Modern Engineering Thermodynamics

Problems 359
85. Create a specific availability vs. temperature curve for saturated
liquid and saturated vapor water. Neglect any kinetic or
potential energy effects.
86. Using the data and situation described in Example 10.6, plot
the specific flow availability vs. (a) water inlet temperature,
(b) water velocity, and (c) height above the ground. Use this
information to create a three-dimensional surface with
availability on the vertical axis and water velocity and height
as the other two coordinates. For each part of this problem,
assume all the variables except those under consideration are
constant at their values given in Example 10.6.
87. Example 10.9 contains a situation involving a large steam
turbine. Using the data provided there and assuming the
values of all the remaining variables are constant, plot the
turbine’s heat loss as a function of
a. The steam flow rate.
b. The surface temperature of the turbine.
c. The ground state temperature (note that changing T 0
changes the a f values).
88. The second law efficiency of heating a liquid in a closed
container is evaluated in Example 10.10. Using the results
obtained there, plot the ratio of second law availability
efficiency to the first law energy efficiency (ε/η T ) vs. ΔT for a
variety of T and T 0 values. Comment on the general trends of
these curves.
Writing to Learn Problems
Provide a coherent 500-word written response to the following questions
on 8½ by 11 in. paper, double spaced, with 1-inch margins on
all sides. Unless your instructor indicates otherwise, your response
should include the following items:
a. An opening thesis statement containing the argument
you wish to support.
b. A body of supporting material.
c. A conclusion section in which you use the supporting
material to substantiate your thesis statement.
89. Availability is the name given to the amount of energy within a
system that can produce useful work. Describe in your own
words what constitutes “useful work” and provide at least
three representative examples.
90. Is the definition of what constitutes useful work a cultural
variable (e.g., could work considered useful in one culture not
be considered useful in another)? Provide specific arguments
and at least three examples to support your contention.
91. If the numerical value of availability represents the amount of
energy within a system that can be converted into useful work,
does this value depend on existing energy conversion
technology accessible for use with the system? Provide specific
arguments and at least three examples to support your
contention.
92. Useful work is associated with the potential of a conservative
force. Provide a definition of potential using only words and
arguments that your (hypothetical) nine-year-old sister would
understand. Provide at least three examples she could relate to.
93. The concept of a conservative force is central to defining useful
work. Describe the differences between conservative and
nonconservative forces using only words and arguments a
second-year college music major would understand. Provide
examples of at least three forces of each type.
94. In the text, it is stated that electrical energy is more available
to do useful work through a rotating shaft of an 90.0%
efficient electrical motor than is an equivalent amount of fuel
chemical energy used to power the rotating shaft of an internal
combustion engine with an energy conversion efficiency of
20.0%. However, if the chemical fuel is supplied to a fuel cell
that converts it directly into electrical energy with a 90.0%
efficiency, would the chemical energy of the fuel now have a
higher availability than an equivalent amount of electrical
energy?
95. Write a letter to your (hypothetical) ten-year-old brother in
which you describe in words he would understand the concept
of a local environment. Be sure to distinguish it from the
complete surroundings, and provide at least three physical
examples he would be able to understand.
Create and Solve Problems
Engineering education tends to focus on the process of solving problems.
It ignores teaching the process of formulating solvable problems.
However, working engineers are never given a well-phrased
problem statement to solve. Instead, they need to react to situational
information and organize it into a structure that can then be
solved using the methods learned in college.
These “Create and Solve” problems are designed to help you learn
how to formulate solvable thermodynamics problems from engineering
data. Since you provide the numerical values for some of
the variables, these problems do not have unique solutions. Their
solutions depend on the assumptions you need to make and how
you set them up to create a solvable problem.
96. You have been hired as a thermal engineer at a company that
manufactures domestic cookware appliances. Your first job is
to analyze a new design for a pressure cooker. It has a volume
of 0.15 ft 3 and initially contains 2.50 lbm of a mixture of
liquid plus vapor water at 14.7 psia. When the pressure cooker
is heated electrically, its internal pressure reaches 35.0 psia. To
understand the design, you need to know the heat transfer
during the process, the irreversibility of the process if the inner
surface of the pressure cooker is constant at 250.°F, and the
change in total availability of the water if the local
environment is at 14.7 psia and 70.0°F. Write and solve a
thermodynamics problem that answers these questions.
97. As the resident thermal engineer at a new company developing
inventive new ideas, you are faced with a scenario where 1.00
lbm of saturated water vapor at 212°F is condensed inside a
flexible balloon to saturated liquid at 212°F. This occurs in a
constant pressure process by a heat transfer from the balloon
to the environment. The balloon has an average surface
temperature during this process of 125°F. Your job is to
determine the irreversibility and the change in total availability
for this process if the local environment is at 14.7 psia and
70.0°F. Write and solve a thermodynamics problem that
provides the answers to these questions.
98.* It is now 1923, and you are working for Thomas Edison in
his New Jersey research laboratory. The surface temperature of
his 50.0 W incandescent lightbulb is 60.0°C. But the surface