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Reg. No. : ..................................... Name : ..........................................
Third Semester B.Tech. Degree Examination, November 2009 (2008 Scheme) Branch : Mechanical – Production Engineering 08.305 : MECHANICAL ENGINEERING Time : 3 Hours
Max. Marks : 100 PART – A (Answer all questions.)
1. Differentiate thermal conductivity and thermal diffusivity. What are the factors affecting thermal conductivity ? 2. What is meant by the term log mean area ? 3. Distinguish between emissive power and emissivity of a body . 4. Explain the term LMTD and NTU as applied to heat exchangers. 5. What is meant by optimum pressure ratio in compressors ? Obtain an expression for the same ? 6. Explain the different methods of improving thermal efficiency of gas turbine cycles. 7. Sketch the layout a vapour compression refrigeration cycle and name the various components . 8. What is meant by sensible heat factor ? How does it differs from by pass factor ? 9. Name the various processes in conditioning of air. Show each on psychrometric chart. 10. Explain the term Ozone depletion and Global warming.
(10×4=40 Marks)
P.T.O.
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PART – B (Answer any one question from each Module). Module – I 11. a) Derive an expression for general heat conduction in Cartesian co-ordinate.
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b) The interior temperature of a refrigerator is maintained at 7°C. The walls are constructed with two mild steel sheets 3 mm thick with 5 cm of glass wool insulation between them. The heat transfer coefficient on inner and outer surface of refrigerator are 10 W/m°K and 12.5 W/m2°K respectively. Find the rate of heat removed from the refrigerator when it is kept in a kitchen room. The temperature of air in kitchen is 28°C given, K(mild steel) = 40 W/mk K(glass wool) = 0.04 W/mk. 12. a) Derive air expression for the LMTD of a counter flow heat exchanger.
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b) Lubricating oil at temperature of 60°C enters into 1 cm diameter pipe with a velocity of 2.5 m/s. The tube surface is maintained at 30°C. Calculate the tube length required to cool the oil to 45°C. Properties of oil are ρ = 865 Kg / m3 , Ω
K = 0.12 W/mk, CP = 1.7 KJ/Kg K and
= 9 × 10−6 m2 / sec.
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Module – II 13. a) Derive an expression for the thermal efficiency of a simple gas turbine working on joule cycle in terms of its pressure ratio.
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b) In an open cycle gas turbine, air enters the compressor at 1 bar and 300 K. The pressure of air after compression is 4 bar. The isentropic efficiencies of compressor and turbine are 78% and 85% respectively. The air fuel ratio is 80:1. Calculate the power developed and thermal efficiency of the cycle if the air flow rate is 2.5 Kg/sec. Take Cp = 1.005 KJ/KgK, γ for air = 1.4 and Cp = 1.147 KJ/KgK and γ = 1.33 for gases. R = 0.287 KJ/KgK. CV of fuel = 42000 KJ/Kg. 12
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14. A single stage, double acting air compressor delivers 15 m3/min at a pressure of 1.013 bar and temperature 27°C and delivers at 7 bar. The conditions at the end of the suction stroke are pressure 0.98 bar and temperature 40°C. The clearance volume is 4% of the swept volume and the stroke/bore ratio is 1.3/1. The compressor runs at 300 rpm. Calculate : i) Volumetric efficiency ii) Cylinder dimensions iii) Indicated power and iv) Isothermal efficiency of the compressor. Take index of compression and expansion as 1.3. 20 Module – III 15. a) Explain the effect of liquid suction heat exchanger on the performance of a simple vapour compression refrigeration cycle.
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b) A sample of moist-air has a dry bulb-temperature of 43°C and a wet bulbtemperature of 29°C. Calculate the following without making use of the psychrometric chart : a) Partial pressure of water vapour b) Specific humidity c) Relative humidity d) Dew point temperature e) Enthalpy and f) Degree of saturation. 12 16. a) With neat sketch explain the working of electrolux system of absorption refrigeration.
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b) In an auditorium which is to be maintained at a temperature not exceeding 24°C and a relative humidity 60%; a sensible heat load of 132 kW and 84 Kg/hr of moisture has to be removed. Air is supplied to the auditorium at 15°C. i) How many Kg of air per hour must be supplied ? ii) What is the dew point temperature of the supply air ? iii) What is the ADP of the coil ? 10 ———————