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8/9/2019 Propulsion & Thermodynamics
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PROPULSIONIntro
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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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