Modern Engineering Thermodynamics

Problems 355
12.5 Btu/hr·ft ·R. If the outer surface temperature of the wire
is maintained constant at 300.°F and the temperature profile
inside the wire is given by T = T w + ρ e J 2 e ðR2 − r 2 Þ/ð4k t Þ, where
T w is the wall temperature of the wire, R is its outside radius,
and r is measured from the center of the wire. Use Eqs. (7.75)
and (10.13b) to determine the steady state irreversibility
production rate within the wire due to the flow of electricity
through it. Assume all the physical properties are independent
of temperature.
20. A constant pressure piston-cylinder apparatus contains 1.30
lbm of saturated liquid water at 212°F. Heat is added to this
system until the contents reach a quality of 85.0%. The surface
temperature of the cylinder is constant at 250.°F. Determine
the irreversibility of this process if the local environment is at
14.7 psia and 70.0°F.
21. 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 (a)
the irreversibility and (b) the change in total availability for this
process if the local environment is at 14.7 psia and 70.0°F?
22.* A closed, rigid container encloses 1.50 kg of air at 0.100 MPa
and 20.0°C. We wish to increase the temperature to 40.0°C
by heat transfer. Assuming constant specific heat ideal gas
behavior, determine (a) the irreversibility and (b) the change
in total availability of the air when this change of state is
accomplished using a constant system boundary temperature
of 100.°C and the local environment is at 0.101 MPa
and 20.0°C.
23. A sealed, rigid kitchen pressure cooker with a volume of 1.00 ft 3
contains 2.20 lbm of a mixture of liquid plus vapor water at 14.7
psia. The pressure cooker is then heated until its internal pressure
reaches 20.0 psia. Determine (a) the heat transfer during the
process, (b) the irreversibility of the process if the inner surface
of the pressure cooker is constant at 250.°F, and (c) the change
in total availability of the water if the local environment is at
14.7 psia and 70.0°F.
24. 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. The vessel is then heated until its contents become a
saturated vapor. If this heating process is done irreversibly,
determine (a) the total irreversibility for this process if the
surface temperature of the vessel is maintained constant at
300.°F and (b) the change in total availability of the water if
the local environment is at 14.7 psia and 70.0°F.
25.* Determine the irreversibility of a 4.00 × 10 −3 kg, 80.0°C lead
bullet traveling at 900. m/s that 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, and the local
environment is at 0.101 MPa and 20.0°C.
26. A small room has a single 60.0 W lightbulb hanging from the
ceiling. The walls are not insulated, so a steady state condition
is reached where the room walls are at 55.0°F. Determine the
irreversibility rate within the room if the local environment is
at 14.7 psia and 40.0°F.
27.* 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
irreversibility rate of each light source when the local
environmental temperature is 20.0°C, and comment on which
is the more efficient.
28.* An automobile engine heater is an electrical resistance heater that
is plugged into a 110. V ac outlet and inserted into the oil
dipstick tube of the engine. Its purpose is to keep the engine oil
warm during the winter when the car is not in use, thus allowing
the engine to start easier. Determine the steady state
irreversibility produced during an 8.00 h period by a 100. W
steady state engine heater whose surface is isothermal at 90.0°C.
29. Determine the irreversibility produced when 3.00 lbm of carbon
dioxide at 70.0°F and 30.0 psia are adiabatically mixed with
7.00 lbm of carbon dioxide at 100.°F and 15.0 psia. The final
mixture pressure is 17.0 psia. Assume the carbon dioxide
behaves as a constant specific heat ideal gas and that the local
environment is at 14.7 psia and 70.0°F.
30.* Determine the irreversibility produced as 10.0 kg of liquid
water at 10.0°C is adiabatically mixed with 20.0 kg of liquid
water at 80.0°C. The specific heat of the water is 4.20 kJ/kg·K,
and the local environment is at 0.101 MPa and 20.0°C.
31. Here is the classical coffee and cream problem. Which of the
following processes produces less irreversibility?
a. Mixing cream with hot coffee and then letting the mixture
cool to the drinking temperature.
b. Letting the coffee cool to a temperature such that, when
the cream is added, the mixture will be at the drinking
temperature.
Do not ignore the cooling heat transfer irreversibility.
32.* An engine operating on a Carnot cycle extracts 10.0 kJ of heat
per cycle from a thermal reservoir at 1000.°C andrejectsa
smaller amount of heat to a low-temperature thermal reservoir
at 10.0°C. Determine the net change in availability of the engine
per cycle of operation when the local temperature is 0.00°C.
33. An engine operating on a Carnot cycle extracts heat at a rate of
500. Btu/s from a thermal reservoir at 1000.°F and rejects heat to
a low-temperature thermal reservoir at 100.°F. Determine the
rate of change in availability of the engine when the local
environmental temperature is 70.0°F.
34. A fire hose is used to extinguish a building fire. The end of the
hose is held 50.0 ft from the ground on a ladder and sprays
50.0°F water at a velocity of 12.0 ft/s. Assuming the water is
an incompressible liquid, determine the specific flow
availability at the exit of the hose when the local environment
(ground state) is at 14.7 psia and 70.0°F.
35. Recompute the specific flow availability in Problem 34 using
Table C.1a of Thermodynamic Tables to accompany Modern
Engineering Thermodynamics when the water exits the fire hose
as a saturated liquid at 50.0°F and the local environment
(ground state) is saturated liquid water at 70.0°F.
36.* A garden hose is used to fill a swimming pool. The hose is
laid on the ground and the water exits at 1.00 m/s at 15.0°C.
Assuming the water is an incompressible liquid, determine the
specific flow availability at the exit of the hose when the local
environment (ground state) is at 0.101 MPa and 20.0°C.
37.* Recompute the specific flow availability in Problem 36 using
Table C.1b when the water exits the garden hose as a saturated
liquid at 15.0°C and the local environment (ground state) is
saturated liquid water at 20.0°C.
38.* Superheated steam at 1000.°C and 1.00 MPa flows through a
pipe located 20.0 m above the floor in a power plant with a
velocity of 50.0 m/s. Determine the specific flow availability of