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 C_p and C_v 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 C_P 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.
Find:
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)