Homework Problems
1 Consider an 8 L evacuated rigid bottle that is surrounded by the atmosphere at 100 kPa and
17oC. A valve at the neck of the bottle is now opened and the atmospheric air is allowed to flow
into the bottle. The air trapped in the bottle eventually reaches thermal equilibrium with the
atmosphere as a result of heat transfer through the wall of the bottle. The valve remains open
during the process so that the trapped air also reaches mechanical equilibrium with the
atmosphere. Determine the net heat transfer through the wall of the bottle during this filling
process.
2 An insulated rigid tank is initially evacuated. A valve is opened and atmospheric air at 95 kPa
and 17oC enters the tank until the pressure in the tank reaches 95 kPa, at which point the valve is
closed. Determine the final temperature of the air in the tank. Assume constant specific heats.
3 A 0.2 m3 rigid tank initially
contains refrigerant-134a at
8oC. At this state 70% of the
mass is in the vapor phase and
the rest is in the liquid phase.
The tank is connected by a
valve to a supply line the
refrigerant at 1 MPa and 100oC
flows steadily. Now the valve
is opened slightly and the
refrigerant is allowed to enter
the tank. When the pressure in
the tank reaches 800 kPa, the
refrigerant is a saturated vapor.
At this point the valve is closed. Determine (a) the final temperature in the tank, (b) the mass of
refrigerant that has entered the tank and (c) the heat transfer between the system and the
surroundings.
4 An insulated 60 ft3 rigid tank contains air at 75 psia and 120oF. A valve connected to the tank is
now opened and air is allowed to escape until the pressure inside drops to 30 psia. The air
temperature during this process is maintained constant by an electrical resistance heater placed
in the tank. Determine the electrical work done during this process.
Pin = 1 MPa
Tin = 100oC
Inflow
Tank
V = 0.2 m3
T1= 8oC
x1 = 0.7
Q
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