Basic

Concepts,

Work

&H

eatTransfer

Dr.

Md.

Zah

uru

lH

aq

Pro

fessorD

epartm

ent

ofM

echanica

lEngin

eering

Bangla

desh

University

ofEngin

eering

&Tech

nolo

gy

(BU

ET

)D

haka

-1000,Bangla

desh

http

://za

huru

l.buet.a

c.bd/

ME

6101:

Cla

ssicalT

herm

odynam

ics

http://zahurul.buet.ac.bd/ME6101/

©D

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1/

29

Overview

1Stead

y-State,

Stead

yFlow

(SSSF)

Pro

cesses

2Isen

tropic

(First

Law

)Effi

ciency

Nozzle

Diff

user

Turb

ine

Com

pressor&

Pum

p

SSSF

Pro

cess

©D

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asic

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Tra

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2/

29

Therm

odyn

amics

isa

funny

subject.

The

first

time

you

go

thro

ugh

it,yo

udon’t

understan

dit

atall.

The

second

time

you

go

thro

ugh

it,yo

uth

ink

you

understan

dit,

except

forone

ortw

osm

allpoin

ts.

The

third

time

you

go

thro

ugh

it,yo

uknow

you

don’t

understan

d

it,but

byth

attim

eyo

uare

soused

toit,

soit

doesn

’tboth

eryo

u

any

more.

Arn

old

Som

merfe

ld

©D

r.M

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asic

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rM

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29

Basic

Concepts

ofT

herm

odynam

ics

Therm

odynam

icSystem

������������

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���� �

T006•

Ath

ermodyn

amic

Syste

mis

simply

any

object,

quan

tityof

matter,

or

regionof

space

that

isselected

forth

ermodyn

amic

study.

Everyth

ing

that

isnot

part

ofth

esystem

isreferred

toas

the

Surro

undin

gs.

•B

oundary

orContro

lSurfa

ce(C

S)

separates

the

systemfrom

itssu

rroundin

gsw

hich

•m

aybe

realor

imag

inary,

atrest

orin

motio

n•

may

chan

ge

itssh

ape

and

size•

neith

erco

ntain

sm

atternor

occu

pies

volu

me

•has

zeroth

ickness

and

apro

perty

value

ata

poin

ton

the

boundary

is

shared

byboth

the

systeman

dits

surro

undin

gs.

©D

r.M

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nsfe

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29

Basic

Concepts

ofT

herm

odynam

ics

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Tra

nsfe

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5/

29

Basic

Concepts

ofT

herm

odynam

ics

T061

Asystem

defi

ned

toco

ntain

allofth

eair

ina

pisto

n-cylin

der

device.

T062

Asystem

defi

ned

toco

ntain

allofth

eair

that

isin

itiallyin

atan

kth

atis

bein

g

filled

.

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29

Basic

Concepts

ofT

herm

odynam

ics

Control

Mass

(CM

)or

Closed

System

T007

T1063

CS

Heat√

Work

√

Mass

×

CM

System

T744

•Contro

lM

ass

(CM

)or

Clo

sed

system:

CS

isclosed

tom

assflow

.

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29

Basic

Concepts

ofT

herm

odynam

ics

Control

Volum

e(C

V)

orO

pen

System

•M

asspasses

throu

ghCS

inContro

lVolu

me

(CV

)or

Open

system.

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Exam

ple

ofa

contro

lvo

lum

e(o

pen

system).

An

auto

mobile

engin

e.

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29

Basic

Concepts

ofT

herm

odynam

ics

Therm

odynam

icSystem

:Special

Cases

T079

Adiab

aticsystem

T1066

Isolated

system

•Adia

batic

system:

CS

isim

perm

eable

toheat.

•Iso

late

dsystem

:a

special

caseof

CM

systemth

atdoes

not

interact

inan

yway

with

itssu

rroundin

gs.

©D

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29

Basic

Concepts

ofT

herm

odynam

ics

State

&Prop

erty

•T

he

descrip

tionof

the

condition

ofa

systemat

agiven

instant

is

calledits

Sta

te.

