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Posted By: sasi kala Member Level: Diamond Posted Date: 01 Jun 2008
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2007 Anna University B.E Mechanical ME 231 — APPLIED THERMODYNAMICS Question paper
B.E./B.Tech. DEGREE EXAMINATION,
Third Semester
Mechanical Engineering
ME 231 — APPLIED THERMODYNAMICS
Time : Three hours Maximum : 100 marks
Answer ALL questions.
Use of Thermodynamic Tables and Charts permitted.
PART A — (10 ? 2 = 20 marks)
1. Explain the phenomenon of supersaturated expansion in steam nozzle.
2. What are the effects of friction on the flow through a steam nozzle?
3. What are the advantages of multi-stage compression with intercooling over single stage compression for the same pressure ratio?
4. Discuss the effect of clearance upon the performance of an air-compressor.
5. What are the assumptions made for air-standard cycle analysis?
6. With the help of p-v and T-s diagrams, show that for the same maximum pressure and temperature of the cycle and the same heat rejection, .
7. Why is Rankine cycle modified?
8. What are the limitations of binary vapour cycle?
9. What are the advantages and disadvantages of air refrigeration system?
10. What are the properties of an ideal refrigerant?
PART B — (5 ? 16 = 80 marks)
11. A mercury cycle is superposed on the steam cycle operating between the boiler outlet condition at 40 bar, 400°C and the condenser temperature of 40°C. The heat released by mercury condensing at 0.2 bar is used to impart the latent heat of vapourisation to the water in the steam cycle. Mercury enters the mercury turbine as saturated vapour at 10 bar. Calculate (i) mass of mercury circulated per kg of water, and (ii) the efficiency of the combined cycle. The property values of saturated mercury are given in Table-11 : (8 + 8)
Table-11
P (bar) T (°C) Enthalpy (kJ/kg) Entropy (kJ/kgK) Specific volume (m3/kg)
10 515.5 72.23 363.0 0.1478 0.5167 80.9 ? 10–6 0.0333
0.2 277.3 38.35 336.55 0.0967 0.6385 77.4 ? 10–6 1.163
12. (a) Steam at 10.5 bar and 0.95 dryness is expanded through a convergent divergent nozzle. The pressure of steam leaving the nozzle is 0.85 bar. Find (i) the velocity of steam at throat for maximum discharge
(ii) the area at the exit (iii) the steam discharge if the throat area is
1.2 cm2. Assume the flow is isentropic and there are no friction losses.
Take . (5 + 8 + 3)
Or
(b) Dry saturated steam at 2.8 bar is expanded through a convergent nozzle to 1.7 bar. The exit area is 3 cm2. Calculate the exit velocity and the mass flow rate, assuming (i) isentropic expansion and (ii) supersaturated flow. (8 + 8)
13. (a) A two-stage compressor delivers 2 m3 free air per minute. The temperature and pressure of air at the suction are 27°C and 1 bar. The pressure at the delivery is 50 bar. The clearance is 5% of the stroke in L.P. cylinder as well as in H.P. cylinder. Assume perfect inter-cooling between the two stages, find (i) the minimum power required to run the compressor. (ii) If the compressor is to run at 200 rpm find the diameters and strokes assuming the strokes of both the cylinders are equal to the diameter of L.P. cylinder. (4 + 12)
Or
(b) A single-acting two-stage compressor with complete inter-cooling delivers 5 kg/min of air at a pressure of 15 bar. The intake state of air is 1 bar and 15°C. The clearance volumes of L.P. and H.P. cylinders are 5% and 6% of the respective cylinder swept volumes. The speed of the compressor is 420 rpm. Assuming the compression and expansion processes are polytropic with . Calculate (i) the power required (ii) the isothermal efficiency (iii) swept and clearance volumes of the L.P. and H.P. cylinders. (4 + 2 + 10)
14. (a) An air standard dual cycle has a compression ratio of 16, and compression begins at 1 bar, 50°C. The maximum pressure is 70 bar. The heat transferred to air at constant pressure is equal to that at constant volume. Estimate (i) the pressures and temperatures at cardinal points of the cycle (ii) the cycle efficiency (iii) the m.e.p. of the cycle. (For air kJ/kg K and kJ/kgK). (8 + 3 + 5)
Or
(b) In a Brayton cycle the air enters the compressor at 1 bar and 25°C. The pressure of air leaving the compressor is 3 bar and temperature at turbine inlet is 650°C. Determine per kg of air (i) cycle efficiency
(ii) heat supplied to air (iii) work output (iv) heat rejected in the cooler and (v) temperature of air leaving the turbine. (3 + 4 + 6 + 3)
15. (a) A dense air machine operates on a reversed joule cycle and is required for a capacity of 12 tonnes. The cooler pressure is 4.2 bar and the refrigerator pressure is 1.4 bar. The air is cooled in the cooler to a temperature 47°C and the temperature of air at inlet is –17°C. Determine for the ideal cycle (i) C.O.P. (ii) mass of air circulated per min (iii) theoretical piston displacement of compressor (iv) theoretical piston displacement of expander and (v) power per tonne of refrigeration.
(4 + 3 + 3 + 3 + 3)
Or
(b) A refrigerator works between –7°C and 27°C. The vapour is dry at the end of adiabatic compression. Assuming there is no under cooling determine (i) the C.O.P. (ii) power of the compressor to remove a
heat load of 12140 kJ/h. The properties of the refrigerant are given in Table 15 b. (8 + 8)
Table 15 (b)
Temperature °C Sensible heat, kJ/kg
Latent heat kJ/kg
Entropy of liquid kJ/kgK
Entropy of vapour kJ/kgK
–7 –29.3 1297.9 –0.109 4.748
27 117.23 1172.3 0.427 4.333
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