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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2008/
Module Title: Thermofluids 3
Module Code: INTR
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering in Mechanical Engineering
Programme Code: EMECH_7_Y
External Examiner(s): Mr. Alan Bateman, Dr. Patrick Delassus Internal Examiner(s): Dr Gerard Kelly, Dr K McMullan
Instructions: Attempt Q 1 (34 marks) and any other two questions (33 marks each)
Duration: 2 hours
Sitting: S emester 2 2009
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
Heat Exchangers Q1.
(a) In the heat transfer relation Q = UA ∆T (^) lm, for a heat exchanger, what is what is ∆T (^) lm, called? How is it determined for a parallelflow and counterflow heat exchanger? ( marks)
(b) A counterflow concentric tube heat exchanger is used to cool the lubricating oil for a large industrial gas turbine. The flow rate of the cooling water through the inner tube (D (^) i = 25mm) is 0.2kg/s, while the flow rate of oil through the outer annulus ( D (^) o =45mm) is 0.1kg/s. The oil and water enter at temperatures of 100○C and 30○C respectively. Take the overall heat transfer coefficient for the exchanger to be 40 W/m 2 K. The properties of the oil and water may be taken from tables 5 and 6 below.
(i.) Sketch the schematics depicting the temperature and flow through the heat exchanger for the parallel and counterflow heat exchanger. (6 marks) (ii.) What length of tube is required to cool the oil temperature at the outlet to 60○C? (20 marks)
Combustion
Q2.
(a) 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 relation to burning a hydrocarbon fuel as the air to fuel ratio is varied from a weak to a rich mixture (9 marks)
(b) A boiler consumes 100 kg/hr coal, the composition by mass being 90% carbon, 5% hydrogen and the remainder ash. Calculate
(i.) The mass of air required per hour for complete combustion of the coal assuming complete combustion of the coal assuming 40% excess air is supplied (12 marks) (ii.) The mass of oxygen per hour leaving the boiler in the flue gases. (12 marks)
Assume Relative atomic masses C=12; O =16; H = 1; N=14. The composition, by mass of air is 77% nitrogen and 23% oxygen.