•A

Pro

perty

isa

quan

tityw

hose

num

ericalvalu

edep

ends

onth

estate

but

not

onth

ehistory

ofth

esystem

.T

he

originof

properties

inclu

de

those

1directly

measu

rable

2defi

ned

bylaw

softh

ermodyn

amics

3defi

ned

bym

athem

aticalco

mbin

ations

ofoth

erpro

perties.

•Two

statesare

iden

ticalif,

and

only

if,th

eprop

ertiesof

thetw

ostates

areid

entical.

•In

tensiv

eprop

ertiesare

indep

enden

tof

the

sizeor

extent

ofth

e

system.Exte

nsiv

eprop

ertiesdep

end

onth

esize

orexten

tof

the

system.

An

extensive

property

isad

ditive

inth

esen

seth

atits

value

for

the

whole

systemis

the

sum

ofth

evalu

esfor

itsparts.

©D

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/29

Basic

Concepts

ofT

herm

odynam

ics

E2 ,V

2 ,T,P

E1 ,V

1 ,T,P

Esyste

m=

E1+

E2

Vsyste

m=

V1+

V2

Tsyste

m=

T1

=T

2

Psyste

m=

P1

=P

2

Exten

sive

Pro

perties

Inten

sive

Pro

perties

}}

System

boundary

T012

Pro

perty

Exten

siveIn

tensive

Mass

mρ

Volu

me

Vv

KE

12mV

212V

2

PE

mgZ

gZ

Total

Energ

yE

e

Intern

alEnerg

yU

u

Enth

alpyH

h

Entro

pyS

s

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/29

Basic

Concepts

ofT

herm

odynam

ics

Pro

cess&

Cycle

•Any

transform

ationof

asystem

fromon

eeq

uilibriu

mstate

toan

other

iscalled

Therm

odynam

icPro

cess.

•Path

ofa

process

isth

esu

ccessionof

statesth

rough

which

the

system

passes.

•A

systempro

cessis

saidto

goth

rough

aT

herm

odynam

icCycle

when

the

final

statean

dth

ein

itialstate

ofth

epro

cessare

same.

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asic

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&H

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/29

Basic

Concepts

ofT

herm

odynam

ics

Therm

odynam

icEquilibrium

Asystem

inT

herm

odynam

icEquilib

rium

satisfies

the

followin

gstrin

gent

requirem

ents:

1M

ech

anica

lEquilib

rium

:no

unbalan

ceforces

acting

onan

ypart

of

the

systemor

the

systemas

aw

hole.

2T

herm

alEquilib

rium

:no

temperatu

rediff

erences

betw

eenparts

of

the

systemor

betw

eenth

esystem

and

the

surrou

ndin

g.

3Chem

icalEquilib

rium

:no

chem

icalreaction

sw

ithin

the

systeman

d

no

motion

ofan

ych

emical

species

fromon

epart

toan

other

part

of

the

system.

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Basic

Concepts

ofT

herm

odynam

ics

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*+

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T1368

Two

systems

1an

d2

canexch

ange

diff

erent

kinds

ofco

nserved

physical

quan

titiesas

they

appro

acheq

uilibriu

m:

(a)system

sin

therm

al

contact

canexch

ange

energ

yth

rough

a

conductive

wall.

(b)

systems

that

share

a

com

mon

boundary

that

canbe

disp

lacedw

ill

exchan

ge

volu

me.

(c)system

ssep

aratedby

asem

i-perm

eable

mem

brane

canexch

ange

molecu

les(m

ass)ofa

particu

larsp

ecies.

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/29

Basic

Concepts

ofT

herm

odynam

ics

T077

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/29

Basic

Concepts

ofT

herm

odynam

ics

•A

systemis

saidto

be

inSta

ble

/Equilib

rium

Sta

tew

hen

no

finite

chan

geof

statecan

occu

runless

there

isan

interaction

betw

eenth

e

systeman

dits

environ

men

tw

hich

leavesa

finite

alterationin

the

state

ofth

een

vironm

ent.

•D

urin

ga

Quasi-sta

ticPro

cess,

the

systemis

atall

times

infinitesim

allynear

astate

ofth

ermodyn

amic

equilibriu

m;th

isim

plies

that

the

process

shou

ldbe

carriedou

tin

finitely

slowly

toallow

the

systemto

settleto

astab

lestate

atth

een

dof

eachin

finitesim

alstep

inth

epro

cess.

