Wright State University
Department of Mechanical and Materials Engineering
Summer 2016
ME 3310: THERMODYNAMICS I, FINAL EXAM
OPEN BOOK, CLOSED NOTES, SHOW ALL WORK FOR PARTIAL CREDIT
Problem 1: (10 points) A heat pump with refrigerant R-134a as the working fluid is used to keep a space at
25°C by absorbing heat from geothermal water that enters the evaporator at 50°C at a rate of 0.065 kg/s and
leaves at 40°C. Refrigerant enters the evaporator at 20°C with a quality of 15 percent and leaves at the same
pressure as a saturated vapor. If the compressor consumes 1.2 kW of power, determine the mass flow rate of the
refrigerant, the rate of heat supplied to the 25°C space, the coefficient of performance of the heat pump, and the
minimum power input to the compressor for the same rate of heat supplied to the 25°C space. Answers: 0.0173
kg/s, 3.92 kW, 3.27, 0.303 kW.
Conditioned
Space at
25°C
𝑄̇
Condenser
Compressor
Throttling
Valve
𝑊̇
Evaporator
Geothermal
Water In
Tin = 50°C
𝑄̇
Water Out
Tout = 40°C
Problem 2: (5 points) Steam expands in a turbine steadily at a rate of 25,000 kg/hour, entering at 6 MPa and
450°C and leaving at 20 kPa as a saturated vapor. If the power generated by the turbine is 4 MW, determine the
̇ = 11.0
rate of entropy generation for this process. Assume the surrounding medium is at 25°C. (Answer: 𝑆gen
kW/K).
Problem 3: (15 points) An air-standard cycle is executed in a closed system with 0.003 kg of air and consists
of the following three processes:
1 to 2: Isentropic compression from 100 kPa and 27°C to 700 kPa
2 to 3: Heat addition to the initial specific volume (P = constant)
3 to 1: Heat rejection to the initial state (v = constant)
Show the cycle on P-v and T-s diagrams. Calculate the heat input, the heat rejected and the thermal efficiency.
Assume constant specific heats at room temperature.
hn = 104 sus
·
W
Q
=
mw(hg
-
ha)
Gradi
Qin
-
Qut-w = Qin
Qut
Qin + w
=
ntw
Cop
=
=
3 262
.