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B.E. (Mechanical Engineering / Power Engineering) Fourth Semester (C.B.S.)

Engineering Thermodynamics

P. Pages : 3 NRJ/KW/17/4424/4448
Time : Three Hours *0217* Max. Marks : 80
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Notes : 1. All questions carry marks as indicated.
2. Solve Six questions as follows:
Question No. 1 OR Questions No. 2.
Question No. 3 OR Questions No. 4.
Question No. 5 OR Questions No. 6.
Question No. 7 OR Questions No. 8.
Question No. 9 OR Questions No. 10.
Question No. 11 OR Questions No. 12.
3. Use of Non programmable Calculator is permitted.
4 Use of Steam Table / Mollier's Charts is permitted.
5. Assume suitable data wherever necessary.
6. The solution must be supported with appropriate p-v/t-s/h-s diagrams.

1. a) Define a thermodynamic system. 4

Differentiate between open system, closed system and an isolated system giving example
of each.

b) Discuss the concept of thermodynamic equilibrium and its importance in engineering 3

thermodynamics.

A non-flow reversible process follows the law P   V 2   .

8
c)
 v 6
3
Where P is in bar and V is in m . Find the work done during the process when the volume
change from 3 m3to1 m3 .

OR

2. a) Derive an expression for work done during a polytropic process. 7

rn
Hence prove that the heat transfer Q     W.
 r 1 

b) The internal energy of a certain system is a function of temperature only and is given by 6
u  25  0.25T, where T is the temperature in ºC. When this executes a certain process,
dW
the work done per degree temperature change is given by  350 T Joule.
dT
Find the heat transferred when the temperature of the system changes from 100ºC to
260ºC.

3. a) State the First law of thermodynamics and explain it when applied to 4

i) Process and ii) Cycle

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b) Air is compressed from 1 bar and 300k to 5 bar isothermally. Then it receives heat as 10
constant pressure and finally returns to its original condition following the constant volume
path. Determine heat transfer, work transfer and change in internal energy for each process
and also for the entire cycle.

OR

4. a) Write down the general steady flow energy equation. Derive the simplified form when 5
used for the following systems.
i) Centrifugal water pump. ii) Steam nozzle.

b) Steam with an enthalpy of 3000 kJ/kg and specific volume of 0.187m3/kg. enters a steam 9
nozzle and leaves with 2762 kJ/kg of enthalpy and 0.498 m3/kg specific volume. The inlet
velocity of steam is 60m/s and inlet area is 0.1m2. Find out velocity of steam at the exit of
the nozzle, steam mass flow rate and exit area.

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5. a) Explain the following. 6

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i) Cyclic Heat Engine ii) Refrigerator

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iii) Heat pump

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b) An inventor makes the following claims. Determine, whether his claims are valid or 7
invalid, why?
w.
i) A building receives heat of 50,000 kJ/hr from a heat pump. The inside temperature is
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maintained at 22ºC and surroundings are at -1ºC. The inventor claims a work input of
7000kJ/hr is sufficient.

ii) An engine operates between 1000K and 400K with a heat transfer into the engine is
500kW. The inventor states that the heat transfer to the low temperature reservoir is
250 kW and the work output is 250 kW.
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e.c

OR
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6. a) State and prove clausius theorem. 6

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b) One kg of ice at -5ºC is exposed to the atmosphere which is at 20ºC. The ice melts and 7
comes into thermal equilibrium with the atmosphere.
w.
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Determine the entropy increase of the universe.

Cp of ice = 2.093 kJ/kg K.
Latent heat of fusion of ice = 333.3 kJ/kg.

7. a) Explain the following terms related to steam formation. 5

i) Dryness fraction ii) Degree of superheat
iii) Triple point iv) Critical point
v) Sensible heat of water

b) Calculate the internal energy per kg of superheated steam at a pressure of 10 bar and a 8
temperature of 300ºC. Also find the change of internal energy if this steam is expanded to
1.4 bar and dryness fraction 0.8.

OR

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8. a) Describe with a neat sketch a throttling calorimeter for measuring the dryness fraction of 6
steam. State its limitations.

b) Water is supplied to the boiler at 15bar and 80ºC and steam is generated at the same 7
pressure at 0.9 dryness. Determine the heat supplied in the boiler and change in entropy.

9. a) In a Rankine cycle, the steam at inlet to turbine is dry saturated at a pressure of 30 bar and 11
the exhaust pressure is 0.25 bar.
Determine.
i) Pump work.
ii) Turbine work.
iii) Rankine efficiency.
iv) Condenser heat flow.
v) Dryness at the end of expansion.

b) Why Carnot cycle is not used in actual power plant? 2

OR

10. a) Explain with the help of a neat diagram a 'Reheat cycle'. Also derive an expression for its 6
thermal efficiency.

b) In a regenerative cycle the inlet conditions are 40 bar and 400ºC. Steam is bled at 10 bar 7
in regenerative heating. The exit pressure is 0.8bar. Neglecting pump work determine the
efficiency of the cycle.

11. a) Prove that the efficiency of an Otto cycle is a function of compression ratio. 5

b) An engine with 200mm cylinder diameter and 300mm stroke works on theoretical Diesel 9
cycle. The initial pressure and temperature of air used are 1bar and 27ºC. The cut off is
8% of the stroke. Determine.
i) Pressure and temperature at all salient points.
ii) Air standard efficiency.
Assume that compression ratio is 15 and working fluid is air.

OR

12. a) Plot a Brayton cycle on p-v & t-s diagrams. Derive an expression for air standard 5
efficiency of Brayton cycle.

b) An engine working on the Otto cycle is supplied with air at 0.1MPa, 35ºC. The compression 9
ratio is 8. Heat supplied is 2100 kJ/kg. Calculate the maximum pressure and temperature of
the cycle, the cycle efficiency and the mean effective pressure.

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