Modern Engineering Thermodynamics

140 CHAPTER 4: The First Law of Thermodynamics and Energy Transport Mechanisms
Finally, we study a special technique that maps the solution of any thermodynamic problem. If you follow
the format given in Figure 4.21, you will breeze through the solution maze. But you must follow it religiously
and take no shortcuts. As an extension of your problem solving skills, you are also shown how to
write and solve your own thermodynamic problems. If you can do this successfully, you have mastered the
subject.
Problems (* indicates problems in SI units)
1.* Determine the energy transport required to increase the
temperature of 3.50 kg of air from 20.0 to 100.°C (Figure 4.23).
Assume the air is stationary and behaves as an ideal gas with
constant specific heats.
u aircraft = 3500. Btu/lbm
Air
3.50 kg
20.0°C
Energy
transport = ?
Air
3.50 kg
100.°C
V = 500. ft/s
Altitude = 30,000. ft
m = 50.0 lbm/min
FIGURE 4.23
Problem 1.
2.* Determine the energy transport necessary to decrease the
temperature of 15.0 kg of methane from 500. to 20.0°C. Assume
the methane is stationary and behaves as an ideal gas with
constant specific heats.
3. Determine the gain in energy of a stationary system of
5.00 lbm of argon whose temperature is increased from
70.0 to 1000.°F. Assume ideal gas behavior with constant
specific heats.
4.* Determine the gain in energy of a stationary system of 11.0 kg
of oxygen whose pressure is increased from 0.100 to 100. MPa
isothermally. Assume ideal gas behavior with constant specific
heats.
5. If 150. Btu are transported into a system via a work mode
while 75.0 Btu are removed via heat transfer and mass flow
modes (Figure 4.24), determine the net energy gain for this
system.
FIGURE 4.25
Problem 6.
7. An automobile transmission has 175 hp of power entering
from the engine, 167 hp leaving to the wheels, while losing
5000. Btu/h to the surroundings as heat. What is the net energy
transport rate of the transmission?
8. To keep the transmission in the previous problem from
overheating, it was decided to cool it by circulating a coolant
through its case. If the coolant enters the transmission with a
mass flow energy rate of 10.0 Btu/s, what is its mass flow energy
rate as it leaves the transmission?
9. Determine the heat transfer rate, in Btu/h, required to cool a
200. kW electric generator that is driven by a 300. hp diesel
engine (Figure 4.26). Note: The generator runs cool if it has a
zero net energy transport rate.
Q = ?
Heat and mass flow = 75.0 Btu
200. kW
Net energy gain = ?
300. hp diesel
W =150. Btu
FIGURE 4.24
Problem 5.
6. A jet aircraft with a constant specific internal energy of
3500. Btu/lbm consumes fuel at a rate of 50.0 lbm/min
while flying horizontally at an altitude of 30,000. ft with a
constant velocity of 500. ft/s (Figure 4.25). Determine the net
energy transport rate of the aircraft.
FIGURE 4.26
Problem 9.
10. In a stationary dynamometer test, an internal combustion
automobile engine has a fuel energy input rate of 1.90 million
Btu/h while producing 150. hp of output power. What other
energy transport mechanisms are present and what are
their magnitudes. Assume that the net energy transport rate is zero.