Heat and Mass Transfer

 

UEME 3213 Heat 1

and Lecture 1Mass Transfer  Jan 2015 Dr. Mah Shee Keat

 

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Teaching stas 

Dr. Mah Shee Keat !ahs"#utar.edu.!

%$ 

&ssistant 'r(fess(r De)art!ent (f *he!ica+ Engineering



S& Le,e+ -



Mr. *h(ng K(" *hung ch(ng"c#utar.edu.!

%$

Lecturer



 

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De art!ent (f *he!ica+ En ineerin

La(rat(r$ de!(nstrat(rs 

Ms. Tan L Lee ee /an tan+f# tan+f#utar utar.edu. .edu.!$% !

%$

Ms. Tan in ing tan$#utar.edu.!

%$La(rat(r$ attendance is c(!)u+s(r$ '+ease register $(ur +a(rat(r$ sessi(ns ith +a (4cer.

 

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*(!!unicati(n channe+ 

You will be made aware of the date of all TESTS, assignments, etc at the start of the unit usually through the WBLE system

 

5

Late su!issi(n (f assign!ent and etc 

Your unit coordinator/lecturer may give you an extension to submit your assignment/reort etc! Submission of your wor" after the due date will attract a enalty of #$% er day for a maximum of & days after which the wor" will no longer be considered and you will attract a 'ero mar" for that submission!

 

Unit +earning (utc(!es

: 



Deter!ine the rates (f heat c(nducti(n and !ass transfer $ diusi(n c(n,ecti(n and radiati(n in si!)+e ge(!etries *a+cu+ate the heat and !ass transfer c(e4cients in 6(ing s$ste!s



&na+$7e the )erf(r!ance (f heat e8changers air9c(nditi(ning and (ther heat9transfer eui)!ent.



*(nduct e8)eri!ent ana+$7e and inter)ret data (f 6uid !echanics

 

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T()ics 

T()ic 1; Heat transfer $ c(nducti(n



T()ic 2; Heat transfer $ c(n,ecti(n T()ic 3; Heat transfer $ radiati(n

 

T()ic -; S)eci<c heat 6( )r(+e!s in se+ected !ateria+s s$ste! 

T()ic 5; Mass transfer

 

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S$n()sis; 

This unit c(,ers heat transfer $ c(nducti(n c(n,ecti( c(n,ecti(n n design and radiati(n their a))+icati(n in the (f heatand transfer eui)!ent.



Mass transfer as a trans)(rt )r(cess in the deter!inati(n (f diusi(n c(e4cients. The re+ati(nshi) eteen heat and !ass transfer is a+s( high+ighted.

 

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Main reference; 

=ncr()era > Deitt 2013. 'rinci)+es (f heat and !ass transfer ?th ed.%. J(hn @i+e$ > S(ns Singa)(re 'te Ltd.



=ncr()era > Deitt 2013. of heat and mass transfer .

Fundamentals

(7th ed.). Massachusetts: John Wiley & Sons.  The content of these two books books is the same.

 

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&ssess!ent Su!!ar$ 

&ssign!ent 15A%



Test 10A% La(rat(r$ 5A%

 

/ina+ E8a!inati(n ?0A%

T (ta+ assess! assess!ent; ent; 100A

 

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Evolution of Process Engineering Disciplines

Material process engineering

Food process engineering

Other  disciplines? Biochemical engineering

 Biological  science

Fluid motion Flow patterns Solid mechanics

 Material  science

Food science

Chemical engineering

Chemical  kinetics

Transfer processes (Heat  Mass Transfer! Transfer!

Mechanical engineering

 

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=ntr(ducti(n

@hat is Heat TransferB Heat transfer transiti(n (f

is the ther!a+

energ$ (r si!)+$ heat fr(! a h(tter (Cect t( a c((+er (Cect .. s(

that

the

(d$

and

the

surr(undings reach

ther!a+ eui+iriu!.

