Anna University Chennai 2005 B.E Electrical and Electronics Engineering Third Semester - Question Paper

B.E./B.Tech. DEGREE exam
Third Semester
Electrical and Electronics Engineering
ME 251 — THERMODYNAMICS
Time : 3 hours Maximum : 100 marks
ans ALL ques..
Use of Steam tables, Mollier chart and HMT databook permitted.
PART A — (10 ? two = 20 marks)
1. What is meant by ‘Perpetual Motion Machine of 1st kind’?
2. Write the 2 statements of the 2nd legal regulations of thermodynamics.
3. What is the effect of cut-off ratio on the efficiency of diesel cycle when the
compression ratio is kept constant?
4. Sketch Otto and Diesel cycle for the identical compression ratio and heat input
and compare the efficiency.
5. 1 kg of steam at 10 bar has an enthalpy of 2500 kJ/kg. obtain its quality.
6. What is the necessity of compounding of steam turbine? Name 2 kinds.
7. What is sub cooling? What is its effect on COP of vapour compression system?
8. Name any 2 rotary kind of air compressors.
9. State Fourier's legal regulations of heat conduction.
D 056
2 D 056
10. describe emissivity.
PART B — (5 ? 16 = 80 marks)
11. (i) Sketch the dual cycle on P-V and T-S co-ordinates and name the different
processes. (4)
(ii) Sketch the Brayton cycle. Air enters the compressor of the cycle at one bar
and 25°C : Pressure after compression is three bar. Temperature at turbine
inlet is 650°C. Determine per kg of air the cycle efficiency, heat supplied
to air, work available, heat rejected in the cooler and temperature of air
leaving the turbine. (12)
12. (a) Air undergoes a cyclic process in a cylinder and piston arrangement.
Atmospheric air at one bar and 27°C is compressed adiabatically to 10 bar,
expanded isothermally to initial pressure and brought to initial condition
at constant pressure. obtain the change in internal energy, enthalpy
change, heat transfer and work transfer for every process and for the
cycle. (16)
Or
(b) 2 reversible heat engines A and B are organizes in series. A rejects
heat directly to B. A receives 200 kJ at 421°C from the hot source while B
rejects heat to cold sink at 5°C. Work output of A is twice that of B. obtain
the intermediate temperature ranging from A and B, efficiency of every engine
and heat rejected to the sink. (16)
13. (a) Steam at 10 bar and 0.95 dry is available. obtain the final dryness fraction
of steam for every of the subsequent operations, using steam tables :
(i) 160 kJ of heat is removed per kg of steam at constant pressure.
(ii) It is cooled at constant quantity till its temperature falls to 140°C.
(iii) Steam expands isentropically in the steam turbine developing
200 kJ of work per kg of steam flow and pressure becomes 0.5 bar.
(7 + five + 4)
Or
(b) A coal fired boiler plant consumes 400 kg of coal per hour. The boiler
evaporates 3200 kg of water at 45°C into superheated steam at a
pressure of 12 bar and 275°C. If the calorific value of fuel is 32760 kJ/kg
of coal, determine ; (i) Equivalent evaporation ‘‘from and 100°C,’’ and
3 D 056
(ii) Thermal efficiency of the boiler. Specific heat of super heated
steam = 2.1 kJ/kgK. (12 + 4)
14. (a) (i) State 4 advantages of multistage compression. (4)
(ii) A 2 stage air compressor compresses air from one bar and 20°C to
42 bar. If the legal regulations of compression is PV1.35 = constant and the
intercooling is complete to 20°C obtain per kg of air the compressor
work. obtain also mass of water circulated in the intercooler if its
temperature raise is 25°C. (12)
Or
(b) (i) List atleast 4 desirable properties of a good refrigerant. (4)
(ii) A refrigeration system operates with condensing and evaporating
temperatures of 30°C and –5°C. There is no sub cooling and
refrigerant is dry saturated at the end of compression. Sketch the
cycle on T-s and P-h diagrams and obtain the theoretical COP. (12)
The properties of refrigerant :
Enthalpy (kJ/kg) Entropy (kJ/kg-K)
Temperature Liquid Vapour Liquid Vapour
–5°C 158.3 1431.9 0.63 5.41
30°C 323. 1465.4 1.202 4.984
15. (a) A furnace wall is made up of 3 layers of thickness 250, 100 and
150 mm with thermal conductivities of 1.65 K and 9.2 W/m°C
respectively. The inside is exposed to hot gases at 1250°C with convection
coefficient of 25 W/m2 °C. Inside surface temperature is 1100°C and
outside surface is exposed to air at 25°C with a convection co-efficient of
12 W/m2 °C. Determine the thermal conductivity, overall heat transfer coefficient
and all the surface temperatures. (16)
Or
(b) A steel rod 22 mm in diameter is to be heated from 420°C to 540°C. It is
placed concentrically in a long cylindrical furnace which has an inside
diameter 180 mm. The inner surface of the furnace is at 1100°C and its
emissivity is 0.82. The rod surface has an emissivity of 0.62. obtain the
time needed for heating operation. Specific heat of steel is 0.67 kJ/kg-K
and density is 7845 kg/m3. (16)
———————

Earning:   Approval pending.