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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011/
Module Title: Thermofluids
Module Code: MECH
School: Mechanical & Process Engineering
Programme Title: Bachelor of Engineering in Mechanical Engineering – Year 2 Bachelor of Engineering (Honours) in Sustainable Energy – Year 2
Programme Code: EMECH_7_Y ESENT_8_Y
External Examiner(s): Mr M. McKeon, Dr E. Ahearne Mr P. Kenny, Mr C. Buckley Internal Examiner(s): Ms M. Kavanagh, Mr M. Walsh
Instructions: Answer any 3 questions.
Please tick off which questions you attempt on the front
of the script.
Duration: 2 hours
Sitting: Summer 2012
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.
Question 1
(a) (i) Draw an indicator diagram for both a petrol engine and a diesel engine and discuss the essential differences between them. (4 marks) (ii)Define what is meant by brake specific fuel consumption. (2 marks) (iii) A six cylinder, four stroke internal combustion engine has a bore diameter of 90mm and a piston stroke of 110mm. The indicated mean effective pressure is 782kN/m^2 , the engine runs at a speed of 3700revs/min and the fuel consumption is 26kg/hour. The mechanical efficiency is 80%. The calorific value of the fuel is 41.86MJ/kg. Calculate the indicated power, the brake power and the brake thermal efficiency. Determine the brake specific fuel consumption. (15 marks)
(b) Write a balanced chemical equation for the combustion of heptane (C 7 H 16 ) and determine the stoichiometric mass of air required to completely burn 1kg of the gas. (12 marks)
Composition of air by mass: 77% Nitrogen 23% Oxygen Atomic mass: Carbon(C) 12 Hydrogen(H) 1 Oxygen(O) 16
Question 2 (a) Sketch a neat line diagram for the water-steam circuit in a steam power plant, indicating the relative positions of the main components. (4 marks)
(b) Draw the Rankine cycle on a T-s diagram for a plant that includes a superheater and a reheater. Clearly identify each process on the diagram. (4 marks)
(c) In a steam plant, the steam leaves the boiler at a pressure of 2MN/m^2 and at a temperature of 250 oC. It is then expanded in a turbine and finally exhausts into a condenser. The pressure of the steam at entry to the condenser is 14kN/m^2. Determine: (i) the specific enthalpy of the steam entering the turbine. (3 marks) (ii) the specific enthalpy of the condensate leaving the condenser. (3 marks) (iii) the dryness fraction of the steam entering the condenser. (6 marks) (iv) the specific enthalpy of steam entering the condenser. (5 marks) (v) the work output at the turbine (4 marks) (vi) the Rankine efficiency of the plant. (4 marks)
