intro propulsion lect 8

7/29/2019 Intro Propulsion Lect 8

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1

8


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In this lecture ...

First law of thermodynamics for closedsystems

Energy balance

Energy change for a system Energy transfer mechanisms

First law for a cycle

First law for a system undergoingchange of state

Perpetual motion machines of the firstkind

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay2

Lect-8


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Joules experiment

Joules experiment (1840-1849) toinvestigate the equivalence of heat andwork.

Prior to Joule, heat was considered to be ainvisible fluid known as caloric and flowsfrom a body of higher caloric to one with alower caloric.

Caloric theory of heat Joules experiment laid the foundation of

the first law of thermodynamics.


Lect-8


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Joules experiment


Lect-8

Pulley

Weight

Adiabaticvessel


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Joules experiment

Work, W1-2 done on the system can bemeasured by the fall of the weight.

The system temperature rises as work isdone on the system.

Let the insulation now be removed.

The system reaches its initial state by heattransfer across the system boundaries.

Therefore the work done is proportional tothe heat transfer.

The constant of proportionality is the Joulesequivalent.


Lect-8


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Joules experiment


Lect-8

x

y

2

1

W1-2Q2-1

The cycle consists of two processes, one anadiabatic work transfer followed by heat transfer

( ) ( ) == dQJdWorQJW cyclecycle


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First law of thermodynamics

Conservation of energy principle Energy can neither be created nor

destroyed, it can only be converted fromone form to another.

For all adiabatic processes between twospecified states of a closed system, thenet work done is the same regardless of

the nature of the closed system and thedetails of the process.


Lect-8


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Lect-8

PE = 12 kJKE = 0 kJ

PE = 8 kJKE = 4 kJ

m

m

z Energy cannot becreated or destroyed; itcan only change forms


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Lect-8

PotatoE = 10 kJ

Qin = 10 kJ

The increase in the energy of apotato in an oven is equal to the

amount of heat transferred to it.

E=Qnet=20 kJ

Qin = 25 kJ

Qout = 5 kJ

In the absence of any workinteractions, the energy change

of a system is equal to the netheat transfer.


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Lect-8

Battery

+_

Win=10 kJ

E=10 kJ

Adiabatic

The work done on an adiabatic system is

equal to the increase in energy of the system. Change in total energy during an adiabatic

process is equal to the net work done.

Win=10 kJE=10 kJ

Adiabatic


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Energy balance

The net change (increase or decrease) inthe total energy of a system during aprocess is equal to the differencebetween the total energy entering and

total energy leaving the system.


Lect-8

systemoutin EEE =

=

or,systemtheofenergy

totalin theChange

systemtheleaving

energyTotal

systemtheentering

energyTotal


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Energy change of a system

Energy change = Energy at the finalstate Energy at the initial state

In the absence of electrical, magnetic or

surface tension effects,

E = U + KE + PE

Thus, for stationary systems,

E = U


Lect-8


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Energy change of a system


Lect-8

Win=8 kJ

E= (Qin-Qout)+Win= (25-5) + 8= 28 kJ

Qin = 25 kJ

Qout = 5 kJ

The energy change of a system during a processis equal to the net work and heat transferbetween the system and its surroundings


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Energy transfer mechanisms

Energy can be transferred to or from asystem by three mechanisms

Heat

Work

Mass flow


Lect-8

systemmass,outmass,inoutinoutinoutin E) E(E-W(W)-Q(Q-EE =++= )


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Energy transfer mechanisms


Lect-8

kW)(/

asform,ratein theor,

kJ)(

energiesetc.potentialkineticinternal,inchangeofRatemassandworkheat,by

nsferenergy tranetofRate

energiesetc.potentialkineticinternal,inChange

massandworkheat,bynsferenergy traNet

dtdEE-E

E-EE

systemoutin

systemoutin

=

=

kJ)()/(and,, tdtdEEtWWtQQ ===

For constant rates, the total quantities during atime interval t are related to the quantities per unittime as


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For a closed system undergoing a cycle,the initial and final states are identical.

Therefore,Esystem= E2 - E1 = 0

The energy balance for a cycle simplifies to

Ein - Eout= 0 or Ein = Eout


Lect-8


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A closed system does not involve any massflow across its boundaries, the energybalance for a cycle can be expressed in

terms of heat and work interactions as


Lect-8

innetoutnetinnetoutnet QWQW ,,,, or

==

That is, the net work output during acycle is equal to net heat input


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Lect-8

Qnet = Wnet

P

V

For a cycle, E = 0, thus Qnet = Wnet


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In processes involving a change of state,heat and work interactions may be unknown.

It is a usual practice to assume the directionof heat and work interactions.

It is usually assumed that heat to betransferred into the system (heat input) inthe amount ofQ and work to be done by thesystem (work output) in the amount ofW.


Lect-8

First law for a system undergoing

a change of state


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First law for a system undergoinga change of state

The energy balance would be:

Obtaining a negative quantity for Q or Wsimply means that the assumed directionfor that quantity is wrong and should bereversed.


Lect-8

output.net worktheisand

inputheatnettheiswhere,

,

,

,,

inoutoutnet

outininnet

systemoutnetinnet

WWWW

QQQQ

EWQorEWQ

====

==


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First law for a system undergoinga change of state


Lect-8

EWQ =

SystemSystem

Surroundings

Q

Q3Q2

Q1

WW1

W2

W4

W3

( ) ( ) EWWWWQQQ =+++ 4321321


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First law for closed systems


Lect-8

dewq

ewqUWQ

EWQ

=

==

=

:formalDifferenti

:massunitPer:systemsStationary

:General


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First law: isolated system

An isolated system has no interactionbetween the system and its surroundings

For an isolated system, Q=0 and W=0

The first law givesdE=0

or, E=constant

The energy of an isolated system is thus,always a constant.


Lect-8


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First law : some observations

The first law cannot be provenmathematically, but no process in natureis known to have violated the first law.

First law of thermodynamics is afundamental physical law in itself.

From the first-law point of view, heat andwork are not different at all.

However heat and work are very differentfrom the second law point of view.


Lect-8


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Perpetual Motion Machine of theFirst Kind (PMM1)

Any device that violates first law is calleda perpetual-motion machine of the firstkind (PMM1).

Such a device will create energy!

Numerous ideas have been proposed overthe years, of devices that generate

energy in some way. These devices of course violate the first

law and hence were never demonstrated.


Lect-8


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Pump



Lect-8

Condenser

Turbine GeneratorPump

Boiler

Resistance heater

System boundaryoutnetW ,

outQ


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The device continuously produces energyat a rate of without receiving

any energy. This is a clear violation of the first law.

Converse of a PMM1: there can be nomachine which would continuouslyconsume work without some other formof energy appearing simultaneously.


Lect-8

outnetout WQ ,

+


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Recap of this lecture

First law of thermodynamics for closedsystems

Energy balance

Energy change for a system Energy transfer mechanisms


First law for a system undergoingchange of state

Perpetual Motion Machines of the FirstKind


Lect-8


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In the next lecture ...

First law of thermodynamics for opensystems/flow processes

Flow work and the energy of a flowing

fluid Total energy of a flowing fluid

Energy transport by mass

Energy analysis of steady-flow systems Some steady-flow engineering devices

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Lect-8

intro propulsion lect 8

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