Thermodynamics - Dec 2011

Mechanical Engineering (Semester 3)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks.

Fill in the blank

1 (a) (i) An engine producing continuous work and no other effect is called as _____(1 marks) 1 (a) (ii) Entropy is a _____ function an _____ differential.(1 marks) 1 (a) (iii) Two carnot engines producing different powers but working between same temperature limits will have _____ thermal efficiency.(1 marks) 1 (a) (iv) For same compression ratio and same heat rejection write the thermal efficiency in decreasing order for otto, diesel and dual cycles.(1 marks) 1 (a) (v) If the state of a system departs from that of the surroundings an opportunity exists for producing _____(1 marks) 1 (b) Represent the following processes on P.V. and T-S diagram starting from the same point.
(i) Isentropic process
(ii) Isobaric process
(iii) Isochoric process
(iv) Isothermal process
(v) Polytropic process(5 marks) 1 (c) 2 kg of steam is at 10 bar and 0.8 dry. Determine its enthlpy, entropy and volume (use steam table)(5 marks) 1 (d) Define thermodynamic property. list the tyes of property giving three examples of each.(5 marks) 2 (a) An air compressor takes in air at 1 bar and 20°C and discharges into a pipe having inlet diameter 20 mm. The average velocity of air at a point in the pipe close to the discharge is 7.7 m/sec and the discharge pressure is 3 bar. Neglecting the air inlet air velocity and assuming the compression of air as adiabatic. calculate the power input to the compressor take ? =1.4 and R=286.7 J/kg°k.(10 marks) 2 (b) Define the following terms -
(i) System
(ii) Quasi-static process
(iii) Reversible process
(iv) Thermodynamic work
(v) Thermodynamic equilibrium.(10 marks) 3 (a) State and prove Carnot Theorem.(8 marks) 3 (b) One kg of water of 0°C is brought into contact with a heat reservoir at 95°C when the water has reached 95°C find -
(i) Enthalpy change of water
(ii) Enthalpy change of heat reservoir
(iii) Enthalpy change of universe.(8 marks) 3 (c) Write the limitations of first law of thermodynamics.(4 marks) 4 (a) Define availability. Derive an expression of availability for steady flow system.(10 marks) 4 (b) Air at the rate 25 kg/min is compressed in a centrifugal compressor from 1 bar to 2 bar. The temperature increases from 15°C to 105°C during compression. Determine actual and minimum power required to run the compressor. The surrounding air temperature is 15°C. Neglect the changes in K.E. and P.E.(10 marks) 5 (a) Steam at 20 bar 350°C is expanded in a steam turbine to 0.1 bar. It them enters a condenser where it is condensed to saturated liquid water. The pump feed back the water into the boiler.
Determine Rankine efficiency and specific steam consuption of the power plant. Also find the Rankine efficiency when turbine and pump have efficiencies 0.8 and 0.75 respectively.(10 marks) 5 (b) Define -
(i) Saturation temperature
(ii) Dryers fraction
(iii) Latent heat of evaporation
(iv) Work ratio
(v) Specific steam consumption.(10 marks) 6 (a) Show that the compression ratio for the maximum work to be done per kg of air in an otto cycle between upper and lower limits of absolute temperatures T₃ and T₁ is given by
$$ R_{C}=\left (\dfrac {T_{3}}{T_{1}} \right )^{1.25}$$(10 marks) 6 (b) The peak pressure in an otto cycle is 21 bar and the minimum pressure is 1 bar with thermal efficiency of 47.5% find -
(i) Compression ratio
(ii) Mean effective pressure.(10 marks) 7 (a) What is the effect of variation in back pressure on C-D nozzle performance.(7 marks) 7 (b) Explain how the normal shock wave is developed.(5 marks) 7 (c) Derive the relation for one dimensional isentropic flow.
$$\dfrac {dA}{A}= \dfrac {dV}{V}(m^2 -1)$$(8 marks)