module 2: worked-out problems
TRANSCRIPT
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MODULE 2: Worked-out Problems
Problem 1:The steady-state temperature distribution in a onedimensional wall of thermal conductivity
50W/m.K and thickness 50 mm is observed to be T(0
C)= a+bx2
, where a=2000
C, B=-20000
c/m2, and x in meters.
(a) What is the heat generation rate in the wall?
(b) Determine the heat fluxes at the two wall faces. In what manner are these heat fluxes
related to the heat generation rate?
Known: Temperature distribution in a one dimensional wall with prescribed thickness andthermal conductivity.
Find: (a) the heat generation rate, q in the wall, (b) heat fluxes at the wall faces and relationto q.
Schematic:
Assumptions: (1) steady-state conditions, (2) one dimensional heat flow, (3) constant
properties.
Analysis:(a) the appropriate form of heat equation for steady state, one dimensionalcondition with constant properties is
3520.
2
.
.
m/W100.2K.m/W50)m/CC2000(2q
bk2]bx2[dx
dk
)bxa(dx
d
dx
dkq
dx
dT
dx
dKq
==
==
+=
=
(b) The heat fluxes at the wall faces can be evaluated from Fouriers law,
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x
''x
dx
dTk)x(q =
Using the temperature distribution T(x) to evaluate the gradient, find
dx
dk)x(q
''x = [a+bx
2]= -2kbx.
The flux at the face, is then x=0
2x
''
20''x
x''
m/W000,10)L(q
m050.0)m/C2000(K.m/W502kbL2)l(q,LatX
0)0(q
=
===
=
Comments: from an overall energy balance on the wall, it follows that
0EEE g.
out
.
in
.
=+ 0Lq)L(q)0(q.
x''''
x =+
352"
x"x
.
m/W100.2m050.0
0m/w00,10
L
)0(q)L(qq =
=
=
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Problem 2:A salt gradient solar pond is a shallow body of water that consists of three distinct fluid layers
and is used to collect solar energy. The upper- and lower most layers are well mixed and
serve to maintain the upper and lower surfaces of the central layer at uniform temperature T1
and T2, where T1>T2. Although there is bulk fluid motion in the mixed layers, there is no such
motion in the central layer. Consider conditions for which solar radiation absorption in thecentral layer provides non uniform heat generation of the form q=Ae-ax, and the temperature
distribution in the central layer is
cbxeka
A)x(T ax
2++=
The quantities A (W/m3), a (1/m), B (K/m) and C(K) are known constants having the
prescribed units, and k is the thermal conductivity, which is also constant.
(a)Obtain expressions for the rate at which heat is transferred per unit area from thelower mixed layer to the central layer and from central layer to the upper mixed layer.
(b)Determine whether conditions are steady or transient.(c)Obtain an expression for the rate at which thermal energy is generated in the entirecentral layer, per unit surface area.
Known: Temperature distribution and distribution of heat generation in central layer ofa solar pond.
Find: (a) heat fluxes at lower and upper surfaces of the central layer, (b) whetherconditions are steady or transient (c) rate of thermal energy generation for the entire
central layer.
Schematic:
Assumptions: (1) central layer is stagnant, (2) one-dimensional conduction, (3)constantproperties.
Analysis (1) the desired fluxes correspond to conduction fluxes in the central layer at the
lower and upper surfaces. A general form for the conduction flux is
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+==
+==
=
=
=
+
Bka
AkqqBe
ka
Akqq
Hence
Beka
Akq
"
)0x(cond
"
u
al"
0Lx(cond
"
l
ax"cond
(b) Conditions are steady ifT/t=0. Applying the heat equation,
t
T1
k
q
t
T.
2
2
=+
t
1e
k
Ae
k
A axax
=+
Hence conditions are steady since
tT
=0 (for all 0
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Problem 3:The steady state temperatures distribution in a one-dimensional wall of thermal conductivity
and thickness L is of the form T=ax3+bx2+cx+d. derive expressions for the heat generation
rate per unit volume in the wall and heat fluxes at the two wall faces(x=0, L).
Known: steady-state temperature distribution in one-dimensional wall of thermalconductivity, T(x)=Ax3+Bx2+CX+d.
