Question Paper: Applied Thermodynamics : Question Paper Dec 2013 - Mechanical Engineering (Semester 4) | Visveswaraya Technological University (VTU)

Applied Thermodynamics - Dec 2013

Mechanical Engg. (Semester 4)

(1) Question 1 is compulsory.
(2) Attempt any four from the remaining questions.
(3) Assume data wherever required.
(4) Figures to the right indicate full marks.
1 (a) Define the therms: i) Stoichiometric air; ii) Enthalpy of combustion iv) Enthalpy of formation; v) Adiabatic flame temperature.(10 marks) 1 (b) The volumetric composition of dry fuel gases obtained by the combustion of an unknown hydrocarbon is 12.7% CO2, 0.9% CO, 3.9% O2 and 82.5% N2 Determine: i) Composition of the fuel; ii) Theoretical air required for complete combustion ; iii) Percentage of excess air required.(10 marks) 2 (a) With the help of P-V and T-S diagrams, derive an equation for theoretical air standard efficiency of a semi diesel (dual) cycle in terms of compression ratio, cut off ratio and explosion ratio, with suitable assumptions.(10 marks) 2 (b) In an air standard diesel cycle the compression ratio is 15 and the fluid properties at the beginning of compression are 100kPa and 300K. For a peak temperature of 1600K calculate i) the percentage of stroke at which cut off occurs; ii) the cycle efficiency and iii) the work output/kg.(10 marks) 3 (a) Describe the following as applied to I.C engine i) Morse test ii) Heat balance sheet.(8 marks) 3 (b) During a test on a single cylinder 4 stroke oil engine the following observation were made Bore=30cm, stroke=45 cm, duration of trail =1hr , total fuel consumption=7.6 kg calorific value of fuel=45,000kJ/kg. Total revolution made=12000, mean effective pressure 6 bar, net brake load=1.47 kN. Brake drum diameter 1.8m rope diameter 3cm. Mass of jacket cooling water circulated=550kg water enters at 15°C water leaves at 60°C. Total air consumption 360kh room temperature 20°C, exhaust gas temperature 300°C, calculate: i) Indicated and break power ii) Indicated thermal efficiency iii) Mechanical efficiency ; iv) Draw the heat balance sheet on minute basis(12 marks) 4 (a) Why Carnot cycle is not used as a reference cycle for steam power plant?(3 marks) 4 (b) Sketch the flow diagram and corresponding T-S diagram,of a reheat vapour cycle and derive an expression for reheat cycle efficiency(7 marks) 4 (c) A 40MW steam power plant working on Rankine cycle operates between boiler pressure of 4MPa and condenser pressure of 10 kPa. The steam leaves the boiler and enters the steam turbine at 400°C. The isentropic efficiency of steam turbine is 85%. determine : i) The cycle efficiency ii)The quality of exhaust steam turbine and iii) Steam flow rate in kg/hr considering pump work.
Properties of steam

Pressure bar ts c Specific volume m3/kg Specific enthalpy kJ/kg Specific entropy kJ/kgK
Vf Vg hf hf hg sf sf sg
40 250.3 0.00125 0.049 1087.4 1712.9 2800.3 2.797 3.272 6.069
0.1 45.83 0.0010


191.8 2392.9 2584.7 0.649 7.502 8.151
(10 marks) 5 (a) State the advantages of multi-stage compression.(4 marks) 5 (b) Derive the relation among volumetric efficiency with clearance for various pressure ratio.(8 marks) 5 (c) Atmospheric air at 1 bar and 27°C is taken into a single stage reciprocating compressor. It is compressed according to the law PV13=C, to the delivery pressure of 6 bar. The compressor takes 1m3 of air/min. The speed of the compressor is 300rpm. Stroke to diameter ratio is equal to 1.5:1 mechanical efficiency of the compressor 0.85 motor transmission efficiency 0.9 calculate:
i)The indicated power and isothermal efficiency.
ii) The cylinder dimensions and power of motor required to drive the compressor.
(8 marks)
6 (a) Discuss with help of T-S diagram the three methods of improving the thermal efficiency of an open cycle gas turbine plant.(12 marks) 6 (b) A gas turbine unit has pressure ratio 6.1 and maximum cycle temperature of 610°C. The isentropic efficiency of the compressor and turbine are 0.80 and 0.82 respectively. Calculate the power output in kilo wants of an electric generator geared to the turbine when the air enters the compressor at 15°C and rate of 16kg/s. Assume C_{\rho}=1.005 for C_{\rho}=1.11 for r=1.4 compression=expansion =n=1.33(8 marks) 7 (a) Write a short note on air cycle refrigeration.(4 marks) 7 (b) With the help of neat flow diagram explain the working of steam jet refrigeration system.(8 marks) 7 (c) An ammonia vapour compression refrigeration machine works between 25°C and -20°C. The ammonia leaves the compressor in dry and saturated condition. Liquid ammonia is under cooled to 21.5°C before passing through throttle valve. The average specific heat of liquid ammonia is 4.75 kJ/kg°C. Find the theoretical cop of machine. The following properties of NH3 are given. If the net refrigeration required is 400 × 103 kj/hr. Find the mass of ammonia circulated /min. Assume cop actual is 75% of cop theoretical:
Temp C Liquid kJ/kg Vapour kJ/kg K
hfkJ/kg sf hg sg
25 537.6 4.612 1708.5 8.534
-20 328.4 3.854 1661.0 9.118
(8 marks)
8 (a) Derive the equation for relative humidity and specific humidity of moist air.(4 marks) 8 (b) With neat sketch explain the working of air conditioning system for hot and dry weather. Present the process involved on a psychometric chart.(6 marks) 8 (c) Calculate i) relative humidity; ii) humidity ratio; iii) dev point temperature; iv)density and v)Enthalpy of atmospheric air when the DBT is 35°C, WBT=23°C and the barometer reads 750mm Hg.(10 marks)

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