Modern Engineering Thermodynamics

Problems 275
15. A 20.0 ft 3 tank contains air at 100. psia, 100.°F. A valve on the
tank is opened and the pressure in the tank drops to 20.0 psia.
If the air that remains in the tank is considered to be a closed
system undergoing a reversible adiabatic process, calculate the
final mass of air in the tank. Assume constant specific heat ideal
gas behavior, and neglect any changes in kinetic and potential
energies.
16. A pressure vessel contains ammonia at a pressure of 100. psia
and a temperature of 100.°F. A valve at the top of the vessel is
opened, allowing vapor to escape. Assume that, at any instant,
the ammonia that remains in the vessel has undergone an
isentropic process. When the ammonia remaining in the vessel
becomes a saturated vapor, the valve is closed. The mass of
ammonia in the vessel at this moment is 2.00 lbm. Find the
mass of ammonia that escaped into the surroundings. Neglect
any changes in kinetic and potential energies.
17.* A 2.00 m 3 tank contains air at 0.200 MPa and 35.0°C. A valve
on the tank is opened and the pressure in the tank drops to
0.100 MPa. If the process is isentropic, calculate the final mass
of air in the tank. Assume the air behaves as a constant specific
heat ideal gas. Neglect any changes in kinetic and potential
energies.
18. A 58.0 ft 3 tank contains air at 30.0 psia and 100.°F. A valve on
the tank is opened and the pressure in the tank drops to 10.0
psia. If the air that remains in the tank has gone through an
adiabatic polytropic process with n = 1.33, calculate the final
mass of air in the tank and the entropy production that
occurred in this mass. Assume the air behaves as a constant
specific heat ideal gas, and neglect any changes in kinetic and
potential energies.
19. An operating gearbox (transmission) has 200. hp at its input
shaft while 190. hp are delivered to the output shaft. The
gearbox has a steady state surface temperature of 140.°F.
Determine the rate of entropy production by the gearbox.
20.* A gearbox (transmission) operating at steady state, receives
100. kW of power from an engine and delivers 97.0 kW to the
output shaft. If the surface of the gearbox is at a uniform
temperature of 50.0°C and the surrounding temperature is
20.0°C, what is the rate of entropy production?
21.* Determine the amount of entropy produced in the process
described in Problem 1 at the end of Chapter 5, when both the
specific internal energy and the specific entropy of the water
have returned to their initial values.
22.* Determine the amount of entropy produced in the process
described in Problem 3 at the end of Chapter 5. Assume that the
system boundary temperature is the same as its bulk isothermal
temperature.
23. Determine the amount of entropy produced in the process
described in Problem 4 at the end of Chapter 5 if the 500. Btu
heat transfer occurred across an isothermal system boundary at
250.°F.
24.* Determine the amount of entropy produced in the process
described in Problem 8 at the end of Chapter 5 if the work
transport is 90.0% of the magnitude of the heat transport.
Assume that the system boundary temperature is the same as its
bulk isothermal temperature.
25.* Determine the amount of entropy produced in the process
described in Problem 13 at the end of Chapter 5. Assume the
human body is a steady state closed system with an isothermal
surface temperature of 36.0°C during the exercise process.
26. Determine the amount of entropy produced during the adiabatic
expansion process described in Problem 17 at the end of
Chapter 5. Discuss the difficulty encountered in determining the
entropy production during the final isobaric compression
process.
27. 1.00 lbm of saturated water vapor at 212°F is condensed in a
closed, nonrigid system to saturated liquid at 212°F ina
constant pressure process by a heat transfer across a system
boundary with a constant temperature of 80.0°F. What is the
total entropy production for this process?
28. A rigid container encloses 150. lbm of air at 15.0 psia and
500. R. We wish to increase the temperature to 540. R.
Assuming constant specific heat ideal gas behavior,
a. Determine the heat transfer to the air for this change of state.
b. Determine the entropy production if this change of state is
accomplished by using a constant system boundary
temperature of 300.°F.
29. A sealed kitchen pressure cooker whose volume is 1.00 ft 3
contains 2.20 lbm of saturated water (liquid plus vapor) at
14.7 psia. The pressure cooker is then heated until its internal
pressure reaches 20.0 psia. Determine
a. The work done during the process.
b. The heat transfer during the process.
c. The entropy produced during the process if the inner surface
of the pressure cooker is constant at 250.°F.
30. A closed, sealed, rigid container is filled with 0.05833 ft 3 of
liquid water and 0.94167 ft 3 of water vapor in equilibrium at
1.00 psia.
a. What is the quality in the vessel at this state?
The vessel is then heated until its contents become a
saturated vapor.
b. What are the temperature and pressure in the vessel at this
state?
The heating process just described is done irreversibly.
c. Determine the total entropy produced for this process if the
surface temperature of the vessel is maintained constant at
300.°F.
31.* Determine the entropy produced as a 4.00 g, 80.0°C lead bullet
traveling at 900. m/s impacts a perfectly rigid surface
aergonically and adiabatically. The specific heat of lead at the
mean temperature of the bullet is 167 J/(kg · K).
32.* Determine the minimum isothermal system boundary
temperature required by the second law as a 1500. kg iron ingot
is heated from 20.0°C to 1000.°C. Assume the ingot is
incompressible.
33.* In the 21st century, the Earth will be terrorized by Zandar the
Wombat, an asexual rebel engineer from the planet Q-dot.
Earth’s only hope for survival lies in your ability to determine
the entropy production rate of Zandar. To do this, you
cleverly trick Zandar into completely wrapping himself
(herself?) with insulation and holding his breath. You then
quickly measure his body temperature and find that it is
increasing at a constant rate of 2.00°C perminute.Zandar
weighs 981 N and has the thermodynamic properties of
liquid water. Determine Zandar’s entropy production rate
when his body temperature reaches 50.0°C.
34.* The surface temperature of a 100. W incandescent lightbulb is
60.0°C. The surface temperature of a 20.0 W fluorescent tube
producing the same amount of light as the 100. W incandescent
lightbulb is 30.0°C. Determine the steady state entropy