heat and mass transfer; example problems
TRANSCRIPT
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PROBLEM 1.5
KNOWN: Inner and outer surface temperatures of a glass window of prescribed dimensions.
FIND: Heat loss through window.
SCHEMATIC:
ASSUMPTIONS: (1) One-dimensional conduction in the x-direction, (2) Steady-state
conditions, (3) Constant properties.
ANALYSIS: Subject to the foregoing conditions the heat flux may be computed from
Fouriers law, Eq. 1.2.
( )
T Tq k
L 15-5 CWq 1.4
m K 0.005m
q 2800 W/m .
1 2x
x
2x
=
=
=
Since the heat flux is uniform over the surface, the heat loss (rate) is
q = qx A
q = 2800 W / m2 3m2
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PROBLEM 1.7
KNOWN: Dimensions of freezer compartment. Inner and outer surface temperatures.
FIND: Thickness of styrofoam insulation needed to maintain heat load below prescribed
value.
SCHEMATIC:
ASSUMPTIONS: (1) Perfectly insulated bottom, (2) One-dimensional conduction through 5
walls of area A = 4m2, (3) Steady-state conditions, (4) Constant properties.
ANALYSIS: Using Fouriers law, Eq. 1.2, the heat rate is
q = q A = kT
LA
total
Solving for L and recognizing that Atotal= 5W2, find
L =5 k T W
q
2
( ) ( )5 0.03 W/m K 35 - -10 C 4mL =
500 W
2
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PROBLEM 1.10
KNOWN: Thickness, diameter and inner surface temperature of bottom of pan used to boil
ater. Rate of heat transfer to the pan.wF
IND: Outer surface temperature of pan for an aluminum and a copper bottom.
SCHEMATIC:
A
SSUMPTIONS: (1) One-dimensional, steady-state conduction through bottom of pan.
ANALYSIS: From Fouriers law, the rate of heat transfer by conduction through the bottomof the pan is
T T1 2q kAL
=
Hence,
qLT T1 2kA
= +
where ( )22 2A D / 4 0.2m / 4 0.0314 m . = = =
Aluminum:( )
( )600W 0.005 m
T 110 C 110.40 C12240 W/m K 0.0314 m
= + =
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PROBLEM 1.18
K
NOWN: Chip width and maximum allowable temperature. Coolant conditions.
F
IND: Maximum allowable chip power for air and liquid coolants.
SCHEMATIC:
ASSUMPTIONS: (1) Steady-state conditions, (2) Negligible heat transfer from sides and
bottom, (3) Chip is at a uniform temperature (isothermal), (4) Negligible heat transfer by
adiation in air.rANALYSIS: All of the electrical power dissipated in the chip is transferred by convection to
he coolant. Hence,t
P = q
and from Newtons law of cooling,
P = hA(T - T) = h W2(T - T).
n air,I
Pmax= 200 W/m2K(0.005 m)
2(85 - 15) C = 0.35 W.
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PROBLEM 1.27
KNOWN: Hot plate suspended in vacuum and surroundings temperature. Mass, specific heat, area
and time rate of change of plate temperature.
FIND:(a) The emissivity of the plate, and (b) The rate at which radiation is emitted from the plate.
SCHEMATIC:
Ts
stE
qrad
qrad
T = 25sur C
Ts = 225C
Plate, 0.3 m 0.3 m
M = 3.75 kg, cp = 2770
J kg K
T(t)
tt0
dT K= -0.022sdt
Ts
stE
qrad
qrad
Ts
stE
qrad
qrad
T = 25sur C
Ts = 225C
Plate, 0.3 m 0.3 m
M = 3.75 kg, cp = 2770
J kg K
T(t)
tt0
dT K= -0.022sdt
T(t)
tt0
dT K= -0.022sdt
ASSUMPTIONS: (1) Plate is isothermal and at uniform temperature, (2) Large surroundings, (3)
Negligible heat loss through suspension wires.
ANALYSIS: For a control volume about the plate, the conservation of energy requirement is
(1)in out stE - E = E
where st pdT
E = Mcdt
(2)
and for large surroundings (3)4 4
in out sur sE - E = A(T - T )
Combining Eqns. (1) through (3) yields
p
4 4sur s
dTMc dt
=A (T - T )
Noting that Tsur= 25 C + 273 K = 298 K and Ts = 225 C + 273 K = 498 K, we find
-8 4 4 4
2 4
J K3.75 kg 2770 (-0.022 )
kg K s= = 0.42
W2 0.3 m 0.3 m 5.67 10 (498 - 298 ) K
m K
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PROBLEM 1.28
KNOWN: Length, diameter, surface temperature and emissivity of steam line. Temperature
and convection coefficient associated with ambient air. Efficiency and fuel cost for gas fired
urnace.fF
IND: (a) Rate of heat loss, (b) Annual cost of heat loss.
SCHEMATIC:
= 0.8= 0.8
ASSUMPTIONS: (1) Steam line operates continuously throughout year, (2) Net radiationransfer is between small surface (steam line) and large enclosure (plant walls).t
ANALYSIS: (a) From Eqs. (1.3a) and (1.7), the heat loss is
( ) ( )4 4conv rad s s sur q q q A h T T T T = + = +
where ( ) 2A DL 0.1m 25m 7.85m .= = =
H
ence,
( ) ( )2 2 8 2 4 4 4q 7.85m 10 W/m K 150 25 K 0.8 5.67 10 W/m K 423 298 K = + 4
( ) ( )2 2q 7.85m 1,250 1,095 W/m 9813 8592 W 18,405 W= + = + =
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PROBLEM 1.59
KNOWN: Thermal conductivity, thickness and temperature difference across a sheet of rigid
extruded insulation. Cold wall temperature, surroundings temperature, ambient temperature and
emissivity.
FIND:(a) The value of the convection heat transfer coefficient on the cold wall side in units of
W/m2C or W/m2K, and, (b) The cold wall surface temperature for emissivities over the range
0.05 0.95 for a hot wall temperature of T1= 30 C.
L=20 mmT1=30CT2=20C=293 K
x
qcond qrad
qconvT
sur=320 K
T=5C
Air
k=0.029
W
m K
L=20 mmT1=30CT2=20C=293 K
x
qcond qrad
qconvT
sur=320 K
T=5C
Air
k=0.029
W
m K
SCHEMATIC:
ASSUMPTIONS: (1) One-dimensional conduction in the x-direction, (2) Steady-state
conditions, (c) Constant properties, (4) Large surroundings.
ANALYSIS:
(a) An energy balance on the control surface shown in the schematic yields
orin outE = E
cond conv rad q = q + q
Substituting from Fouriers law, Newtons law of cooling, and Eq. 1.7 yields
4 41 22 2
T - Tk = h(T - T ) + (T - T )
L sur (1)
or 4 41 2 2 sur 2
T - T1h = [k - (T - T )]
(T - T ) L
Substituting values,o
-8 4 4 4
o 2
1 W (30 - 20) C Wh = [0.029 - 0.95 5.67 10 (293 - 320 ) K ]
m K 0.02 m(20 - 5) C m K 4
h = 12.22
W
m K