# Final exam ENGI 2800 - Engineering Thermodynamics I

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```					                         Final exam
ENGI 2800 – Engineering Thermodynamics I

Date: December 11th, 2007                                     Room: Dalplex
Time: 8h35 – 11h30                                            Professors: Dr. Dominic Groulx
Open book, no question allowed                                            Dr. George Jarjoura

Question #1 (5 points)
Give a short written answer to this question:

a) Explain why the compression ratio in a diesel engine can be much higher than the
compression ration of a spark-ignition engine.
b) How do internal and external combustion engines differ? Give one example for each.

Question #2 (12 points)

Air as an ideal gas flows through the compressor and heat exchanger shown in the following
figure. A separate liquid water stream also flows through the heat exchanger. The data given on
the figure are for operation at steady state. Stray heat transfer to the surroundings can be
neglected through the entire system. Determine:

a) the compressor power, in kW;
b) the mass flow rate of cooling water, in kg/s;
c) the rates of entropy production, each in kW/K, for the compressor and heat exchanger.

1
Question #3 (20 points)
The figure to the right shows the schematic
diagram of a cogeneration cycle. In the
steam cycle, superheated vapor enters the
turbine with a mass flow rate of 5 kg/s at 4
MPa, 440°C and expands isentropically to
150 kPa. Half of the flow is extracted at
150 kPa and used for industrial process
heating. The rest of the steam passes
through a heat exchanger, which serves as
the boiler of the R-134a cycle and the
condenser of the steam cycle.            The
condensate leaves the heat exchanger (state
3) as saturated liquid at 100 kPa before it is
combined with the return flow from the
process, at 60°C and 100 kPa, before being
pump isentropically to the steam generator
pressure of 4 MPa.

The R-134a cycle is an ideal Rankine cycle
with refrigerant entering the turbine at 1.6
MPa, 100°C and saturated liquid leaving the
condenser at 900 kPa. Draw the two T-s
diagrams needed and determine, in kW:

a) the rate of heat transfer to       the
working fluid passing through      the
steam generator.
b) the net power produced by          the
binary cycle
c) the rate of heat transfer to       the
industrial process.

Hints: 1) start your analysis at state 1, before the steam turbine 2) Steam pressure goes down
through the heat exchanger but R-134a pressure stays constant in the same heat exchanger, so
don’t forget to account for it.
Question #4 (13 points)
Air, treated as an ideal gas, enters the compressor of a simple gas turbine at 100 kPa, 300 K, with
a volumetric flow rate of 5 m3/s. The compressor pressure ration is 10 and its isentropic
efficiency is 85%. At the inlet of the turbine, the pressure is now 950 kPa, and the temperature is
1400 K. The turbine has an isentropic efficiency of 88% and the exit pressure is 100 kPa.

On the basis of an air-standard cycle analysis, draw the T-s diagram for this cycle and determine:

a) the net work, in kW, produced by this cycle;
b) the thermal efficiency of this cycle;
c) the total amount of entropy generated during the cycle, in kW/K, assuming there’s no
heat transfer in the compressor or the turbine and the high and low temperature reservoir
are at temperatures of 1500 K and 250 K respectively.

2

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