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B.E./B.Tech. DEGREE EXAMINATION, APRIL/MAY 2011
Fifth Semester
Mechanical Engineering
ME 2301 - THERMAL ENGINEERING
(Regulation 2008)
(Common to PTME 2301 Thermal Engineering for B.E. (Part-Time)
Mechanical Engineering Fourth Semester – Regulation 2009)
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PART A — (10 × 2 = 20 marks)
1. Which air standard cycle (Otto / Diesel / Dual) is more efficient for the same heat input? Justify.
2. Define cut off ratio for a diesel cycle.
3. State the merits of a Diesel engine over a petrol engine.
4. List out the major pollutants from a petrol engine.
5. Define Free Air Delivered with regard to a compressor.
6. State the effect of clearance on work done in a reciprocating compressor.
7. State the necessity for governing of steam turbines.
8. What is supersaturated flow in a nozzle?
9. What is the effect of sub-cooling a refrigerant in a vapour compression cycle?
10. Define Bypass factor of a heating coil.
PART B — (5 × 16 = 80 marks)
11. (a) (i) Draw the actual p-v diagram of an Otto cycle and discuss the deviation from an ideal cycle. (4)
(ii) For air standard diesel cycle the following data is available (12)
Compression ratio = 16
Heat added/kg = 2500 kJ/kg
Lowest temperature in the cycle = 300 K
Lowest pressure in the cycle = 1 bar
Calculate :
(1) Pressure and temperature at each point in the cycle
(2) Thermal efficiency
(3) Mean effective pressure if air flow rate of 0.25 kg/sec
Assume p C = 1 kJ/kg K and v C = 0.714 kJ/kg K.
Or
(b) (i) Define mean effective pressure? Give its expression and state its significance. (4)
(ii) For air standard Otto cycle the following data is available : (12)
Compression ratio = 9
Heat added/kg = 1200 kJ
Lowest temperature in the cycle = 300 K
Lowest pressure in the cycle = 1 bar
Calculate
(1) Pressure and temperature at each point in the cycle
(2) Thermal efficiency
(3) Mean effective pressure if air flow rate of 0.25 kg/sec
Assume p C = 1 kJ/kg K and v C = 0.714 kJ/kg K.
12. (a) (i) Distinguish between Impulse and reaction turbines. (4)
(ii) Steam issues from the nozzles of a De Laval turbine with a velocity of 1000 m/sec. The nozzle angle is 20°. Mean bade velocity is 400 m/sec. The blades are symmetrical. The mass flow rate is 1000 kg/h. Friction factor is 0.8, nozzle ? = 0.95. Determine
(1) Blade angles.
(2) Axial thrust on the rotor turbine. (12)
(3) Work done per kg of steam.
(4) Power developed.
(5) Blade efficiency.
(6) Stage efficiency.
Or
(b) (i) Derive the expression for critical pressure ratio in a steam nozzle.
(8)
(ii) Steam at a pressure of 10 bar and 0.98 dry is passed through a CD nozzle to a back pressure of 0.1 bar. The mass flow rate of steam is 0.55 kg/s. Find
(1) pressure at throat
(2) no. of nozzles used if each nozzle has a throat area of 0.52 cm .
The enthalpy drop used for reheating the steam by friction in the divergent part is 10% of overall isentropic drop. Take index of expansion n = 1.13. (8)
13. (a) (i) Sketch the typical valve timing diagram of a high speed 4 stroke petrol engine. (6)
(ii) Explain the functioning of a water cooling system with a neat sketch. (10)
Or
(b) (i) Describe the construction functioning of magneto ignition system used in modern two wheeler engines. (8)
(ii) Sketch the jerk type diesel fuel injection pump and discuss how it injects diesel different load conditions. (8)
14. (a) (i) A single stage double acting air compressor delivers 15 m min 3 of air measured at 1.013 bar, 27°C. The air is delivered at 7 bar.
The conditions at the end of suction stroke are pressure 0.98 bar and temperature 35°C. The clearance volume is 4% of stroke volume, the L/D ratio is 1.3 and the compressor runs at 300 rpm. Calculate the volumetric efficiency, cylinder dimensions and isothermal efficiency of the compressor. Take index of expansion and compression as 1.3 and R = 0.287 kJ/kg. K (12)
(ii) State the merits of multistage compression. (4)
Or
(b) (i) Derive the necessary condition for minimum work input in a multistage compression process. (6)
(ii) The free air delivery of a single stage reciprocating air compressor is 2.5 m min 3 . The ambient air is at N.T.P condition and the delivery pressure is 7 bar. The clearance volume is 5% of stroke volume and law of compression and expansion is 1.25 pv = C. If L = 1.2 D, and the compressor runs at 150 rpm, determine the dimensions of the cylinder. (10)
15. (a) (i) A refrigerant plant using 2 CO as a refrigerant works between 298 K and 268K. The dryness fraction of 2 CO is 0.8 at entry of compressor. Find out the ice formed per month if the relative
efficiency is 50%. Take that ice is formed at 0°C from water at 10°C. The quantity of 2 CO circulated is 6 kg/min. Assume p C for water as 4.187 kJ/kg. K and latent heat of fusion of ice as 335 kJ/kg. K(12)
Properties of 2 CO are given below Temperature K Liquid Heat Latent Heat Entropy of liquid
kJ/kg kJ/kg kJ/kg. K
298 81.25 121.5 0.2513
268 –7.53 245.8 –0.04187
(ii) Discuss the merits and demerits of a vapour absorption system over a vapour compression system. (4)
Or
(b) (i) Discuss the requirement of a winter air-conditioning system. Draw a schematic of the same. (6)
(ii) Saturated air at 2°C is required to be supplied to a room where the temperature must be held at 20°C with a relative humidity of 50%. The air is heated and then water at 10°C is sprayed to
give the required humidity. Determine the temperature to which the air must be heated and the mass of spray water required per 3 m of air room conditions. Assume that the total pressure is constant at 1.013 bar and neglect fan power. (10)
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