FINAL EXAM Course : Thermodynamics Course Code ... - ITS
FINAL EXAM Course : Thermodynamics Course Code ... - ITS
FINAL EXAM Course : Thermodynamics Course Code ... - ITS
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<strong>FINAL</strong> <strong>EXAM</strong><br />
<strong>Course</strong> : <strong>Thermodynamics</strong><br />
<strong>Course</strong> <strong>Code</strong> : ME091307<br />
Day/date : Tuesday, January 10, 2012<br />
Time : 08.00-09.45<br />
Conditions : Closed (Open table, summary in folio all pages and dictionary)<br />
Lecture : Aguk Zuhdi MF<br />
1. An ideal diesel engine has a comparison ratio 20 and uses air as the working fluid. The state of air at<br />
the beginning of the compression process is 95 kPa and 20 O C. If the maximum temperature in the<br />
cycle is not to exceed 2200 O C K, determine:<br />
a) the P-V diagram, (10%)<br />
b) the thermal efficiency of the cycle, (20%)<br />
c) the mean effective pressure. (10%)<br />
Assume constant specific heats for air at room temperature.<br />
The Answer:<br />
An ideal diesel engine with air as the working fluid has a compression ratio of 20.<br />
P1= 95 kPa; T1= 20 O C<br />
T3= 2200 O C<br />
The P-V diagram, the thermal efficiency and the mean effective pressure are to be determined.<br />
Assumptions:<br />
a)<br />
1 The air-standard assumptions are applicable.<br />
2 Kinetic and potential energy changes are negligible.<br />
3 Air is an ideal gas with constant specific heats.<br />
b) Properties: The properties of air at room temperature are cp = 1.005 kJ/kg·K, cv = 0.718 kJ/kg·K,<br />
R =0.287 kJ/kg·K, and k = 1.4 (Table A-2).<br />
Analysis (a) Process 1-2: isentropic compression.
Process 2-3: P = constant heat addition.<br />
Process 3-4: isentropic expansion.<br />
c)<br />
2. A combined gas-steam power cycle uses a simple gas turbine for the topping cycle and Rankine cycle<br />
for the bottoming cycle. Atmospheric air enters the gas turbine at 101 kPa and 20 O C, and the<br />
maximum gas cycle temperature is 1100 O C. The compressor pressure ratio is 8; the compressor<br />
isentropic efficiency is 85 percent; the gas turbine isentropic efficiency is 90 percent. The gas steam<br />
leaves the heat exchanger at the saturation temperature of the steam flowing through the heat<br />
exchanger. Steam flows through the heat exchanger with a pressure 6000 kPa, and leaves at 320 O C.<br />
The steam-cycle condenser operates at 20 kPa, and the isentropic efficiency of steam turbine is 90<br />
percent. Determine:<br />
a. the block diagram and T-s diagram, (20%)
. the mass flow rate of air through the air compressor required for this system to produce 100 MW<br />
of power. (40%)<br />
Use constant specific heats for air at room temperature.<br />
The answer:<br />
A combined gas-steam power cycle uses a simple gas turbine for the topping cycle and simple Rankine<br />
cycle for the bottoming cycle. The mass flow rate of air for a specified power output is to be determined.<br />
Assumptions :<br />
1 Steady operating conditions exist.<br />
2 The air-standard assumptions are applicable fo Brayton cycle.<br />
3 Kinetic and potential energy changes are negligible.<br />
4 Air is an ideal gas with constant specific heats.<br />
a) block diagram & T-s diagram:<br />
5<br />
Comperessor<br />
2<br />
Pump<br />
1<br />
9<br />
Combustion<br />
chamber<br />
6 7<br />
Heat Exchanger<br />
3<br />
Block Diagram<br />
Gas<br />
turbine<br />
Steam<br />
turbine<br />
Condenser<br />
b)<br />
Properties: The properties of air at room temperature are cp = 1.005 kJ/kg·K and k = 1.4 (Table A-2a).<br />
Analysis Working around the topping cycle gives the following results:<br />
8<br />
4<br />
T-s Diagram
Fixing the states around the bottom steam cycle yields (Tables A-4, A-5, A-6):
The net work outputs from each cycle are<br />
An energy balance on the heat exchanger gives<br />
That is, 1 kg of exhaust gases can heat only 0.1010 kg of water. Then, the mass flow rate of air is