978-0078027680 Chapter 19 Part 6

subject Type Homework Help
subject Pages 14
subject Words 1807
subject Authors John Cimbala, Robert Turner, Yunus Cengel

Unlock document.

This document is partially blurred.
Unlock all pages and 1 million more documents.
Get Access
page-pf1
page-pf2
19-102
Te [C]
Q
[W]
0.1
45.82
2495
0.2
45.45
2560
0.3
45.1
2622
0.4
44.77
2680
0.5
44.46
2735
0.6
44.16
2787
0.7
43.88
2836
0.8
43.61
2883
0.9
43.36
2928
1
43.12
2970
44
44.5
45
45.5
46
2600
2700
2800
2900
3000
Te [C]
Te
Q
page-pf3
19-103
19-104 The components of an electronic system located in a rectangular horizontal duct are cooled by forced air. The exit
temperature of the air and the highest component surface temperature are to be determined.
Assumptions 1 Steady flow conditions exist. 2 The inner surfaces of the duct are smooth. 3 The thermal resistance of the duct
is negligible. 4 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm. 6 Flow is fully developed in the
channel.
Properties We assume the bulk mean temperature for air to be 35C (based on the problem statement) since the mean
temperature of air at the inlet will rise somewhat as a result of heat gain through the duct whose surface is exposed to a
constant heat flux. The properties of air at 1 atm and this temperature are (Table A-22)
7268.0Pr
CJ/kg. 1007
C W/m.02625.0
kg/m 145.1
25-
3
p
c
k
Analysis (a) The mass flow rate of air and the exit
Air
27C
L = 1 m
Air duct
16 cm 16 cm
180 W
page-pf4
19-104
19-105 The components of an electronic system located in a circular horizontal duct are cooled by forced air. The exit
temperature of the air and the highest component surface temperature are to be determined.
Assumptions 1 Steady flow conditions exist. 2 The inner surfaces of the duct are smooth. 3 The thermal resistance of the duct
is negligible. 4 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm. 6 Flow is fully developed in the
channel.
Properties We assume the bulk mean temperature for air to be 35C (based on the problem statement) since the mean
temperature of air at the inlet will rise somewhat as a result of heat gain through the duct whose surface is exposed to a
constant heat flux. The properties of air at 1 atm and this temperature are (Table A-22)
7268.0Pr
CJ/kg. 1007
C W/m.02625.0
kg/m 145.1
25-
3
p
c
k
Analysis (a) The mass flow rate of air and the exit temperature are
C39.3
C)J/kg. kg/s)(1007 0124.0(
W)(0.85)(180
+C27)(
p
ieiep cm
Q
TTTTcmQ
m/s 0.613=m/min 8.36
/4m) (0.15
m/min 65.0
2
avg
c
A
V
V
and
C W/m.02625.0 2
k
Air
27C
0.65 m3/min
Electronics, 180 W
L = 1 m
D = 15 cm
page-pf5
19-105
19-106 Water flows through a concentric annulus tube with constant inner surface temperature and insulated outer surface,
the length of the annulus tube is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Properties are constant. 3 Constant inner tube surface temperature. 4
Insulated outer tube surface. 5 Fully developed flow.
page-pf6
19-106
Review Problems
19-107 The heat generated by four transistors mounted on a thin vertical plate is dissipated by air blown over the plate on
both surfaces. The temperature of the aluminum plate is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5105. 3 Radiation effects are
page-pf7
page-pf8
19-108
19-109E A minivan is traveling at 60 mph. The rate of heat transfer to the van is to be determined.
Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5105. 3 Radiation effects are
negligible. 4 Air flow is turbulent because of the intense vibrations involved. 5 Air is an ideal gas with constant properties. 5
The pressure of air is 1 atm.
Properties Assuming a film temperature of Tf = 80F, the properties of air are
evaluated to be (Table A-22E)
7290.0Pr
/sft 10697.1
FBtu/h.ft. 01481.0
24-
k
Analysis Air flows along 11 ft long side. The Reynolds number in this case is
6
ft) (11]ft/s )3600/528060[(
VL
Air
V = 60 mph
T = 90F
L = 11 ft
Minivan
page-pf9
page-pfa
page-pfb
page-pfc
page-pfd
19-113
19-114 Warm air blowing over the inner surface of a windshield is used for defrosting. The required inner convection heat
transfer coefficient to cause the accumulated ice to begin melting is to be determined.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer through the windshield is one-dimensional. 3 Thermal
page-pfe
page-pff
page-pf10
page-pf11
19-117
19-118 Air is flowing across a cylindrical pin fin that is attached to the hot surface. The maximum possible rate of heat
transfer from the pin fin is to be determined.
Pr = 0.7177.
Analysis The Reynolds number for the air flowing across the pin fin is
)m 005.0)(m/s 10(
VD
5/4
8/5
4/13/2
3/12/1
cyl 282000
Re
1
]Pr)/4.0(1[
PrRe62.0
3.0Nu
k
hD
2506
1
)7177.0()2506(62.0
3.0
K W/m02881.0
5/4
8/5
4/13/2
3/12/1
.
h
page-pf12
page-pf13
page-pf14

Trusted by Thousands of
Students

Here are what students say about us.

Copyright ©2022 All rights reserved. | CoursePaper is not sponsored or endorsed by any college or university.