 2000 m/s only 1 kg/s is required but

the propulsive efficiency is 0.04


Conclusions


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Propulsion is always performed by action and reactionon dry land, on the water or in the air

Action on a fixed support is always preferable as far as energy is concerned

Action on a moving support is done by augmenting the

momentum of the support

The moving support consumes a certain amount of energy

Minimizing losses implies maximizing the mass flow and

minimizing the momentum increase

Working rangeof the different configurations is limited in terms of altitude

and speed


Flight envelope


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g p


Rolls Royce Trent


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Pratt & Whitney JT9D


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CFM 56


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General Electric GE 90


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Leduc 021-1


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Leduc 021-2


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Leduc 022


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Daedalus


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Airbreathing engines : piston engines


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The pionneers : Lorin


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The pionneers : Whittle


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The pionneers : Jumo


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Airbreathing engines : Evolution


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October 2008 Slide 57/61

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TOC


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Aircraft Design : Propulsion Systems : Contents

Action - Reaction

Propulsion Airbreathing engines : historical perspectives

Airbreathing engines : the pulse jet engine

Airbreathing engines : the ram jet engine


Airbreathing engines : synthesis

Thrust production process

Airbreathing engines : performance

Braytons cycle efficiency


Global efficiency


Integration issues


Conclusions


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conception aero turbo machine

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