•T

heoretical

calculation

sm

ust

relateto

Stab

lestates,

sinceit

ison

ly

forth

esewe

have

therm

odyn

amic

data.

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Basic

Concepts

ofT

herm

odynam

ics

Categories

ofT

hermodynam

icsQ

uantities

1Sta

tefu

nctio

ns:

allprop

ertiesare

statefu

nction

s.

2Pro

cess

or

Path

functio

ns:

quan

titiesw

hose

values

dep

end

onth

e

path

ofth

epro

cess.

T038

∫21dy=

y2−y1=

∆y

⇒∮dy=

0

State

functio

n

∫2

1δZ

≡Z

126=

∆Z

T013

Path

functio

n

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/29

Work

&H

eat

Tra

nsfe

r

Work

inM

echanics

��

��

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T1366•

The

product,

W=

Fs

offorce

Fan

ddisp

lacemen

ts

isdefi

ned

asth

e

work

don

eby

the

forceF

and

against

the

force−F

.

•W

ork

,is

defi

ned

asth

escalar

product

offorce

and

disp

lacemen

t:

δW

=F·ds

→W

=

∫F·d

s

W=

F·s

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/29

Work

&H

eat

Tra

nsfe

r

Acceleration

&G

ravitationalW

ork

•Acce

lera

tion

work

(Wacc )

isdon

eon

systemto

chan

geits

velocity.

⇒W

acc=

∫t2t1m

d~V

dt·(~Vdt)

=∫~V2

~V1m~V·d~V

⇒W

acc,1

2=

mV

222

−mV

212

=∆KE

•mV

2/2≡

translation

alkin

eticen

ergy,K

E.

•G

ravita

tionalwork

(Wgrav )

isth

ework

don

eagain

stgravity

to

chan

geth

eelevation

ofa

system.

⇒W

grav=

∫t2t1−m~g·~Vdt=

∫z2

z1mgdz

⇒W

grav,1

2=

mgz2−mgz1=

∆PE

•mgz≡

gravitational

poten

tialen

ergy,PE.

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asic

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/29

Work

&H

eat

Tra

nsfe

r

Som

eform

sof

work

andtheir

rateequations

T1067

©D

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ET

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asic

Concepts,

Work

&H

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/29

Work

&H

eat

Tra

nsfe

r

Som

eform

sof

work

andtheir

rateequations

···

T1068

©D

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asic

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Work

&H

eat

Tra

nsfe

rTherm

odynam

icW

ork&

Heat

Work

isperform

edby

asystem

on

itssu

rroundin

gsdurin

ga

process

ifth

eon

ly

effect

external

toth

esystem

could

be

the

raising

ofa

weigh

t.

Heat

isen

ergyin

transition

fromon

ebody

orsystem

toan

other

solelybecau

se

ofa

temperatu

rediff

erence

betw

eenth

esystem

s.

•T

he

magn

itudes

ofheat

and

work

dep

end

onth

earb

itraryselection

of

bou

ndaries

betw

eenin

teracting

systems.

These

arenot

properties,

and

itis

improp

erto

speak

ofheat

orwork

’contain

ed’in

asystem

.

•H

eatan

dwork

transfers

areth

eonly

mech

anism

sby

which

energy

can

be

transferred

acrossth

ebou

ndary

ofa

closedsystem

.

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asic

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/29

Work

&H

eat

Tra

nsfe

r

T010

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asic

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&H

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/29

Work

&H

eat

Tra

nsfe

r

Com

parisonof

Heat

&W

ork

T023

An

example

ofth

ediff

erence

betw

eenheat

and

work.

•H

eatan

dwork

areboth

transien

tphen

omen

a.System

snever

possess

heat

orwork.

•Both

heat

and

work

arebou

ndary

phen

omen

a.Both

areob

servedon

ly

atth

ebou

ndary

ofth

esystem

,an

dboth

represent

energy

crossing

the

bou

ndary.

