Wright State University ME 315, Spring 2000
Department of Mechanical and Materials Engineering
FINAL EXAM, THERMODYNAMICS I
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1. (25 points) A closed copper vessel has a volume of 70 ft3. In the vessel is 9 lbm of water (liquid plus vapor) at 30 psia. The surroundings are at 320°F. There is a heat transfer to the vessel until the water is all just evaporated.
a. What is the final pressure in the vessel?
b. What is the entropy change of the water in the vessel during the heat transfer process?
c. What is the entropy change of the surroundings due to the heat transfer process?
d. What is the total entropy generation for this process?
2. (25 points) Steam enters an adiabatic turbine at 8 MPa and 500°C with a mass flow rate of 3 kg/s and leaves at 30 kPa. The isentropic efficiency of the turbine is 0.90. Neglecting the kinetic energy change of the steam, determine the rate of entropy generation of the turbine.
3. (25 points) For an adiabatic air turbine, determine the mass flow rate required to produce 9 kW (Pi = 10 MPa, Ti = 1500°C, Pe = 2.1 MPa, h = 0.7). Do not assume constant specific heats, i.e., use the following equation:
s2 - s1 = s2° - s1° - R ln(P2/P1)
4. (25 points) A 3-ft3 rigid tank initially contains R-134a at 120 psia and 100 percent quality. The tank is connected by a valve to a supply line that carries R-134a at 160 psia and 80°F. The valve is now opened, allowing the refrigerant to enter the tank, and is closed when it is observed that the tank contains only saturated liquid at 140 psia. Determine the mass of the refrigerant that entered the tank, the amount of heat transfer with the surroundings at 120°F, and the entropy generated during this process.