Find: expressions for the heat generation rate in the wall and the heat fluxes at the two wallfaces(x=0, L).
Assumptions: (1) steady state conditions, (2) one-dimensional heat flow, (3) homogeneousmedium.
Analysis:the appropriate form of the heat diffusion equation for these conditions is
0kq
dxTd
.
2
2
=+ Or2
2.
dxTdkq =
Hence, the generation rate is
]B2Ax6[kq
]0CBx2Ax3[dx
dk
dx
dT
dx
dq
.
2.
+=
+++=
=
which is linear with the coordinate x. The heat fluxes at the wall faces can be evaluated fromFouriers law,
]CBx2Ax3[kdx
dTkq 2"x ++==
Using the expression for the temperature gradient derived above. Hence, the heat fluxes are:
Surface x=0; (0)=-kC"xq
Surface x=L;"xq (L) = -K [3AL
2+2BL+C]
COMMENTS: (1) from an over all energy balance on the wall, find
BkL2AkL3E
0E]CBL2AL3)[K()kC()L(q)0(q
0EEE
2''
g
.
g
.2
x"
x"
g
.
out
.
in
.
=
=+++=
=+
From integration of the volumetric heat rate, we can also find
BkL2AkL3E
]Bx2Ax3[kdx]B2Ax6[kdx)x(qE
2g
''.
L
0
L0
2.
L0
''
g
.
=
+=+==
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Problem 4:The one dimensional system of mass M with constant properties and no internal heat
generation shown in fig are initially at a uniform temperature Ti. The electrical heater is
suddenly energized providing a uniform heat flux qo at the surface x=0. the boundaries at
x=L and else where are perfectly insulated.
(a)Write the differential equation and identify the boundary and initial conditions thatcould be used to determine the temperature as a function of position and time in the
system.
(b)On T-x coordinates, sketch the temperature distributions for the initial condition(t
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Boundary conditions: x=0 0t)x/Tkq 0"o >=
x=L 0)x/T L = Insulated
(b) The temperature distributions are as follows:
No steady-state condition will be reached since is constant.in..
outin
.
EandEE
(c) The heat flux as a function of time for positions x=0, L/2 and L appears as:
( d) If the heater is energized until t=to and then switched off, the system will eventually reach
a uniform temperature , Tf. Perform an energy balance on the system, for an interval of time
t=te,
.
stin
.
EE = es"os
"0
t0inin tAqdtAqQE
e === )TT(McE ifst =
It follows that = OR es"o tAq )TT(Mc if += if TT
Mc
tAq es"o
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Problem 5:
A 1m-long steel plate (k=50W/m.K) is well insulated on its sides, while the
top surface is at 100
0
C and the bottom surface is convectively cooled by a fluidat 200C. Under steady state conditions with no generation, a thermocouple at the
midpoint of the plate reveals a temperature of 850C. What is the value of the
convection heat transfer coefficient at the bottom surface?
Known: length, surfacethermal conditions, and thermal conductivity of aPlate. Plate midpoint temperature.
Find:surface convection coefficient
Schematic:
Assumptions: (1) one-dimensional, steady conduction with no generation, (2)Constant properties
Analysis: for prescribed conditions, is constant. Hence,
2/L
TTq 21"cond
= = 20
m/W1500k.m/W50/m5.0
C15=
K.m/W30h
m/W1500
W/K.m)h/102.0(
C30
)h/1()k/L(
TT"q
2
2
2
01
=
=+
=+
=
Comments: The contributions of conduction and convection to the thermal
resistance are
W/K.m033.0
h
1R
W/K.m02.0K
LR
2cond,t
"
2cond,t
"
==
==
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Problem 6:
The wall of a building is a composite consisting of a 100-mm layer of common
brick, a 100-mm layer of glass fiber(paper faced. 28kg/m
2
), a 10-mm layer ofgypsum plaster (vermiculite), and a 6-mm layer of pine panel. If the inside
convection coefficient is 10W/m2.K and the outside convection coefficient is
70W/m2.K, what are the total resistance and the overall coefficient for heat
transfer?
Known: Material thickness in a composite wall consisting of brick, glass fiber,
and vermiculite and pine panel. Inner and outer convection coefficients.
Find: Total thermal resistance and overall heat transfer coefficient.