•Both

heat

and

work

arepath

function

san

din

exactdiff

erentials.

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asic

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/29

Work

&H

eat

Tra

nsfe

r

An

energ

ytran

sfercan

be

heat

orwork,

dep

endin

gon

systemselectio

n.

����������� �

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�����������

��

T081

Energ

ygen

eratedin

the

heatin

gelem

ent

istran

sferreddue

totem

peratu

re

diff

erence

betw

eenth

eheatin

gelem

ent

and

the

airin

side

oven

heat

interactio

n

����������� �

�������������

�����������

��

T080

Energ

ytran

sferis

not

caused

by

temperatu

rediff

erence

betw

eenth

eoven

and

the

surro

undin

gs,

rather

iscau

sedby

electrons

crossin

gboundary

work

interactio

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Work

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Tra

nsfe

rWork

&H

eat:Sign-C

onvention

T004

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Work

&H

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nsfe

r

Macroscopic

work

T1364

Pressu

reon

apisto

n(a)

and

the

macro

scopic

pisto

ndisp

lacemen

t(b

)

Energy

transfer

aswork

refersto

energy

transfer

(acrossa

bou

ndary)

associated

with

microscop

icm

otions

which

areob

servable

macroscop

ically.

The

observab

ilityof

the

motion

mean

sth

atth

em

otions

ofin

dividual

particles

have

some

overallcoh

erence.

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Work

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nsfe

r

Energy

Transfer

asH

eat

T1365

Energy

transfer

asheat

refersto

that

energy

transfer,

acrossa

system

bou

ndary,

associated

with

microscop

icdisp

lacemen

tsth

atare

not

observab

lem

acroscopically.

Heat

transfer

results

inm

icroscopic

vibrations

ofth

ewall

atoms,

but

not

inm

acroscopically

observab

lem

otion,so

we

cannot

compute

itas

aforce

times

anob

servable

disp

lacement.

©D

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Work

&H

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Tra

nsfe

rT

herm

odynam

icW

ork

General

Work

Expressions

•T

herm

odynam

icwork

isgen

eralizedto

inclu

de

allform

sof

work,

and

agen

eralizedforce

Fk

and

generalized

disp

lacemen

tdδk

canbe

iden

tified

:

Wk=

∫Fkdδk

•A

general

systemcou

ldhave

man

ypossib

lework

modes,

soa

general

work

expressionis:

W=

Ws+W

b+W

f+···

=∑

Wk

•W

s≡

shaft

work:

rotary

usefu

lwork

•W

b≡

boundary

work:

due

toexp

ansio

n/co

mpressio

nofsystem

•W

f≡

flow

work:

interactio

nreq

uired

forth

em

assto

cross

CS

•W

I≡

electricalwork

•···

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Work

&H

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Tra

nsfe

rT

herm

odynam

icW

ork

Flow

Work

���������

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�����

�

T1360

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�����

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��

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T1361

Flo

wwork

,W

fis

associated

with

mass

crossing

the

CS,an

drepresen

ts

the

work

that

must

be

don

eby

fluid

outsid

eth

econ

trolvolu

me

topush

m

acrossth

econ

trolvolu

me

bou

ndary.

•W

f,i=

−(PAi )L

i:

asdisp

lacemen

tagain

stPAi

•W

f,e=

+(PAe )L

e:

asdisp

lacemen

talon

gPAe

⇒W

f=

Wf,i+W

f,e=

−P(V

i−Ve )

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)B

asic

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Work

&H

eat

Tra

nsfe

rT

herm

odynam

icW

ork

Moving

System

Boundary

Work

��

��

��

T1362•

Force

acting

onan

elemen

tdA

ofCS

=PdA

•W

orkperform

edas

dA

recedes

bya

distan

cedx

=(P

dA)dx

•W

orkdon

e=

δW

b=

∫∫A

PdAdx=

P∫∫A

dAdx=

PdV

δW

b=

PdV⇛

Wb=

∫21PdV

dV

isth

ech

ange

involu

me

represented

byth

evolu

me

enclosed

betw

een

the

full

and

the

brokencu

rvesin

figu

re.©

Dr.

Md.

