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ME 330 Fluid Mechanics
Spring 2007
Department of Engineering
Western Kentucky University
CHAPTER 5B HW  Homework Solutions
Problem #’s 514, 515C, 516C, 519, 521 and 524, Due March 2, 2007
514
Solution
The minimum fresh air requirements of a residential building is specified to
be 0.35 air changes per hour. The size of the fan that needs to be installed and the
diameter of the duct are to be determined.
Analysis
The volume of the building and the required minimum volume flow rate of
fresh air are
L/min
3150
=
=
=
=
×
=
=
=
L/h
189,000
h
/
m
189
)
/h
35
.
0
)(
m
540
(
ACH
m
540
)
m
m)(200
7
.
2
(
3
3
room
3
2
room
V
V
V
&
The volume flow rate of fresh air can be expressed as
)
4
/
(
2
D
V
VA
π
=
=
V
&
Solving for the diameter
D
and substituting,
m
0.106
=
=
=
m/s)
(6
)
/s
m
3600
/
189
(
4
4
3
V
D
V
&
Therefore, the diameter of the fresh air duct should be
at least 10.6 cm
if the velocity of
air is not to exceed 6 m/s.
Discussion
Fresh air requirements in buildings must be taken seriously to avoid health
problems.
515C
Solution
We are to discuss mechanical energy and how it differs from thermal
energy.
Analysis
Mechanical energy
is the
form of energy that can be converted to
mechanical work completely and directly by a mechanical device
such as a propeller
.
It differs from thermal energy in that
thermal energy cannot be converted to work
directly and completely
. The forms of mechanical energy of a fluid stream are kinetic,
potential, and flow energies.
Discussion
It would be nice if we could convert thermal energy completely into work.
However, this would violate the second law of thermodynamics.
House
200 m
2
0.35 ACH
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View Full DocumentME 330 Fluid Mechanics
Spring 2007
Department of Engineering
Western Kentucky University
516C
Solution
We are to define and discuss mechanical efficiency.
Analysis
Mechanical efficiency
is defined as
the ratio of the mechanical energy
output to the mechanical energy input
. A mechanical efficiency of 100% for a
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