Propulsion & Thermodynamics

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PROPULSIONIntro

Friday 28 August 2009


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Introduction

Newtons First Law

Every object continues to remain in its state i.eeither rest or uniform velocity unless a force !pull orpush"acts on it to change its state

This state is called inertia. !Inert: lacking the ability or strength tomove"

It works only in inertial reference frame



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So we need some force to

move the objectSo object/vehicle in order to move need some force!pull or push"

Object needs to force itself to accelerate and alsocounteract frictional forces/gravity/other forces thatoppose it

F= m.a = !dv/dt".m

a=dv/dt ... v => velocity it has a direction!vector"so doforce and we move forward by increasing the velocity inforward direction



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Now we know how to move an

object... we force( pull/ push)Now to move to an object called Aircraft

Di#erent ways of moving an aircraft !apoweredflying vehicle with fixed wingsand aweight greater than that of the air it displaces"

Say some 100people/passengers can flap their hands

Tie some birds to it and make them flap

Take it to the edge of the mountain and drop it !may fly/glide "

Or Use an Engine !a machine with moving parts that convertspowerinto motion"



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Propulsion system require a source ofpower and means of accelerating

airflowSources

Hydrocarbon fuels and a heat engine

Batteries and an Electric Motor

Solar Cells and an Electric Motor

Human powered ... Propeller by bicycle gearing!possible!!!!! ... check internet for record breakingsuch flight"



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Human- Powered, MIT

Daedalus



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So by two means we accelerate/moveforward throughpropellersandjet

ex ansion

Opposing forces for aircraft

Drag!pull (someone or something) along forcefully, roughly, or with difficulty"of the aircraft

Drag.... friction

We have to provide su$cient forceto the aircraft such

that it is greater than drag and also to sustain theweight of the a/c ... then it moves



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Thrust

It is the force that moves and aircraft through the airand is generated by the propulsion system.



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The PropellerFirst Generation a/c powered propellersand internal !fuel burning"orcombustion engines !I.C Engines"

I.C Engine

Take the air from surroundings !we have to move in air, so we use air"

Mixes it with fuel

Thereby releasing the energy in the fuel

Uses the heated gas tomov!the piston that is attached to the shaft

In Automobiles the shaft turns the wheels of the car

In Aircraft the shaft turns a propeller



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We got move move it...

Automobile

Piston%Shaft%Wheels and then whole car

Aircraft

Piston%Shaft%Propeller and then the plane !pushingthe air back"



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I.C Engine Fig.Intake stroke: Air and vaporized fuel are drawn in.

Compression stroke: Fuel vapor and air are compressed and ignited.

Combustion stroke: Fuel combusts and piston is pushed downwards.

Exhaust stroke: Exhaust is driven out.



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Thermodynamics

Thermal !of or relating to heat"... heat ... we generated byburning something

Dynamics!(of a process or system) characterized by constant change, activity, orprogress"

. moving the pistons/shafts



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PressureWe are working with air/gases and they exertpressur#, we need to know why

If our ears are more sensitive we hear a perpetual!never ending"noise

Why Noise %%%%the eardrum is in contact with air %%air is a lot of molecules in continuousmotion

So they keep banging eardrum !we dont hear because atoms are too small"

Then how do we hear?? The atoms on one side exert more pressure!the amount ofbombardment on one side is more"than the other side.

Silence%%%the force on both sides is same

We can feel the di#erence %%%%traveling faster cars/airplanes/Elevators ....

When we get cold .... similar e#ect... imbalance in pressure



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Atoms in a Box

Imagine a volume of gas in box, at end a piston that can move

We know atoms are moving with some velocity , so they will bebanging the piston

Suppose the other side is vacuum, nothing to oppose, every time

the atoms the piston it picks up momentum and starts moving

If we oppose the motion, say hold it, we need to apply a Force Fover the area of piston A

So Pressure..... F/A

FvGasVacuum



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Other way

Work done

The di#erential workdWdone on gas by compressing it by moving the pistonin small amountdxwill be force times the distance

dW=F!%dx"=P.A.!%dx"=PdV

%minus sign because we compressed it, the $olume decreased

Ideal Gas Law

PV=nRT



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First Law

Internal Energy

Heat

Work Done

Change in U = Q+W

First Law

Heat put into a System + Work done on a system= Increase inInternal Energy of the system

dQ+dW=dU



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Second Law

Actually discovered before first law

If we do work against friction....the work is lost andheat is produced.... work is converted to heat at roomtemperature.

What about reversibility? .. i.e. heat to workAccording to second law ...NO



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Convert heat to work by reversing a process likefriction... not possible

It is impossible to extract heat at a single temperature.

If the whole world is at same temperature we cannotconvert heat energy into work

Work can be converted to heat at same temperature

but not reverse.... %ith no other change in th#surroundings



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Example... Can of compressed gas

Heat flow only from higher temperature to lower temperature T2 to T1, say.

!S=!!!Q/T2"%!!Q/T1"

!

is always positive....

Reversible process Entropy is constant

irreversible always increasing

First Law: Energy is constant

Second Law: Entropy always increasing



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Jet Engines

Often referred to as GAS TURBINE ENGINE

Core Elements

Intake: Brings free stream air in to the engine

Compressor: Increases the pressure of the incoming air

Burner: Fuel with compressed is burned here

Turbine:resulting high%temperature exhaust gas is used to turnthe power

Nozzle: Flow oes throu h at hi her velocities



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Jet Engine & Brayton Cycle



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Inlet

Is Inlet........



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Turbine& Nozzle

Extracts the work through expansion

Turbine drives Compressor by the shaft



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Twin-Spool/shaft



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Turbofan



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Turbofans (in Black&

White)



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Turbo-prop



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again in B&W



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Ramjet

No moving Parts



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Propulsion & Thermodynamics

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