Schematic:
Kb Kgl
Brick Gypsumglass
Pine panel,Kp
100mm 10mm 6mm
hi=10W/m
2.
K
h0=70W/m2.K
0h
1
b
b
K
L
gl
lg
k
L
gy
gy
k
L
p
p
K
L
ih
1
Assumptions: (1) one dimensional conduction, (2) constant properties, (3)
negligible contact resistance.
Properties: T= 300K: Brick, kb=1.3 W/m.K: Glass fiber (28kg/m3), kg1=
0.038W/m.K: gypsum, kgy=0.17W/m.K: pine panel, kp=0.12W/m.K.
Analysis: considering a unit surface Area, the total thermal resistance
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W/K..m93.2R
W/K.m)1.00500.00588.06316.20769.00143.0(R
W
K.m
10
1
12.0
006.0
17.0
01.0
038.0
1.0
3.1
1.0
70
1R
h
1
K
L
k
L
k
L
K
L
h
1R
2"tot
2"tot
2"tot
ip
p
gy
gy
1g
1g
B
B
0
"tot
=
+++++=
+++++=
+++++=
The overall heat transfer coefficient is
.K.m/W341.0U
)W/K.m93.2(R
1
AR
1U
2
12
tot"
tot
=
===
Comments: an anticipated, the dominant contribution to the total resistance is
made by the insulation.
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Problem 7:
The composite wall of an oven consists of three materials, two of which are
known thermal conductivity, kA=20W/m.K and kC=50W/m.K, and known
thickness, LA=0.30m and LC=0.15m. The third material, B, which is sandwiched between materials A and C, is of known thickness, LB=0.15m, but unknown
thermal conductivity k
B
B. Under steady-state operating conditions, measurements
reveal an outer surface temperature of Ts,0=200C, an inner surface temperature
of Ts,i=600 C and an oven air temperature of T =800 C. The inside convection
coefficient h is known to be 25W/m .K. What is the value of k
0 0
2BB?
Known: Thickness of three material which form a composite wall and thermal
conductivities of two of the materials. Inner and outer surface temperatures of
the composites; also, temperature and convection coefficient associated with
adjoining gas.
Find: value of unknown thermal conductivity, kB.
Schematic:
KA KB
LA
T=8000C
h= 25W/m2.K
LB LC
KC
LA=0.3mLB=LC=0.15mkA=20W/m.KkC=50W/m.K
Ts,i=6000C
Ts,0=200C
Assumptions: (1) steady state conditions, (2) one-dimensional conduction, (3)
constant properties, (4) negligible contact resistance, (5) negligible radiation
effects.
Analysis: Referring to the thermal circuit, the heat flux may be expressed as
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2
B
B
0
C
C
B
B
A
A
0,si,s"
m/WK/15.0018.0
580
K.m/W50
m15.0
K
m15.0
018.0
m3.0
C)20600(
K
L
K
L
K
L
TTq
+=
++
=
++
=
The heat flux can be obtained from
2''
02i,s
"
m/W5000q
C)600800(K.m/W25)TT(hq
=
==
Substituting for heat flux,
.K.m/W53.1K
098.0018.05000
580018.0
q
580
K
15.0
B
"B
=
===
Comments:radiation effects are likely to have a significant influence on the
net heat flux at the inner surface of the oven.
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Problem 8:
A steam pipe of 0.12 m outside diameter is insulated with a 20-mm-thick layer
of calcium silicate. If the inner and outer surfaces of the insulation are at
temperatures of Ts,1=800K and Ts,2=490 K, respectively, what is the heat lossper unit length of the pipe?
Known: Thickness and surface temperature of calcium silicate insulation on
a steam pipe.
Find: heat loss per unit pipe length.
Schematic:
D2=0.16m
D1=0.12m
Steam
Ts,1=800K
Ts,2=490K
Calcium silicate insulation
Assumptions: (steady state conditions, (2) one-dimensional conduction, (3)constant properties.
Properties: calcium silicate (T=645K): k=0.089W/m.K
Analysis: The heat per unit length is
m/W603q
)m12.0/m16.0ln(
K)490800)(K.m/W089.0(2q
)D/Dln(
)TT(K2
q
qq
'r
'r
12
2,s1,s
L
r'r
=
=
==
Comments: heat transferred to the outer surface is dissipated to the surroundings
by convection and radiation.
