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Unit +earning (utc(!es
Deter!ine the rates (f heat c(nducti(n and !ass transfer $ diusi(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 eui)!ent.
*(nduct e8)eri!ent ana+$7e and inter)ret data (f 6uid !echanics
T()ic 1; Heat transfer $ c(nducti(n
T()ic 2; Heat transfer $ c(n,ecti(n T()ic 3; Heat transfer $ radiati(n
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 eui)!ent.
Mass transfer as a trans)(rt )r(cess in the deter!inati(n (f diusi(n c(e4cients. The re+ati(nshi) eteen heat and !ass transfer is a+s( high+ighted.
=ncr()era > Deitt 2013. 'rinci)+es (f heat and !ass transfer ?th ed.%. J(hn @i+e$ > S(ns Singa)(re 'te Ltd.
=ncr()era > Deitt 2013. of heat and mass transfer .
(7th ed.). Massachusetts: John Wiley & Sons. The content of these two books books is the same.
Test 10A% La(rat(r$ 5A%
/ina+ E8a!inati(n ?0A%
T (ta+ assess! assess!ent; ent; 100A
Evolution of Process Engineering Disciplines
Material process engineering
Food process engineering
Other disciplines? Biochemical engineering
Fluid motion Flow patterns Solid mechanics
Transfer processes (Heat Mass Transfer! Transfer!
@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(
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&&
'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
Energy associated with microscopic behavior of matter
! or u
A means of indirectly assessing the amount of thermal energy stored in matter
Thermal energy transport due to temperature gradients
$ or $%&g
Amount of thermal energy transferred over a time interval t >
Thermal energy transfer per unit time
Thermal energy transfer per unit time and surface area
+ ! → Thermal energy of system u → Thermal energy per unit mass of system
Tut(ria+ gr(u) 5 and :. Ti!eta+e issue that aected n(r!a+ inta"e students.
Students are reuired t( f(++( (dd and e,en ee" tut(ria+ gr(u).
L( nu!er (f student in Tut(ria+ gr(u) 1 and 2. Tut(ria+ gr(u) 1 and 2 i++ e cance++ed.
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
!e)uires material medium
Transport does not Transport re)uire material
*riven by temperature difference
*(nducti(n; Transfer (f energ$
fr(! !(+ecu+e !(+ecu+e due t( ,irati(n (f !(+ecu+es. !(+ecu+es.
*(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.
@hat is a 6uid 6uidB B
& sustance hich underg(es c(ntinu(us def(r!ati(n hen def(r!ati(n hen suCected 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$
*(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 .
!(+ecu+es "ee) their re+ati,e )(siti(n t(
such an e8tent that bulk movement (r 6( is )r(hiited
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.
c(n,ecti(n;; c(n,ecti(n 6uid u+" !(ti(n due t( !echanica+ !eans
such as a fan )u!)
Energ$ is radiated fr(! a++ !ateria+s in the f(r! (f a,es hen this radiati(n is as(red $ !atter it a))ears as heat. (
necessar$ f(r necessar$
radiati(n t( (ccur radiati(n (r"s e,en in and thr(ugh a )erfect ,acuu! ,acuu!..
Conduction !" heat trans#er across $lanar sla% Fourier’s La
' $% x
$% x&, x&, x
∆" ∆ x )egative as T decrease with increase of *
lim , x
$ x - = ' = − k dx
" " ,"
(eat flux )W/m*+
" ( , x x
conductivity Thermal conductivity
-mlies directional directional .uantity heat flux normal normal to to lane of constant
$ x - = $ x = − k d" dx ' .f the temperature distribution is linear/
$ x - =
= − k
$ x - = k $ - = k x
" ( − " + )
" + − " ( ) ∆" )
Although & is a function of temperature/ it is normally assumed to be constant in narrow temperature range0
s ∞ or if T - T 1eendent on boundary layer roerties roerties
& s ' & ∞: eat trans#er #rom sur#ace to %ul
& ' & s: eat trans#er #rom %ul to sur#ace
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(>
(/ 6 (;/ '%m(> (
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
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$
$emit - = σ " s
/(r a gra$ surface; surface;
all radiation and radiation and reflects none i!e! a blac"body
$emit - = εσ " s
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
$r′′ad = ε , ( "s ) − α = εσ ( " s − "sur )
- = εσ 2" 9 − "
rad emit$ s *
asor * + emit$ sur
"or combined convection and radiation/
$total - = $con/ -+ $rad 9
$total = h2" s − " ∞ 4 + εσ 2" s − " sur surr r 4
Conservation of ,nerg#
The principal of conservation of energ" states that# 7lthough energ" assumes forms$ 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 +
Each term has units of ;<s or '&
The Surface Energ" Balance 48
- in − - out + - g =
d- st dt
reduced to 0
- in − - out = $c′′ond − $c′′on/ − $r′′ad = k
"+ − " (
− h ( "( − "∞ ) − ε (σ
− " 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
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
*ue to random motion "ouriers :aw
of constituent • ∆T as driving force •
Associated with bul& motion/ forced % free
as driving force
$ x - = k ∆" ) Bewtonss :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 ;