Chapter 1 Part3

8/18/2019 Chapter 1 Part3

1/33

1

Chapter 1: Introductionand Basic Concepts

Copyright © The McGraw-Hill Companies, Inc. Permission required or reproduction or display.


2/33

Objectives

When you finish studying this chapter, you should beable to:

Understand the basic mechanisms of heat transfer, which

are conduction, convection, and radiation, and Fourier'slaw of heat conduction, Newton's law of cooling, and theStefan–olt!mann law of radiation,

"dentify the mechanisms of heat transfer that occur

simultaneously in practice

#evelop an awareness of the cost associated with heatlosses, and Solve various heat transfer problems

encountered in practice$

2


3/33

HEAT AND ENERGY

DEFINITION OF HEAT AND ENERGY

Heat is the form of energy that can be

transferred from one system to another as aresult of temperature difference.

Energy is the capacity of a physical system to

perform work. The simplest definition of energyor work is that 1 Joule is the work done by a force

of 1 Newton acting over a distance of 1 meter.

3


4/33

Units Of Energy And Heat

%he international unit of energy is joule &( or kilojoule &)* + ) ($

"n the -nglish system, the unit of energy is the British thermal

unit &tu(, which is defined as the energy needed to raise thetemperature of ) lbm of water at ./F by )/F$

%he magnitudes of * and tu are almost identical

&) tu + )$000. *($

1nother well2*nown unit of energy is the calorie

&) cal + 3$)4.4 (, which is defined as the energy needed to

raise the temperature of ) gram of water at )3$0/5 by )/5$


5/33

TABLE 1

nits and Conversion Factors for Heat

Measurements

!I "nits #nglish "nits

Thermal #nergy $Q% & ' (.)*+*&-) tu

Heat Transer /ate $ % & '0s or & 1 2.)&32 tu0h

Heat 4lu5 $q% & 10m3 .2&*& tu0h t3


6/33

THERMODYNAMICS

AND HEAT TRANSFER

6ou may be wondering why we need to underta*e adetailed study on heat transfer$ 1fter all, we can determinethe amount of heat transfer for any system undergoing anyprocess using a thermodynamic analysis alone$

%he reason is that thermodynamics is concerned with theamount of heat transfer as a system undergoes a

process from one equilibrium state to another, and it

gives no indication about how long the process will

take$

1 thermodynamic analysis simply tells us how much heatmust be transferred to reali!e a specified change of state tosatisfy the conservation of energy principle$


7/33

"n practice we are more concerned about the rate of heattransfer &heat transfer per unit time( than we are with the amountof it$

For e7ample, we can determine the amount of heat transferredfrom a thermos bottle as the hot coffee inside cools from 8/5 to4/5 by a thermodynamic analysis alone$

ut a typical user or designer of a thermos is primarily interestedin how long it will be before the hot coffee inside cools to 4/5,and a thermodynamic analysis cannot answer this 9uestion$

#etermining the rates of heat transfer to or from a system andthus the times of cooling or heating, as well as the variation ofthe temperature, is the subect of heat transfer


8/33

We are normally interested in how long it

ta*es for the hot coffee in a thermos to coolto a certain temperature, which cannot bedetermined from a thermodynamic analysisalone


9/33

Application AreasOf Heat Transfer

;eat transfer is commonly encountered inengineering systems and other aspects of life

%he human body is constantly reecting heat to itssurroundings, and human comfort is closely tied tothe rate of this heat reection$ We try to control this

heat transfer rate by adusting our clothing to theenvironmental conditions$


10/33

Household appliances :electric or gas range, the heatingair2conditioning system, the refrigerator and free!er,

the water heater, the iron,and even the computer, the %<

Devices : car radiators, solar collectors,various components of power plants, andeven spacecraft


11/33

11


12/33

Heat Transfer

12

%;- 1S"5 =->U"=-?-N% F@= ;-1% %=1NSF-= "S%;- A=-S-N5- @F 1 %-?A-=1%U=- #"FF-=-N5-$

%;- S-5@N# B1W =->U"=-S %;1% ;-1%- %=1NSF-==-# "N %;- #"=-5%"@N @F#-5=-1S"NC %-?A-=1%U=-$

%;- %-?A-=1%U=- #"FF-=-N5- "S %;- #="


13/33

;eat %ransfer ?echanisms

;eat can be transferred in three basic modes:conduction,convection,radiation$

1ll modes of heattransfer re9uire the

e7istence of a temperature difference$

1ll modes are from the high2temperature medium to a

lower 2temperature one.

13


14/33

Conduction

5onduction is the transfer of energy from the moreenergetic particles of a substance to the adacent lessenergetic ones as a result of interactions between theparticles$

5onduction can ta*e place in solids,

li9uids, or gases "n gases and li9uids conduction is due to

the collisions and diffusion of the

molecules during their random motion$ "n solids conduction is due to the

combination of vibrations of the

molecules in a lattice and the energy

transport by free electrons.14


15/33

Conduction

15

where the constant o proportionality k is the

thermal conductivity o the material.

In dierential orm

which is called Fourier’s law of heat conduction.