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Problem 9:
A long cylindrical rod of 10 cm consists of a nuclear reacting material
(k=0.0W/m.K) generating 24,000W/m3
uniformly throughout its volume. This
rod is encapsulated within another cylinder having an outer radius of 20 cm anda thermal conductivity of 4W/m.K. the outer surface is surrounded by a fluid at
1000C, and the convection coefficient between the surface and the fluid is
20W/m2.K. Find the temperatures at the interface between the two cylinders and
at the outer surface.
Known: A cylindrical rod with heat generation is cladded with another cylinder
whose outer surface is subjected to a convection process.
Find: the temperature at the inner surfaces, T1, and at the outer surface, Tc.
Schematic:
Assumptions: (1) steady-state conditions, (2) one-dimensional radialconduction, (3), negligible contact resistance between the cylinders.
Analysis: The thermal circuit for the outer cylinder subjected to the
convection process is
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o
'2
2
1o'1
r2h
1R
k2
r/rlnR
=
=
Using the energy conservation requirement, on the inner cylinder,
g
.
out
.
EE =
Find that
21
.
1'
rqq =
The heat rate equation has the form hence,R/Tq '.
=
'''
2'1
'i R/Tandq)RR(qTT ==
Numerical values:
m/W0.754m)1.0(m/W000,24q
W/m.K0398.0m20.02K.m/W20/1R
W/m.K0276.0K.m/W42/1.0/2.0lnR
223'
2'2
'1
==
==
==
Hence
C13030100W/m.K0398.0m/W0.754C100T
C8.1508.50100W/m.K)cccc0276.0(m/W0.754C100T
00C
00i
=+=+=
=+=++=
Comments: knowledge of inner cylinder thermal conductivity is not
needed.
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Problem 10:
An electrical current of 700 A flows through a stainless steel cable having a
diameter of 5mm and an electrical resistance of 6*10-4
/m (i.e. perimeter of
cable length). The cable is in an environment having temperature of 300C, andthe total coefficient associated with convection and radiation between the cable
and the environment is approximately 25W/m2.K.
(a) If the cable is bar, what is its surface temperature?
(b) If a very thin coating of electrical insulation is applied to the cable, with a
contact resistance of 0.02m2K/W, what are the insulation and cable surface
temperatures?
(c) There is some concern about the ability of the insulation to withstand
elevated temperatures. What thickness of this insulation (k=0.5W/m.K) will
yields the lowest value of the maximum insulation temperature? What is the
value of the maximum temperature when the thickness is used?
Known: electric current flow, resistance, diameter and environmental
conditions associated with a cable.
Find: (a) surface temperature of bare cable, (b) cable surface and insulation
temperatures for a thin coating of insulation, (c) insulation thickness which provides the lowest value of the maximum insulation temperature.
Corresponding value of this temperature.
Schematic:
T
Ts
Eg
q
Assumptions: (1) steady-state conditions, (2) one-dimensional conduction in r,(3) constant properties.
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Analysis: (a) the rate at which heat is transferred to the surroundings is fixed by
the rate of heat generation in the cable. Performing an energy balance for a
control surface about the cable, it follows that or, for the bare cable,qEg.
=
.m/W294)m/106()A700(RIwithq).TT)(LD(hLRI 42'e2'
si'e
2 ====
It follows that
C7.778T
)m005.0()K.m/W25(
m/W294C30
Dh
qTT
0s
2
0
i
'
s
=
+=
+=
(b) With thin coating of insulation, there exists contact and convection
resistances to heat transfer from the cable. The heat transfer rate is
determined by heating within the cable, however, and therefore remains the
same,
h1R
)TT(Dq
LDh
1
LD
R
TT
LDh
1R
TTq
c,t
si'
ii
c,t
s
i
c,t
s
+
=
+
=
+
=
And solving for the surface temperature, find
C1153T
C30W
K.m04.0
W
K.m02.0
)m005.0(
m/W294T
h
1R
D
qT
0s
022
c,t
i
'
s
=
+
+
=+
+
=
The insulation temperature is then obtained from
e,t
s
R
TTq
=
Or
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C7.778T
)m005.0(
W
K.m02.0
m
W294
C1153LD
RqC1153qRTT
0i
2
0
i
c,t"
0c,tsi
=
=
==
(c) The maximum insulation temperature could be reduced by reducing the
resistance to heat transfer from the outer surface of the insulation. Such a
reduction is possible Di Di =0.005m. To minimize the maximum temperature, which
exists at the inner surface of the insulation, add insulation in the amount.