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(BU

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Work

&H

eat

Tra

nsfe

rT

herm

odynam

icW

ork

Exp

ansio

n&

Com

pression

Work

T064

−→

δW

=Fdx

=PAdx

=PdV

⇒W

12

=

∫PdV

T011

©D

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Work

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Tra

nsfe

rT

herm

odynam

icW

ork

�

� �

�

�

�

���

��

��

���

T1363

Wi→

f=

Wif

=32PoVo

Wia+W

af

=2PoVo+

0=

2PoVo

Wib+W

bf

=0+PoVo=

PoVo

Work

don

eby

asystem

dep

ends

not

only

onth

ein

itialan

dfinal

statesbut

alsoon

the

interm

ediate

states⇒path

functio

n.

©D

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rT

herm

odynam

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T1373

Adiab

aticpro

cess

T1371

T1372

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Work

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nsfe

rT

herm

odynam

icW

ork

T066

P=

f(V

):not

specifi

ed.

T067

P=

Patm

W12

=

∫21PdV

=Patm(V

2−V

1 )

©D

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Work

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rT

herm

odynam

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ork

Pro

cessin

volvin

ga

chan

ge

invo

lum

ew

ithW

=0

T022•

Work

canon

lybe

iden

tified

atth

esystem

bou

ndary.

•If

gas&

vacuum

space

iscon

sidered

assystem

:no

work

isdon

eas

no

work

canbe

iden

tified

atth

esystem

bou

ndary.

•If

the

gasis

the

system:

there

isa

chan

gein

volum

e,but

no

resistance

atth

esystem

bou

ndary

asth

evolu

me

increases,

and

sono

work

is

don

ein

the

process

offillin

gth

evacu

um

.

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Zero

’thLaw

ofT

herm

odynam

ics

Zero’th

Law

ofT

hermodynam

ics

Zero’th

Law

ofT

herm

odyn

amics

Two

systems

with

therm

aleq

uilibriu

mw

itha

third

arein

therm

al

equilibriu

mw

itheach

other.

A

BC

Therm

alEquilib

irum

Therm

alEquilib

irum

0thLaw

T745

T002

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Mass

Conse

rvatio

nLaw

Mass

Continuity

Equation

T444

T443

⇒m

cv(t)

+m

i=

mcv(t

+∆t)

+m

e

⇒m

cv(t

+∆t)

−m

cv(t)

=m

i−m

e

⇒m

cv(t+

∆t)−m

cv(t)

∆t

=m

i −m

e

∆t

•if∆t→

0:⇛

dm

cv

dt

=m

i−m

e

dm

cv

dt

=∑

im

i−∑

em

e

m=

ρAV=

AVv

(for1D

flow

)

©D

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Mass

Conse

rvatio

nLaw

Mass

Balance:

Transient

Flow

dm

cv

dt

=∑

i

mi−∑

e

me

⇛dm

cv=

dm

i−dm

e

⇒∫t0

(

dm

cv

dt

)

dt=

∫t0( ∑

im

i )dt−∫t0( ∑

em

e )dt

⇒∆m

cv=

mcv(t)

−m

cv(0)=

∑i

(∫t0m

i dt)

−∑

e

(

∫t0m

e dt)

∆m

cv=

mcv(t)

−m

cv(0)=

∑

i

mi−∑

e

me

©D

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asic

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Mass

Conse

rvatio

nLaw

Moran

Ex.

4.1

:⊲

Feed

-water

heater

atstead

y-state.D

etermin

em

2&

V2 .

Assu

me,

v2≃

vf (T

2 ).

T125

•dm

cv

dt

=∑

i mi−∑

em

e

⇒dm

cv/dt=

0

⇒∑

im

i=

m1+m

2

⇒∑

em

e=

m3

•m

=ρAV

⇒m

3=

ρ3 (A

V)3

⇒ρ

2=

ρ(T

=T

2 ,P=

P2 )

ρ3=

ρ(x

=0.0

,P=

P3 )

⇒m

2=

14.1

5kg/s,

V2=

5.7

m/s⊳

.

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Mass

Conse

rvatio

nLaw

Ahyp

otheticalexp

eriment

T1370

©D

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asic

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