CORK INSTITUTE OF TECHNOLOGY
Bachelor of Engineering (Honours) in Mechanical Engineering – Stage 2
(EMECH_8_Y2)
Summer 2009
Thermodynamics – Old Syllabus
Time: 3 Hours
Answer Question 1 and four other
questions.
All questions carry an equal 20 marks.
Examiners:
(1) The Second Law of Thermodynamics is a Natural Law. Discuss.
Dr. R.K. McMullan
Prof. R. Clarke
Mr. P. Clarke
(4 marks)
Prove the existence of the property entropy through consideration of a reversible adiabatic
process and the Second Law.
(4 marks)
Derive a relationship between the COP of a refrigerator and heat-pump operating at the same
conditions.
(4 marks)
Explain the importance of the Carnot Cycle, and how it is used in identifying whether a
process is a perpetual-motion machine.
(4 marks)
Describe in detail a device that could nearly produce a reversible isothermal expansion. Using
a sketch show any interactions with the surroundings.
(4 marks)
(2) Sketch an ideal vapour-compression refrigeration cycle on P-v, T-s and P-h diagrams, and
describe the processes that make up this cycle.
(3 marks)
On a further P-h diagram show how a typical real vapour-compression refrigeration cycle
differs from the ideal one described above, and clearly explain the reason for each difference.
(3 marks)
A heat pump uses groundwater at 12degC as an energy source. If the energy delivered by the
heat pump is to be 60MJ/h, and the compressor operates with Freon 12 between pressures of
100kPa and 1.0MPa;
a) estimate the minimum mass-flow rate of groundwater
b) calculate the minimum compressor power
Assume that the refrigerant enters the compressor dry saturated, and is nowhere sub-cooled.
(14 marks)
(3) Sketch a Regenerative cycle that uses three closed feed-water heaters
a) on a clearly annotated T-s diagram
b) schematically, showing all components and pipe-work, and directions of flow
(6 marks)
A steam power plant based on the above supplies steam at 3MPa, 450degC to the turbine;
condenser pressure is 4kPa.
Choose bleed-pressures so that the temperature-differences between each stage are equal.
Calculate
a) the ideal cycle-efficiency
b) the Specific Steam Consumption.
(14 marks)
(4) Compare typical Spark-Ignition and Compression-Ignition engines under the following
headings:
•
•
Initiation of combustion
Fuel
•
•
Suitable applications
Cost & complexity
(4 marks)
Sketch an Indicator Diagram for a four-stroke cycle spark-ignition engine, annotating valve
opening and closing positions and showing representative pressure values for each part of the
cycle
(6 marks)
A four-stroke 3-liter V6 spark ignition petrol engine has a maximum power output of 100 kW
at 5500 rpm, and a maximum torque of 236 Nm at 3000 rpm. The minimum sfc is 0.090 kg/MJ
at 3000 rpm, and the air flow rate is 0.068 m3/s. The compression ratio is 8.9:1 and the
mechanical efficiency is 90 per cent. The engine was tested under ambient conditions of
20degC and 1 bar; take the calorific value of the fuel to be 44 MJ/kg.
(a) Calculate the power output at 3000 rpm and the torque output at 5500 rpm.
(b) Calculate for both speeds the bmep and the imep.
(c) How does the overall efficiency at 3000 rpm compare with the corresponding air standard
Otto cycle efficiency?
(d) What is the volumetric efficiency and air/fuel ratio at 3000 rpm?
(10 marks)
(5) Give two examples of where and why flue/exhaust-gas analysis is performed.
Outline how exhaust-gas composition depends on mixture strength.
(4 marks)
(4 marks)
Methane is burned with atmospheric air. The analysis of the products on a dry basis, percent
by volume is as follows:
Product
CO2
O2
CO
N2
% by volume
10.00
2.37
0.53
87.10
Calculate:
a) The AFR (air-fuel ratio)
b) Percentage theoretical air
Constituents of air:
By Volume
By Mass
N2 (%)
79
76.7
O2 (%)
21
23.3
Atomic Weight
Hydrogen
1
Carbon
12
Nitrogen
14
Oxygen
16
(12 marks)
(6) In an ideal Rankine cycle with superheat and re-heat, steam enters the first-stage turbine at
8.0MPa, 480degC, and expands to 0.7MPa. It is then re-heated to 440degC before entering the
second-stage turbine, where it expands to the condenser pressure of 0.008MPa. The net power
output is 100MW.
Sketch this cycle on both P-v and T-s diagrams and schematically, showing all components,
pipework and directions of flow.
(5 marks)
Determine:
a) the thermal efficiency of the cycle
b) the mass flow-rate of the steam in kg/h
c) the rate of heat-transfer from the steam in the condenser in MW
(15 marks)
(7) Derive an expression for the air-standard Brayton Cycle that expresses efficiency as a function
of isentropic pressure-ratio.
(6 marks)
Sketch the cycle on P-v and T-s diagrams, and show on the T-s diagram the effect of:
• compressor inefficiency
• pressure-drop between compressor and turbine
• turbine efficiency
(2 marks)
An air standard Brayton cycle has air enter the compressor at 27degC and 100 kPa. The
pressure ratio is 10 and the maximum allowable temperature in the cycle is 1350 K. Determine
the compressor work, turbine work and the cycle efficiency per kilogram of air.
Compare the results of the previous problem to the results obtained if the compressor and
turbine efficiencies are 85% and a pressure drop of 27kPa exists between the compressor
discharge and the turbine inlet.
(12 marks)
Assume constant for air Cp=1.005kJ/kgK, Cv=0.718kJ/kgK