Thermodynamics - Dec 2014
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) State limitations of first law of thermodynamics. Explain how
Clausius and Kelvin Planck statements overcome these limitations
using the heat engine, heat pump and refrigerator concept.
Define thermal efficiency of heat engine and COP for refrigerator
and heat pump.(6 marks)
1 (b) 1 kg of ice at -5°C is exposed to atmosphere at 20
deg. C. The ice melts and attains thermal equilibrium with
surrounding. Determine :
(i) Change in entropy of the universe
(ii) Total heat transfer during the process.
Cp ice = 2.093 kJ/kg K, Latent heat of fusion = 333.3 kJ/kg. Cp water = 4.187 kJ/kg K.(6 marks) 2 (a) Derive expression for the following quantities for an ideal gas undergoing a constant pressure process:
i) Heat transfer
ii) Non-flow work transfer
iii) Steady flow work transfer
iv) Change in entropy
v) Change in internal energy and change in enthalpy during the process.(6 marks) 2 (b) A reversible heat engine working as a refrigerator absorbs heat from low temperature reservoir of 650 kJ, when work input is 250 kJ:
(i) Find its COP and heat transferred to the surrounding.
(ii) If the same device works as a heat engine, find out its thermal efficiency.
(iii) If the same device works as a heat pump, estimate the COP.(6 marks)
Solve any one question from Q3 and Q4
3 (a) Draw P-v and T-s diagram for Otto cycle and derive the efficiency
equation for Otto cycle.(6 marks)
3 (b) Steam initially at 1.5 MPa, 300 deg. C expands isentropically
in a steam turbine to 40 deg. C. Determine the ideal work
output of the steam per kg of steam.(6 marks)
4 (a) Define dryness fraction. Explain throttling calorimeter with neat
diagram for estimating the dryness fraction.(6 marks)
4 (b) 1000 kJ of heat leaves the hot gases at 1400 deg. C from
a fire box and goes to a steam at 250 deg. C. Atmospheric
temperature is 20 deg. C. Divide the energy into available
and unavailable part as it:
(i) Leaves the hot gases
(ii) Enters the system.(6 marks)
Solve any one question from Q5 and Q6
5 (a) Show block diagram of a boiler plant showing location of air-
preheater, superheater, economizer clearly indicating the air
and water circuit flow.(6 marks)
5 (b) The following particulars refer to a steam power plant consisting
of a boiler, superheater and economizer:
Steam pressure = 20 bar, Mass of steam generated = 10000 kg/hr, Mass of coal used = 1300 kg/hr, CV for coal = 29000 kJ/kg. Temperature of feed water entering the economizer = 35 deg. C, temperature of feed water leaving the economizer = 105 deg. C. Dryness fraction of the steam leaving the boiler = 0.98. Temperature of steam leaving the superheater = 350 deg. C. Determine:
(1) Overall efficiency of the boiler plant.
(2) Equivalent evaporation of the given boiler from and at 100 deg. C in kg of steam generated/kg of coal burnt And
(3) Percentage of heat utilised in economizer, boiler and super- heater.(7 marks) 6 (a) Define equivalent evaporation and boiler efficiency. Explain heat balance sheet for boiler.(7 marks) 6 (b) How must air per kg of coal is burnt in a boiler having chimney height of 32.3 m to create a draught of 19 mm of water column when the temperature of the flue gases leaving chimney is 370 deg. C and temperature of boiler house is 29.5 deg. C. Also calculate the draught produced in terms of hot gas Column.(6 marks)
Solve any one question from Q7 and Q8
7 (a) Define:
(i) Mass fraction
(ii) Mole fraction
(iii) Stoichiometric air
(iv) Actual and excess air
(v) Air-fuel ratio and mixture strength.(6 marks) 7 (b) The ultimate analysis of solid fuel is as follows : C = 78%, O2 = 3%, H2 = 3%, S = 1%, moisture = 5% and ash = 10%. Calculate the mass of air supplied also individual and total mass of products of combustion per kg of fuel if 30% of excess air is supplied for combustion.(7 marks) 8 (a) Explain working of a bomb calorimeter with neat sketch for estimating the CV of solid for liquid fuels.(6 marks) 8 (b) In a bomb calorimeter the following observations were recorded:
Mass of coal burnt = 3 gm
Mass of water in calorimeter = 1.4 kg
Water equivalent of the calorimeter = 0.9 kg/K
Rise in temperature of water jacket = 9 deg. C
The coal contains 3% moisture by weight and R.T. = 25 deg. C.
Calculate the HCV and LCV. Consider latent heat of condensation of steam 2470 kJ/kg.(7 marks)