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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2010/
Module Title: Thermofluids
Module Code: MECH
School: Mechanical and Process Engineering
Programme Title: Bachelor of Engineering in Mechanical Engineering
Programme Code: EMECH_7_Y
External Examiner(s): Dr. E. Ahearne, Mr. M. McKeon Internal Examiner(s): Ms. M. Kavanagh
Instructions: Answer any 4 questions.
Please tick off which questions you attempt on
the front of the script.
Duration: 2 hours
Sitting: Summer 2011
Requirements for this examination: Log tables, Steam 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 Gases
(a) A gas occupies a volume of 1.5m^3 in a cylinder at a pressure of 250kN/m^2. The gas is compressed at constant temperature until the volume is 0.5m^3. Calculate the final pressure. (4 marks)
(b) A compressor takes in carbon dioxide at a pressure of 100kN/m^2 and temperature 20 oC and compresses it to 600kN/m^2. The initial volume in the compressor cylinder is 0.2m^3 and the piston compresses it into a volume of 0.05m^3. (i) Find the temperature of the compressed gas. (6 marks) (ii) Determine the mass of gas delivered assuming the characteristic gas constant for carbon dioxide to be 0.189kJ/kgK. (5 marks)
(c) An engine compresses the air – fuel intake charge through a volume ratio of 7.25 to 1. The initial pressure is 105 kN/m^2 and the temperature is 27oC. The mixture is compressed according to PV1.3^ = C. Calculate the pressure and temperature at the end of the compression. (10 marks)
Q2 Heat
(a) A cylindrical stainless steel process vessel is used as a storage device for heating water. The vessel is 0.65m in diameter and 1.5m long and is half full of water. The water is to be heated from 15oC to 800oC in 30mins.
(i) Calculate the required kW rating of the heating device. (8 marks) (ii) Calculate the time taken in hours to heat the water if an 80kW heating device is used. (4 marks)
The heat gained by tank and heat loss to surroundings may be neglected. For water take, specific heat capacity = 4200J/kgoC, density = 1000kg/m^3.
(b) The specific heat capacity of a metal specimen of mass 16g is to be determined by method of mixtures. The specimen is placed in water at 100°C until it reaches the same temperature as the water. It is then quickly transferred to a copper calorimeter containing 26g of water at 24°C. The mass of the copper calorimeter is 28g and the final temperature of the water in the calorimeter is found to be 28oC. The specific heat capacity of the calorimeter material (copper) is 400J/kgK and that of water is 4.2kJ/kgK.
(i) Calculate the specific heat capacity of the metal. (9 marks) (ii) If the mass of the copper calorimeter is neglected, what would be the calculated value of the specific heat capacity of the metal. (4 marks)
Q5 Fluid Pressure (a) The height of a column of mercury in a barometer is 755mm. Determine the atmospheric pressure in kN/m 2. (3 marks)
(b) A square tank measuring 2m x 2m contains oil of relative density 0.88 to a depth of 3m when full. Find;
(i) the gauge pressure at the base of the tank when full (3 marks) ( ii) the force acting on each side of the tank when the tank is full (3 marks) (iii) the force acting on each side when the tank is half full. (2 marks)
(c) Each side of a cubical block of wood is 10m long. It is floating in sea water of density 1025kg/m^3 with one surface horizontal and 2m above the surface of the water. The atmospheric pressure is 100kN/m^2. Determine:
(i) The total pressure at the upper surface of the block. (2 marks) (ii) The total pressure at its lowermost surface. (3 marks) (iii) The weight of the block. (5 marks) (iv) The density of the block. (4 marks)
Take the density of mercury as 13600kg/m^3 and that of water as 1000kg/m^3.