 

13

What is thermal energy? energy? Thermal energ"# associated with the translation$$ rotation$ rotation$ vi%ration  vi%ration  and translation electronic states  states  of the atoms and molecules that comprise matter matter&&

 

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M(des (f heat transfer

 

E8a!)+es (f Heat Transfer 15



Ther!a+ design )r(+e!s  =nsu+ati(n (f ui+dings in e8tre!e c+i!ates 

Ther!a+ shie+ding (n the s)ace shutt+e

 

E8a!)+es (f Heat Transfer 16



Ther!a+ c(ntr(+ 

Maintaining the ()ti!u! te!)erature in )r(cesses



De,e+()!ent (f faster c(!)uter )r(cess(rs +i!ited $ the inai+it$ t( dissi)ate heat

 

E8a!)+es (f Heat Transfer 17



Design (f heat e8changers 

&ut(!(i+e radiat(r



(i+ers and c(ndensers in che!ica+ )+ants

 

&))+icati(n (f Heat and Mass Transfer in /((d Engineering

18



Design (f heat e8changer 

(i+ers and c(ndensers in f((d )r(cessing )+ants

Source: www.tetrapak.com

 

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&))+icati(n (f Heat and Mass Transfer in En,ir(n!enta+ Engineering  Design (f fer!enter 

i(!ass )(er )+ant

Source: (left) http://www.tohoku-epco.co.jp (right) http://siemens.com  

E8a!)+e (f Heat and Mass Transfer in Fenea+e Engineering

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

Design (f s(+ar energ$ )+ant  S(+ar su)erheater and stea! generat(r

Source: (left) http://green-la.com (right) http://www.nexteraenergyresources.com  

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&))+icati(n (f Heat and Mass Transfer in i(che!ica+ Engineering 

Ther!a+ design )r(+e!s 

Ther!a+ and !ass circu+ati(n in i(react(r f(r +arge sca+e !(n(c+(na+ anti(dies )r(ducti(n

Source: (left) www.leinco.com, (right)

http://sachemdisplacementchromatography.typepad.com  

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'h" is heat and mass transfer important  Almost all the industries industries involve heat and mass transfer operations.  Heat (and mass) transfer can sometimes e coupled !ithin one unit operation. "nderstanding these processes can save energ#$ resources and %%%.

 

DG GT c(nfuse (r interchange the !eanings (f Ther!a+   Ther!a+ Quantity Meaning Symbol Units Energ$ Te!)erature Energ$ Te!)erature and  and Heat Transfer

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Thermal Energy+

Energy associated with microscopic  behavior of matter 

!  or u

Temperature

A means of indirectly assessing the amount of thermal energy stored in matter 

" 

Heat Transfer 

Thermal energy transport due to temperature gradients

$ or $%&g

 

 

Amount of thermal energy transferred over a time interval  t > 

#

$

Heat !ate

Thermal energy transfer per unit time

$

'

Heat "lu#

Thermal energy transfer per unit time and surface area

$ ′′

'%m(

Heat



+ !  →  Thermal energy of system u →   Thermal energy per unit mass of system

 

2-

&nn(unce!ent 

Tut(ria+ gr(u) 5 and :. Ti!eta+e issue that aected n(r!a+ inta"e students.



Students are reuired t( f(++( (dd and e,en ee" tut(ria+ gr(u).

 

2 5

&nn(unce!ent 

L( nu!er (f student in Tut(ria+ gr(u) 1 and 2. Tut(ria+ gr(u) 1 and 2 i++ e cance++ed.

 

26

 

2 ?

&nn(unce!ent 

L( nu!er (f student a+s( (ser,ed in Tut(ria+ gr(u) 3 and -. The nu!ers (f student are 1: and 1- res)ecti,e+$.

 

Modes of Heat Transfer

28

•

!e)uires material medium

•

Transport does not Transport re)uire material

*riven by temperature difference

 

medium

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*(nducti(n; Transfer (f energ$

fr(! !(+ecu+e !(+ecu+e due t( ,irati(n (f !(+ecu+es. !(+ecu+es.

t(

*(nducti(n is the transfer (f heat $ direct c(ntact (f )artic+es (f !atter. !atter. *(nducti(n is )articu+ar+$ i!)(rtant ith !eta+s.

 

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@hat is a 6uid 6uidB B

& sustance hich underg(es c(ntinu(us def(r!ati(n hen def(r!ati(n hen suCected t( a shear stress I shearing f(rce.

