Thermodynamics - Jun 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.
Answer any one question from Q1 and Q2
1 (a) State Kelvin-Planck and Clausius statement of the second law of thermodynamics and prove that the violation of Kelvin-Planck statement results into violation of Clausius statement.(6 marks)
1 (b) In a certain heat exchanger, 50 kg of water is heated per minute from 50°C to 110°C by hot gases which enter the heat exchanger at 250°C. If the flow rate of gases is 100 kg/min, estimate the net change of entropy. Assume no loss of heat to surrounding. Cp(water)=4.186 kJ/kg-K, Cp=1 kJ/kg-K.(6 marks)
2 (a) Derive expression for the following quantities for an ideal gas undergoing a constant temperature process:
(i) Non-Flow System-Work done, Change in internal energy, Heat transfer
(ii) Flow System Work done, Heat transfer, Entropy change.(6 marks) 2 (b) A heat engine working on Carnot cycle absorbs heat from three thermal reservoirs at 1000 K, 800 K and 600 K. The engine does 10 kW of net work and rejects 400 kJ/min. of heat to a heat sink at 300 K. If the heat supplied by the reservoir at 1000 K is 60% of the heat supplied by the reservoir at 600 K, make calculations for the quantity of heat absorbed by each reservoir.(6 marks)
Answer any one question from Q3 and Q4
3 (a) State the assumptions made for air standard cycle. Derive an expression for the air standard efficiency and mean effective pressure of an Otto cycle.(6 marks)
3 (b) Steam of mass 10 kg and pressure 1000 kPa, 0.85 dry, is heated at constant pressure till the volume is doubled. Determine:
i) Final quality of steam
(ii) Heat added
(iii) Change in Internal Energy.(6 marks) 4 (a) Sketch and explain the construction and working of a separating and throttling calorimeter used for determining the dryness fraction of steam in a boiler.(6 marks) 4 (b) A system at 450 K receives 225 kJ/s of heat energy from a source at 1500 K, and the temperature of both the system and source remains constant during the heat transfer process. Represent the process on temperature-entropy diagram and determine:
(i) Net change in entropy
(ii) Available energy of heat source and system
(iii) Decrease in available energy.
Take atmospheric temperature equal to 300 K.(6 marks)
Answer any one question from Q5 and Q6
5 (a) Describe briefly the advantages which you would expect to be gained from incorporating an economizer, air preheater, and a superheated in a steam generating plant. By line diagram, indicate the position of these accessories in a typical boiler plant.(6 marks)
5 (b) The following data relates to a trial on boiler using economizer, air preheater and superheater:
Condition of steam at exit of boiler = 20 bar, 0.96 dry Temperature of steam at exit of superheater = 300°C
Steam evaporation rate/kg of fuel = 12 kg
Room temperature, t0 = 25°C
Temperature of feed water at exit of economizer, t1 = 50°C
Temperature of air at exit of air preheater, ta = 70°C
The temperature of flue gases at inlet to superheater, economizer, air preheater and exit of air preheater are respectively 650°C, 430°C, 300°C and 180°C respectively. Assume that air supplied is 19 kg/kg of fuel of calorific value of 45,000 kJ/kg, find:
(i) Equivalent evaporation with and without economizer, from and at 100°C.
(ii) Thermal efficiency of the boiler with and without economizer.
(iii) Thermal efficiency of superheater, economizer and air Preheater.(7 marks) 6 (a) Define steam generator and write down the classification of Boilers.(6 marks) 6 (b) In a certain boiler installation, a steel chimney of 30 m height produces and natural draught equivalent to 17.75 mm of water column. The mean temperature of the boiler house is 298 K and that of hot gases leaving the chimney is 633 K. If the boiler uses 1350 kg of coal per hour, make calculations for:
(i) Air supplied per kg of coal burnt on the grate,
(ii) Draught in terms of column of hot flue gases,
(iii) Density and mass flow rate of hot gas.(7 marks)
Answer any one question from Q7 and Q8
7 (a) Define mass fraction and mole fraction with example and explain the method of writing the complete combustion equation of a C8H18 with air.(6 marks)
7 (b) A sample of coal supplied to a boiler has the following composition by mass: Carbon = 87%, Hydrogen = 3%, Oxygen = 3%, Nitrogen = 1%, Sulphur = 1% and the remainder is ash. If 15% of excess air is supplied for combustion. Find:
(i) The theoretical amount of air required for complete combustion of fuel
(ii) The mass analysis of flue gas per kg of fuel.(7 marks) 8 (a) For what purpose a Bomb calorimeter is used ? Discuss its working with the help of a neat sketch(6 marks) 8 (b) The following data pertains to a test run made to determine the calorific value of a sample of coal:
Mass of coal burnt = 0.85 gm, Mass of fuel wire burnt and its calorific value is 0.028 gm and 6700 kJ/kg respectively, mass of water in calorimeter = 1800 gm, initial and final temperature of water = 16.5°C and 20.25°C, water equivalent of calorimeter = 350 gm, the coal contains 3% moisture by weight and R.T. = 20°C. Make calculations for the higher and lower calorific values of the coal sample. Consider latent heat of condensation of steam 2460 kJ/kg.(7 marks)