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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2009/
Module Title: Introductory Thermofluids
Module Code: MECH 6010
School: Mechanical and Process Engineering
Programme Title: Bachelor of Engineering (Honours) in Mechanical Engineering – Year 1 Bachelor of Engineering (Honours) in Biomedical Engineering – Year 1
Programme Code: EMECH_8_Y1, EBIOM_8_Y1, EOMNI_8_Y
External Examiner(s): Mr. John J. Hayes, Prof. Robin Clarke
Internal Examiner(s): Mr. W. M. Corr
Instructions: Answer any Three Questions. All Questions carry equal marks.
Duration: 2 Hours
Sitting: Summer 2010
Requirements for this examination: Steam Tables
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.
Q1. (a) Describe throttling of steam: sketch the process on a Temperature-Enthalpy diagram of properties and show how each of the following is altered:
- Pressure
- Temperature
- Enthalpy
- Dryness (6 marks)
(b) Steam at a pressure of 15 bars and a temperature of 250°C, flowing at 1.5 kg/s, is throttled to 7 bars and mixed in adiabatic steady flow with steam also at 7 bars, 50% dry and flowing at 4.6 kg/sec. (i) Calculate the dryness of resulting mixture (8 marks) (ii) Size the pipe to carry the mixed steam if the recommended steam velocity is 35 m/s. (6 marks)
Q3. (a) Demonstrate the necessary and sufficient condition for stability of a hot-air balloon/basket. (4 marks)
(b) (i) Determine the submerged depth h of a solid wooden cylinder of diameter 0.666 m and height 1.300 m with density 610 kg/m^3 when placed in oil of density 850 kg/m^3. (4 marks) (ii) Will the cylinder float stably in this position? Justify your answer. (8 marks) (iii) As oil density decreases (but block density remains constant) how is stability affected? Hint: consider the limiting case where oil density has fallen to 610 kg/m^3. (4 marks)
Q4. (a) Why is cooling necessary in air compressor design? (5 marks)
(b) A compressor takes in 500 kg/min of air at 0.98 bar and 18º C and delivers it at 5.5 bar and 68º C. The diameters of the intake and delivery pipes are respectively 450 mm and 200 mm and the compressor is driven by a motor of 1,000 kW. (i) Demonstrate that kinetic energy change is negligible. (5 marks) (ii) Assuming that all the energy from the motor is transmitted to the air, estimate the rate in kW at which heat is transferred to or from the air during compression.(10 marks) Take Cp , Cv for air 1.005, 0.718 (^) kgKkJ^ , respectively.
Neglect change in Potential Energy.
Hint: change in internal energy (or enthalpy) must be evaluated from first principles.
Q5. (a) Define a polytropic process and demonstrate the appropriate polytropic index for the following process types:
- Constant pressure
- Constant volume
- Constant temperature^ (5 marks)
(b) 2.4 m^3 of dry air initially at 1 bar, 127°C is to be compressed to a volume of 1.0 m^3 either isothermally or adiabatically. (i) Sketch these two possible process options on a P-V diagram and (ii) for each case determine: (2 marks)
- Final pressure (3 marks)
- Final temperature (4 marks)
- Work done (6 marks)
Assume the air to act as an ideal gas and use properties at 127°C from steam tables.
