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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2010/
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering in Mechanical Engineering Bachelor of Engineering (Honours) in Sustainable Energy
Programme Code: EMECH_7_Y2, ESENT_8_Y
Module Title: Thermofluids
Module Code: MECH
External Examiner(s): Dr. E. Ahearne, Mr. M. McKeon Internal Examiner(s): Ms. M. Kavanagh, Mr. Michael Walsh
Instructions: Answer any 3 questions. Please tick off which
questions you attempt on the front of the script.
Duration: 2 hours
Sitting: Summer 2011
Requirements for this examination: Steam Tables, Log Tables
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you are attempting the correct examination. If in doubt please contact an Invigilator.
Q1 Combustion of fuels The efficiency of combustion is generally a function of the degree to which CO 2 , CO and O 2 appear in the products of combustion. Discuss this, in detail, in relation to burning a hydrocarbon fuel as the air to fuel ratio is varied from a weak mixture to a rich mixture. (8 marks)
The fuel supplied in a boiler has the following composition by mass; Carbon 90% Hydrogen 6% Oxygen 4% The fuel is burned with an air to fuel ratio of 18:1. Per kg of fuel burnt, determine: (a) The stoichiometric air to fuel ratio. (14 marks) (b) The percentage excess air. (3 marks) (c) The analysis by mass and as a percentage of the wet products of combustion. (8 marks)
Composition of air by mass: 77% Nitrogen 23% Oxygen Atomic mass: Carbon(C) 12 Hydrogen(H) 1 Oxygen(O) 16
Q2 Steam Plant
Draw the Carnot cycle on the T-S diagram. Discuss briefly the practical problems with this cycle in a steam plant and show the modifications to the cycle to produce the Rankine cycle. (6 marks)
Steam is supplied dry saturated at 36 bar to a turbine and exhausts from the turbine into a condenser at a pressure of 0.03bar. Calculate:
(a) The dryness fraction of the steam entering condenser. (9 marks) (b) The work output at the turbine. (7 marks) (c) If the steam flow rate is 15000kg/hour, determine the power output developed. (4 marks) (d) The Rankine efficiency. (7 marks)
Q4 Expansion and Compression of gases (a) From point 1 on the pv diagram draw each of the following polytropic processes indicating the n value for each; Constant pressure cooling Constant volume heating Adiabatic compression Isothermal compression
(4 marks)
(b) 0.68m^3 of gas at a pressure of 0.11MN/m^2 and a temperature of 15oC is compressed isothermally until the volume becomes 0.1m^3 Determine;
(i) the final pressure (4 marks) (ii) the final temperature (3 marks) (iii) the heat transfer (7 marks)
If the compression had been adiabatic, determine
(iv) the final pressure (4 marks) (v) the final temperature (3 marks) (vi) the work transfer (8 marks)
For the gas take cp = 0.92kJ/kgK and cv = 0.66kJ/kgK.
