Thermodynamics - Dec 2015
Mechanical Engg (Semester 3)
TOTAL MARKS: 100
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.
Solve any one question from Q1 and Q2
1 (a) Explain the following concepts of thermodynamics:
i) Thermodynamics cycle
ii) Flow work
iii) P-dV work.(6 marks) 1 (b) Find the change in entropy of universe when 1 kg of ice at -5°C is exposed to atmosphere which is at 30°C, ice melts and comes in thermal equilibrium with atmosphere. Consider specific heat of ice 2.1 kJ/kgK, latent heat of fusion of ice 330 kJ/kg.(6 marks) 2 (a) What are the Kelvin-Planck and Clausius statements of second law of thermodynamic? Also establish their equivalence.(6 marks) 2 (b) Air is initially at 1 bar and 27°C is compressed reversibly and adiabatically in reciprocating engine to final pressure of 25 bar. Find work done, change in enthalpy and change in entropy per kg of air. Assume Cp and Cv of air 1.005 kJ/kg K and 0.717 kJ/kgK respectively.(6 marks)
Solve any one question from Q3 and Q4
3 (a) Draw P-V and T-S diagram of Otto cycle and derive an expression to find its thermal efficiency.(6 marks)
3 (b) A sample of steam generated in a boiler at a pressure of 12 bar is passed through separating and throttling calorimeter to measure its dryness fraction. The following observations were recorded during the test:
i) Pressure of steam after throttling - 1.2 bar,
ii) Temperature of steam after throttling - 120°C
iii) Mass of steam collected after throttling -1 kg
iv) Mass of water collected through separating calorimeter -15gm.
Find the quality of steam generated by boiler. Assume CP of superheated steam 2.1 kJ/kgK.(6 marks) 4 (a) Prove that when heat is transfer through finite temperature difference it causes loss of available energy.(6 marks) 4 (b) A thermal power plant works on Rankine Cycle has a boiler pressure of 120 bar and condenser pressure of 5 kPa. Steam is superheated in the superheater to 400°C. Find per kg of steam generated by boiler:
i) Net work output
ii) Rankine efficiency
iii) Specific team consumption.(6 marks)
Solve any one question from Q5 and Q6
5 (a) With the help of suitable diagram explain the construction and working of Cochron boiler.(6 marks)
5 (b) Determine the air-fuel ratio for an oil fired steam generator with the following data:
Chimney height - 32 m,
Chimney draught - 12 mm of water column,
Flue gas temperature through chimney - 279°C
Ambient air temperature - 27°C
Also calculate the velocity of flue gas through chimney neglecting losses in the flow of flue gas through chimney.(7 marks) 6 (a) Write the function and locations of the following boiler mountings:
i) Blow off cock
ii) Fusible plug
iii) Steam safety value.(6 marks) 6 (b) The following readings were recorded during boiler trail of 6 hour duration:
Pressure of steam generated - 12 bar,
Mass of steam generated - 4000 kg
Dryness fraction of steam generated - 0.85
Feed water temperature - 30°C,
Coal used - 4000 kg.
Calorific value of coal - 33400 kJ/kg.
i) Factor of equivalent evaporation
ii) Equivalent evaporation from and at 100°C
iii) Efficiency of boiler.(7 marks)
Solve any one question from Q7 and Q8
7 (a) Write the complete combustion equations for C8H18 and C12H26. Also find theoretical air required for complete combustion of 1 kg of these fuels.(6 marks)
7 (b) If 30% excess air is supplied for the combustion of dry anthracite of the following compositions by mass: Carbon - 88%, Hydrogen - 4%, Oxygen - 3.5%, Sulphur - 0.5%, Ash - 3% and Nitrogen - 1%, determine:
i) Air-fuel ratio
ii) Dry analysis of product of combustion by volume.(7 marks) 8 (a) With the help of suitable diagram explain, how Junker gas calorimeter is used to measure Calorific value of gases fuel?(6 marks) 8 (b) Diesel fuel (C12H26) reacts with 80% theoretical air. Determine the product of combustion on percentage of volume basis. Considering complete combustion of hydrogen of fuel.(7 marks)