Modern Engineering Thermodynamics

534 CHAPTER 13: Vapor and Gas Power Cycles
user’s electrical bill (like wind power devices today). If 100
million people exercised for 15 min every day of the year at a
power level of 100. W each, how many barrels of crude oil
(at 5.80 × 10 6 Btu per barrel) and how many tons of coal
(at 26.0 × 10 6 Btu per ton) could be saved annually?
Computer Problems
The following computer programming assignments are designed to
be carried out on any personal computer using a spreadsheet or
equation solver. They are meant to be exercises in the manipulation
of some of the basic formulae of this chapter. They may be used as
part of a weekly homework assignment.
81. Write a computer program or use an equation solver or
spreadsheet to carry out a parametric study of the isentropic
Rankine cycle with regeneration given in Example 13.6, part
b. Allow the regenerator pressure p 5s to vary from 1.00 to
200. psia (this simulates opening a valve in the regenerator
steam line from the turbine). Maintain the regenerator outlet
state as a saturated liquid (i.e., set h 6 = h f (p 5s )), because if
h 6 < h f (p 5s ), the regenerator is not operating as effectively
as possible; and if h 6 > h f (p 5s ), there will be vapor at the
entrance of the boiler feed pump and its performance will be
seriously degraded.
a. Determine the values of p 5s and mass fraction y that
maximize the cycle efficiency.
b. Produce a plot of cycle efficiency η T and mass fraction y on the
y-axis vs. regenerator pressure p 5s on the x-axis. As part of this
solution, note that, when p 5s is equal to the turbine inlet
pressure (200. psia) or the condenser inlet pressure (1.00 psia),
the regenerator has no effect and the cycle efficiency is the same
as without regeneration (i.e., 29.7%).
82. Repeat Problem 78 using the cycle data from Example 13.6,
except now let the boiler pressure range from 100. to 1000. psia
and keep the regenerator pressure at 25.0% of the boiler
pressure. Plot the cycle efficiency and the regenerator mass
fraction vs. boiler pressure. Does the cycle efficiency have a
maximum value under these conditions?
83. Develop a computer program or use a spreadsheet or equation
solver to calculate the Otto ASC thermal efficiency as the
compression ratio ranges from 1.00 to 12.0 for the following
values of k: 1.40, 1.35, 1.30, 1.25, and 1.20 (note that values of
k less than 1.40 give an approximate hot ASC result). Plot these
results (either manually or with a computer) with the thermal
efficiency on the vertical axis and the compression ratio on the
horizontal axis, using k as a parameter for the family of curves.
Utilize at least 25 points per curve.
84. Develop a computer program or use a spreadsheet or equation
solver to calculate the Diesel cold ASC thermal efficiency as
the compression ratio (CR) ranges from 12.0 to 30.0 for the
following cutoff ratios: 1.50, 2.00, 2.50, 3.00, and 3.50. Plot
these results (either manually or with a computer) with the
thermal efficiency on the vertical axis and the compression
ratio on the horizontal axis, using the cutoff ratio as a
parameter for the family of curves. Utilize at least 25 points
per curve.
85. Develop a computer program or use a spreadsheet or
equation solver that determines the value of the cutoff ratio
that causes the Diesel ASC thermal efficiency to become zero
as the compression ratio ranges from 12.0 to 30.0 for the
following values of k: 1.40, 1.35, 1.30, 1.25, and 1.20. Plot
these results (either manually or with a computer) with the
cutoff ratio on the vertical axis and the compression ratio on
the horizontal axis, using k as a parameter for the family of
curves.