(1-21)

(1-22)

& 3 $1%cond T T T

Q kA kA

x x

− ∆= = −

∆ ∆

&

( ) ( )6rea Temperature dierence/ate o heat conductionThic7ness

µ

$1%cond dT

Q kAdx

= −&


16/33


17/33

EX!"#E $ %he &ost of Heat #oss

through a 'oof

%he roof of an electrically heated home is . m long, 4 mwide, and $D0 m thic*, and is made of a flat layer ofconcrete whose thermal conductivity is * + $4 WEm /5$%he temperatures of the inner and the outer surfaces of

the roof one night are measured to be )0/5 and 3/5,respectively, for a period of ) hours$

#etermine &a( the rate of heat loss through the roof that

night and &b( the cost of that heat loss to the home

owner if the cost of electricity is G$4E*Wh.

22


18/33


19/33

1nalysis

&a( Noting that heat transfer through the roof is byconduction and the area of the roof is 1 + . m 7 4 m7 34 mD, the steady rate of heat transfer through theroof is determined to be

24


20/33

25

(b) The amount of heat lost through the roof during a

10-hour period and its cost are determined from

#iscussion

%he cost to the home owner of the heat loss through theroof that night was G)$H0$ %he total heating bill of thehouse will be much larger since the heat losses throughthe walls are not considered in these calculations$


21/33

Convection

Convection is the mode of energy transfer

between asolid surface and theadjacent liquid

or gas that is inmotion.Convection is commonly classified into three

sub-modes:Forced convection,

Natural(or free) convection,

Change of phase (liquid/vapor,

solid/liquid, etc.)26

Convection Conduction ! "dvection $luid motion%


22/33

%he rate of convection heat transfer is e7pressed by(ewton)s law of cooling as h is the convection heattransfer coefficient in *Em+&$

h depends on variables such as thesurface geometry, the nature of fluid

motion, the properties of the fluid,

and the bul* fluid velocity$

2#

$ % $1%conv s sQ hA T T ∞= −&


23/33

E-ample $ !easuring &onvection Heat%ransfer &oefficient

1 D2m2long, $H2cm2diameter electrical wire e7tendsacross a room at )0/5, as shown in Figure D$ ;eat isgenerated in the wire as a result of resistanceheating, and the surface temperature of the wire is

measured to be )0D/5 in steady operation$ 1lso, thevoltage drop and electric current through the wire aremeasured to be . < and )$0 1, respectively$#isregarding any heat transfer by radiation,

determine the convection heat transfer coefficient forheat transfer between the outer surface of the wireand the air in the room$

2$


24/33

.O#/%0O( %he convection heat transfer coefficient forheat transfer from an electrically heated wire to air is tobe determined by measuring temperatures when steady

operating conditions are reached and the electric powerconsumed$

ssumptions ) Steady operating conditions e7ist since

the temperature readings do not change with time$ D=adiation heat transfer is negligible$

nalysis When steady operating conditions are reached,

the rate of heat loss from the wire will e9ual the rate ofheat generation in the wire as a result of resistanceheating$ %hat is,

2%


25/33

3&

%he surface area of the wire is

NewtonIs law of cooling for convection heat transferis e7pressed as


26/33

Disregarding any heat transfer by radiation and thus

assuming all the heat loss from the wire to occur by

convection, the convection heat transfer coefficient is determined to be

31


27/33

=adiation

=adiation is the energy emitted by matter in the form ofelectromagnetic waves &or photons( as a result of thechanges in the electronic configurations of the atoms ormolecules$

;eat transfer by radiation does not re9uire the presence ofan intervening medium$

"n heat transfer studies we are interested in thermal radiation

&radiation emitted by bodies because of their temperature($

=adiation is a volumetric phenomenon$ ;owever, radiation isusually considered to be a surface phenomenon for solidsthat are opa9ue to thermal radiation$

32


28/33

33

%he ma7imum rate of radiation that can be emitted from a surface at athermodynamic temperature T s &in J or =( is given by the Stefan–

olt!mann law as

s +0$.KL)24 WEmDJ3 is the Stefan–Boltzmann constant.

%he ideali!ed surface that emits radiation at this ma7imum rate iscalled a blackbody1

%he radiation emitted by all real surfaces is less than the radiation

emitted by a blac*body at the same temperature, and is e7pressed as

e is the emissivity of the surface$

=adiation 2 -mission

),ma5 $1%emit s sQ A T σ =&

),ma5 $1%

- &

emit s sQ A T εσ

ε

=

≤ ≤

&


29/33

34


30/33

Example : Radiation Effect on ThermalComfort

"t is a common e7perience to feel Mchilly in winter andMwarm in summer in our homes even when the thermostatsetting is *ept the same$ %his is due to the so calledMradiation effect resulting from radiation heat e7change

between our bodies and the surrounding surfaces of thewalls and the ceiling$ %he inner surfaces of the walls,floors, and the ceiling of the house are observed to be atan average temperature of )/5 in winter and D0/5 insummer$

36


31/33

#etermine the rate of radiation heat transfer between thisperson and the surrounding surfaces if the e7posed surfacearea and the average outer surface temperature of theperson are )$3 mD and H/5, respectively

3#


32/33

.O#/%0O( %he rates of radiation heat transfer

between a person and the surrounding surfaces atspecified temperatures are to be determined in summer

and winter$

Assumptions

1 Steady operating conditions exist.

2 Heat transfer by convection is not considered$2 %he person is completely surrounded by the interior

surfaces of the room$

3 %he surrounding surfaces are at a uniform

temperature1

Properties The emissivity of a person is !."#

$table 1%&'3$


33/33

Documents

Chapter 1 Part3