Université d’Ottawa
Faculté de génie
University of Ottawa
Faculty of Engineering
Department of Mechanical Engineering
MCG 2131 Thermodynamics II XXXXXXXXXXInstructor: Nabel Sadek
Final Exam XXXXXXXXXXTime Allowed: 3 Hours
Date: July 31/2018 XXXXXXXXXXClosed Book Exam.
Non-Programmable Calculators Allowed
NOTE: Every student is allowed to use a one page, size A4, double sided formula sheet. The
formula sheet has to be submitted with the answer booklet. Students should write their name on
the formula sheet.
1. Concept questions, Use
ief description only to answer them. (2 marks for each part).
(a) Humid air A has the same moisture content as humid air B (ωA=ωB) but the dry-bulb
temperature of B is higher than that of A. What would you say about the relative humidity
and dew point temperature of A compared to those of B (higher, lower, or the same).
(b) What is the importance of identifying the dew point temperature of the combustion
products?
(c) State three reasons that make the air-standard refrigeration cycle more suitable than vapor
compression cycles for airplane cooling applications.
(d) Explain why is the typical compression ratio of the compression-ignition engine much
higher than that of the spark-ignition engine. What is the limiting factor in each case?
(e) State two advantages of the open feed water heaters.
2. A gas tu
ine plant consists of one tu
ine for compressor drive and another for output power.
Both tu
ines are served by one compressor and one combustion chamber (burner) as shown in
figure 1. Air enters the compressor at 100 kPa and 15oC and is compressed with isentropic
efficiency of 0.76. Gas inlet temperature and pressure in both tu
ines are 680oC and 500 kPa,
espectively. Gas expands to 100 kPa in both tu
ines. The isentropic efficiency of each tu
ine
is 0.86. Assuming cp=1.005 kJ/kg.K and k=1.4 for air and cp=1.128 kJ/kg.K and k=1.34 for
gases,
(a) Find T2 and T XXXXXXXXXXmarks
(b) For each kg of compressed air entering the burner, mf kg of fuel is added. The heat
added in the combustion process 2-3 is 42000 kJ/kg of fuel. Use the energy balance
over the combustion chamber to find mf and the air to fuel ratio. 4 marks
(c) Find the fraction of flue gases that passes through the tu
ine driving the compressor,
(shown as x in figure XXXXXXXXXXmarks
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(d) If the mass flow rate of air through the compressor is 23 kg/s, find the power output of
the plant (shown as �̇� in figure XXXXXXXXXXmarks
(e) State all your assumptions XXXXXXXXXXmark
3. A steam power plant equipped with regenerative as well as reheat a
angement, as shown in
figure 2, is supplied with steam to the H.P. tu
ine at 80 bar 470°C. For open feed water
heating, a part of steam is extracted at 7 bar and the remainder of the steam is reheated to
350°C in a reheater and then expanded in L.P. tu
ine down to 0.03 bar.
(a) Draw a T-S diagram of the cycle assuming isentropic processes throughout. 2 marks
(b) Find h1, h2, h3, h4, and h5, using steam tables XXXXXXXXXXmarks
(c) If the isentropic efficiency of the pump is 0.8, find the specific work of each pump.
XXXXXXXXXXmarks
(d) Find the amount of steam bled-off for feed water heating (m) per kilogram of steam flow
such that saturated liquid comes out of the regenerator. Assume adiabatic mixing in the
egenerator XXXXXXXXXXmarks
(e) Find the cycle efficiency XXXXXXXXXXmarks
4. Saturated air at 3ºC is required to be supplied to a room at 22ºC with a relative humidity of
55%. The air is heated and then water at 10ºC is adiabatically sprayed to give the required
humidity. Neglect the fan power. Assume that the total pressure is constant at XXXXXXXXXXbar and
for dry air consider cp=1.005 kJ/kg.K. Determine :
(a) The mass of spray water required per kg of dry air XXXXXXXXXXmarks
(b) The mass of spray water required per m3 of dry air XXXXXXXXXXmarks
(c) The temperature to which the air must be heated. 3 marks
5. A four-cylinder petrol engine has a bore of 57 mm and a stroke of 90 mm. Its rated speed is
2800 rpm and it is tested at this speed against a
ake which has a torque arm of 0.356 m. the
net
ake load is 155 N and the fuel consumption is 6.74 l/hr. the specific gravity of the fuel
used is 0.735 and its lower heat value is 44200 kJ/kg. The mechanical efficiency at this speed
is measured to be 82.3%. Calculate for this speed:
(a) The
ake mean effective pressure, XXXXXXXXXXmarks
(b) The
ake thermal efficiency, XXXXXXXXXXmarks
(c) The
ake specific fuel consumption in kg/kWh, 2 marks
(d) The indicated mean effective pressure, XXXXXXXXXXmarks
(e) If the engine is idling at this speed (
ake load is removed), what would be its mechanical
efficiency? XXXXXXXXXXmark
(f) The analysis of dry exhaust from this engine is as follows:
Ca
on dioxide (CO2) = 15 per cent Ca
on monoxide (CO) = 3 per cent
Methane (CH4) = 3 per cent, Hydrogen (H2) = 1 per cent
Oxygen (O2) = 2 per cent Nitrogen (N2) = 76 per cent
The molecular weight in kg/kmol is C=12, H=1, N=14, O=16
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Calculate the proportions by mass of ca
on to hydrogen in the fuel, assuming it to be a
pure hydroca
on XXXXXXXXXXmarks
Fig.1
Fig.2
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Psychrometric chart
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