Def(r!ati(nB  *hange in the re+ati,e )(siti(ns (f )arts (f a (d$

 

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*(n,ecti(n; Transfer (f energ$ due t( u+" !(,e!ent (f 6uid. 6uid. *(n,ecti(n is the transfer (f heat $ !(,e!ent (f the heated 6uid. 6uid. The faster   the heat faster 6uid transfer. !(ti(n. the greater !(ti(n the c(n,ecti(n the  c(n,ecti(n transfer *(n,ecti(n ecause .

d(es

n(t

(ccur

in

s(+id

 

32

!(+ecu+es "ee) their re+ati,e )(siti(n t(



such an e8tent that bulk movement   (r 6( is )r(hiited

 

33

2 t$)es (f *(n,ecti(n; 

atura+ c(n,ecti(n; c(n,ecti(n; due t( u+" !(ti(n (f 6uid fr(! high t( +(er te!)erature regi(n.

 /(rced

c(n,ecti(n;; c(n,ecti(n 6uid u+" !(ti(n due t( !echanica+ !eans

such as a fan )u!)  

34

Fadiati(n;

Energ$ is radiated fr(! a++ !ateria+s in the f(r! (f a,es hen this radiati(n is as(red $ !atter it a))ears as heat. (

!ediu!

is

necessar$  f(r necessar$ 

radiati(n t( (ccur radiati(n (r"s e,en in and thr(ugh a )erfect ,acuu! ,acuu!..

 

35

Conduction !" heat trans#er across $lanar sla% Fourier’s La

$ x

 ' $% x

 '

$% x&,  x&, x



∝−

∆"  ∆ x )egative as T decrease with increase of *

lim , x  

(

, x

 

$x

d" 

$ x - =  '   = − k  dx

"  "  ," 

(eat flux )W/m*+

" ( , x  x

 x(  x

conductivity  Thermal conductivity 

-mlies directional directional   .uantity heat flux normal normal to  to lane of constant

)W/m+  

36

$ x - =  $  x = − k  d"  dx  ' .f the temperature distribution is linear/  

$ x - =

$x    '

= − k 

$ x - = k  $ - = k   x

" ( − " +  )

" + − " (  ) ∆"   )

temerature

 

37

Conduction

Although & is a function of temperature/ it is normally assumed to be constant in narrow temperature range0

1roperty Tables 2.ncropera 3 *e'itt45 Solids5 Tables Tables A0 6 A07 8ases5 Table A09 :i)uids5 Tables Tables A0; 6 A0<

 

Convection 38

Develop when there is fluid flow over a surface

Develop if the fluid free stream and surface temperatures differ

! *

" *+temperature of the fluid sufficientl" far from the surface

(ydrodynamic boundary layer  Thermal boundary layer  " *

$

 y

"  s

 x

eton’s La o# 'ooling

$- = h( "    − "   s

∞

) 

if TS + T,

0onvection heat transfer  coefficient )W/m*+

$- = h  ( "  − "  )

 s ∞ or  if T  - T 1eendent on boundary layer  roerties  roerties

& s ' & ∞: eat trans#er #rom sur#ace to %ul

S

,

&  ' & s: eat trans#er #rom %ul to sur#ace  

39

Convection Typical values of convection heat transfer coefficient "ree convection 2air4

; 6 = '%m(> 

"orced convection 2air4

(; 6 7 '%m(> 

"orced co convection 2w 2water4

( 6 / '% '%m(> 

?oiling water

(/ 6 (;/ '%m(>  (

@ondensing steam

9/ 6 / '%m > 

 

E*tra notes a%out convection 40

.& Con Convec vection tion invo involves lves the com%ine com%ined d ef effec fects ts of of conduction  conduction  motion&& and %ul/  fluid motion 0& Thus$ in the a%sence of an" %ul/ fluid motion$ motion $ heat transfer %etween a solid surface and the ad1acent fluid is %" pure conduction& 2& Con Consid sider er the the co coolin oling g of a hot %loc/ %loc/ %" %low %lowing ing ccool ool ai airr over its top surface& Energ" is transferred to the air la"er ad1acent to the %loc/ %" conduction& 3& Thi Thiss energ" energ" iiss th then en carrie carried d awa" awa" from from the the surfac surfacee %" convection$ convection& either %" forced convection or natural

convection&  

41

E*tra notes a%out convection 4& Fluid is forced to flow flow over  over the surface  forced convection Fluid motion is caused %" %uo"anc" forces   natural convection 5& How However ever$$ iiff )6 e*ter e*ternal nal mean meanss (to forc forcee the fflow low!! 7)D the temperature difference %etween the 0 %odies is not large enough to overcome the resistance of air to movement heat transfer %etween the %odies will %e carried out %" conduction&

