Homework 6 Due Friday April 11, 2008
1.
Solve the following problems from S. R. Turns, "Thermodynamics: Concepts
and Applications."
6.4 COPHP
= 4.03
6.7 step
1: w1 = -1435 kJ/kg; step 2: q2 = 3738 kJ/kg; step 3: w3
= 7173 kJ/kg; step 4: q4 = -3738 kJ/kg; hth
= 0526
2.
A rigid tank contains an ideal gas at 40 °C that is being stirred by a paddle wheel. The paddle wheel does 200 kJ of work on the
ideal gas. It is observed that the
temperature of the ideal gas remains constant during this process as a result
of heat transfer between the system and the surroundings at 30 °C. Determine the entropy change of the ideal
gas. (C&B)
3.
A well-insulated rigid tank contains 2 kg of a saturated liquid-vapor mixture
of water at 100 kPa. Initially, three-quarters of the mass is in
the liquid phase. An electric resistance
heater placed in the tank is now turned on and kept on until all the liquid in
the tank is vaporized. Determine the
entropy change of the steam during the process.
(C&B) DS = 8.10 kJ/K
4.
An insulated cylinder-piston device contains 0.05 m3 of saturated
refrigerant-134a vapor at 0.8 MPa pressure. The refrigerant is now allowed to expand in a
reversible manner until the pressure drops to 0.4 MPa. Determine (a) the final temperature in the
cylinder and (b) the work done by the refrigerant. (C&B)
W = 27.2 kJ
5.
A 50-kg copper block initially at 80 °C is dropped into an insulated tank that contains 120 L of
water at 25 °C. Determine the final equilibrium temperature
and the total entropy change for this process.
(C&B) Tf = 300 K,
DStotal
= 0.215 kJ/K
6.
Air is compressed steadily by a 5-kW compressor from 100 kPa
and 17 °C to 600 kPa and 167 °C at a rate of 1.6 kg/min. During the process, some heat transfer takes
place between the compressor and the surrounding medium at 17 °C. Determine the rate of entropy change of air
during this process. (C&B) DS-dot = -0.0025 kJ/s K.
7.
An insulated rigid tank is divided into two equal parts by a partition. Initially, one part contains 5 kmol of an ideal gas at 250 kPa
and 40 °C, and the
other side is evacuated. The partition
is now removed, and the gas fills the entire tank. Determine the total entropy change during
this process. (C&B) DS = 28.81 kJ/K
8.
Helium gas is compressed from 90 kPa and 30 °C to 450 kPa in a reversible, adiabatic process. Determine the final temperature and the work
done, assuming the process takes place (a) in a cylinder-piston device and (b)
in a steady-flow compressor.
(C&B) T2 = 577 K, Wrev
= 853.7 kJ/kg, Wrev-flow = 1423
kJ/kg
"(C&A)"
indicates a problem taken from Y. A. çengel and M. A. Boles, "Thermodynamics – An
Engineering Approach," fifth edition.