mt
mDDDDt icri
0175.0
2
)005.004.0(
22
0
=
=
=
=
The cable surface temperature may then be obtained from
( )
C318.2T
(0.005m)
W
.Km0.02
m
W294
C692.5LD
R
qTqRTT
/RTTq,thatgrecognizin
C692.5T
2.25m.K/W
C30T
0.32)m.K/W0.66(1.27
C30T
m
W294
hence,
(0.04m).Km
W25
1
)2(0.5W/.
005)ln(0.04/0.
(0.005m)
.K/W0.02m
C30T
h
1
2
)/Dln(D
D
R
TTq
0
i
2
0
i
"
ct,
sc,t,si
c,t,is
0
s
0
s
0
s
2
2
0
s
rc,
irc,
i
"
ct,
s'`
=
===
=
=
=
++
=
++
=
++
=
Comments: use of the critical insulation in lieu of a thin coating has the
effect of reducing the maximum insulation temperature from 778.70C to
318.20C. Use of the critical insulation thickness also reduces the cable
surface temperatures to 692.50C from 778.7
0C with no insulation or fro
11530C with a thin coating.
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Problem 11:
The steady state temperature distribution in a complete plane wall of three
different methods, each of constant thermal conductivity, is shown below.
(a)On the relative magnitudes of and of ."3"2andqq
"4
"3andqq
(b)Comment on the relative magnitudes of kA and kB and ok kB BB and kC.(c)Plot the heat flux as a function of x.
Known: Temperature distribution in a composite wall.
Find: (a) relative magnitudes of interfacial heat fluxes, (b) relative magnitudes
of thermal conductivities, and (c) heat fluxes as a function of distance x.
Schematic:
1 2 3 4A B C
Assumptions: (1) steady-state conditions, (2) one-dimensional conduction, (3)constant properties.
Analysis: (a) for the prescribed conditions (one-dimensional, steady state,
constant k), the parabolic temperature distribution in C implies the existence of
heat generation. Hence, since dT/dx increases with decreasing x, the heat flux inC increases with decreasing. Hence,
"
4
"
3 qq >
However, the linear temperatures distributions in A and B indicate no
generation, in which case
"
3
"
2 qq =
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(b) Since conservation of energy requires that and dT/dx/",3"
,3 CB qq = B
Similarly, since thatfollowsit,dT/dx)dT/dx)andqq BA"
B2,
"
A2, >=
AK < KB.
(d)It follows that the flux distribution appears as shown below.
"
xq
x2 x3O x
"
2q
Comments: Note that, with dT/dx)4,C=0, the interface at 4 is adiabatic.
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Problem 12:
When passing an electrical current I, a copper bus bar of rectangular cross
section (6mm*150mm) experiences uniform heat generation at a rate
. If the bar is in ambient air with h=5W/m.
222 A.m/W015.0where,alq == 2. K andits maximum temperature must not exceed that of the air by more than 300C,
what is the allowable current capacity for the bar?
Known: Energy generation, q (I), in a rectangular bus bar.
Find:maximum permissible current.
Schematic:
Assumptions: (1) one-dimensional conduction in x (W>>L), (2) steady-state conditions, (3)constant properties, (4) negligible radiation effects.
Properties: copper: k 400W/m. K
Analysis: the maximum mid plane temperature is
sT
K
qLT +=
2
.2
0
Or substituting the energy balance results,
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,/.
hLqTTs +=
AI
KmWKmW
mmAmW
CI
hkLL
TTI
hence
hk
LLI
hk
LLqTT
o
o
1826
./5
1
./800
003.0003.0)./(015.0
30
)/12/(015.0
,
.1
2015.0
1
2
max
2
1
2
23
0
max
2
1
2.
=
+
=
+
=
+=
+=