 

Radiation 42 "  sur 

$ "  s

8et exchange between exchange between surroundings   blac"body and its surroundings )infinite enclosure+

 '

Ste#an*+olt,mann La 9  s

$ =  σ   '" 

Stefan2Bolt'mann constant )&!34 x #$25 W/m*6+  7ssumes body absorbs

9 $ ∝ σ   '   ("  s9  − "  sur  )

 7ssumes all radiation leaving one surface will reach the other surface  1 ;

/(r a +ac"(d$ +ac"(d$

$emit  - =  σ  "  s

9

/(r a gra$ surface; surface;

all radiation and radiation and reflects none i!e! a blac"body

$emit -  =  εσ  "  s

9  

8 emissivit"

 

43

Special case of surface e#posed to large surroundings of surroundings of uniform temperature/ "  sur 

.f α = ε / the net radiation heat flu# from the surface due to e#change with the surroundings is5 9

9

$r′′ad  = ε , ( "s ) − α   = εσ ( "  s −  "sur )    

$

- = εσ      2"  9 − " 

9

4

rad  emit$ s * 

 s

asor * + emit$ sur 

 

44

"or combined convection and radiation/

$total - = $con/ -+ $rad 9

9

 sur r 

$total  = h2"  s − " ∞ 4  + εσ  2"  s − "  sur  surr  r  4  

45

 Conservation of ,nerg#

 

46

The principal of conservation of energ" states that# 7lthough energ" assumes forms$ the total 9uantit" of energ"man" is constant$ and when energ" disappears in one form$ it appears simultaneousl" in other forms& (The First :aw of Thermod"namics!

 

Conservation of Energ" for a Control =olume 47

-t an instant (t) the rate o# increase o# energy stored in the control /olume must e0ual the e0ual the rate at minus  the hich energy enter the control /olume minus  $lus   rate at hich energy lea/e the control /olume $lus the rate at hich energy is generated ithin the control /olume.

 - in − - out   +   -  g  = ∆-  st  0

0 in −

0 out +

 g 

=

d-  st 

0

≡

 st 

 - 

 - 

 -  

dt 

- 

Each term has units of ;<s or '&  

The Surface Energ" Balance 48

0

0

0

 - in −  - out +  -  g   =

d-  st  dt 

0

≡

-  st 

reduced to 0

0

 - in −  -    out  =  $c′′ond − $c′′on/ − $r′′ad   =  k

"+ − " (  

  ("

− h ( "( − "∞ ) − ε (σ  

9 (

9

)

− " sur  = 

 

Method for Solving Heat Transfer

49 • • •

State concisel" State  concisel" what is /nown

Problems State what State  what is to %e solved Draw a Draw  a schematic#  –

>dentif" control surface<volume

 –

>dentif" relevant heat transfer tran sfer processes

•

:ist appropriate :ist  appropriate assumptions

•

7nal"sis#

•

 –

7ppl" relevant conservation laws

 –

'rite down rate e9uations

 –  –

Develop anal"sis and solution techni9ue Su%stitute numerical values

Discussion of results#  –  –

Summarise /e" conclusions Criti9ue original assumptions

 –

>nfer trends %" carr"ing out a sensitivit" anal"sis on the parameters

 

 -ummar# 50

  •

@onduction

*ue to random motion "ouriers :aw

of constituent •   ∆T as driving force   •

@onvection

Associated with bul& motion/ forced % free

•   ∆T

!adiation

 

•

as driving force

$ x - = k  ∆"   )  Bewtonss :aw  Bewton

$- = h( "    s − " ∞ )

Emitted due to shift of "or gray surface electronic state of constituents 9 $emit -  =  εσ  "  s • transmitted by electromagnetic wave % photon 9 9 $   "  "  2 = εσ     −  propagation 2no rad   s  surr   sur r  4 medium4 ;

;

 

51

 eferencee  eferenc

.ncropera 3 *e'itt/ @hapter 

materials rials  eading mate

.ncropera 3 *e'itt/ @hapter  and @hapter (